Class 10th Science Chapter 12 – Magnetic Effects of Electric Current | Solenoid and Its Magnetic Field

Magnetic Effects of Electric Current

Solenoid and Its Magnetic Field

सोलनॉइड तथा उसका चुंबकीय क्षेत्र

Class 10 Science | CBSE + Foundation + Competitive

Solenoid and Its Magnetic Field

1. Introduction | परिचय

English:
A solenoid is a long cylindrical coil consisting of a large number of closely wound turns of insulated conducting wire. When electric current passes through a solenoid, it produces a magnetic field around it.

हिन्दी:
सोलनॉइड एक लंबी बेलनाकार कुंडली होती है, जिसमें विद्युतरोधी चालक तार के बहुत सारे पास-पास लपेटे हुए फेरे होते हैं। जब सोलनॉइड में विद्युत धारा प्रवाहित की जाती है, तो उसके चारों ओर चुंबकीय क्षेत्र उत्पन्न होता है।

2. Structure of a Solenoid | सोलनॉइड की संरचना

A solenoid generally consists of:

1. Long cylindrical arrangement
2. Large number of turns
3. Insulated conducting wire
4. Current supplied through the coil
5. It may contain a soft iron core when a strong electromagnet is required.

सोलनॉइड में तार के बहुत से पास-पास फेरे होते हैं। आवश्यकता के अनुसार इसके अंदर नरम लोहे का कोर भी रखा जा सकता है।

3. Animated Diagram of Solenoid

Orange = Current-carrying turns | Blue = Magnetic field

4. Magnetic Field Pattern of a Solenoid

The magnetic field produced by a current-carrying solenoid resembles the magnetic field of a bar magnet. Inside a sufficiently long solenoid, the magnetic field lines are nearly parallel and equally spaced. Therefore, the magnetic field inside a long solenoid is approximately uniform.

हिन्दी:
धारा-वाहक सोलनॉइड का चुंबकीय क्षेत्र बार मैग्नेट के चुंबकीय क्षेत्र के समान होता है। लंबे सोलनॉइड के अंदर चुंबकीय क्षेत्र रेखाएँ लगभग समानांतर और समान दूरी पर होती हैं। इसलिए अंदर का चुंबकीय क्षेत्र लगभग एकसमान होता है।

5. Uniform Magnetic Field | एकसमान चुंबकीय क्षेत्र

A magnetic field is called approximately uniform when its magnitude and direction are nearly the same throughout a region. Inside a long solenoid, the field lines are nearly parallel and equally spaced.

लंबे सोलनॉइड के अंदर चुंबकीय क्षेत्र लगभग एकसमान होता है क्योंकि वहाँ क्षेत्र रेखाएँ लगभग समानांतर तथा समान दूरी पर होती हैं।

6. Magnetic Field Due to a Long Solenoid

Magnetic Field inside a long solenoid:

B = μ₀ n I

Where:
B = Magnetic field
μ₀ = Permeability of free space
n = Number of turns per unit length
I = Current through solenoid

If the solenoid has N turns and length L:

n = N/L

Therefore:

B = μ₀(N/L)I

B = μ₀NI/L

This relation applies to an ideal/long solenoid in free space or approximately in air.

7. Number of Turns per Unit Length

The number of turns per unit length is represented by n.

n = N/L

Here:
N = Total number of turns
L = Length of solenoid
n = Turns per metre

Important:
More turns per unit length → stronger magnetic field.

अधिक turns per unit length → अधिक प्रबल चुंबकीय क्षेत्र।

8. Factors Affecting Magnetic Field of a Solenoid

Factor	Effect on Magnetic Field
Current I increases	B increases
Turns per unit length n increases	B increases
Current direction reverses	Magnetic field direction reverses
Suitable soft iron core is inserted	Field becomes much stronger

9. Solenoid as a Bar Magnet

A current-carrying solenoid behaves like a bar magnet. It has two magnetic poles:

North Pole (N)
South Pole (S)

Outside the solenoid, magnetic field lines emerge from the North pole and enter the South pole. Inside the magnet/solenoid, the field lines complete the closed path.

हिन्दी:
धारा-वाहक सोलनॉइड बार मैग्नेट की तरह व्यवहार करता है और इसके दो ध्रुव होते हैं—उत्तर तथा दक्षिण।

10. Finding Polarity of Solenoid

The polarity of a solenoid can be determined using the right-hand grip rule. Curl the fingers of your right hand in the direction of current through the turns. The thumb points towards the North pole of the solenoid.

Right-Hand Grip Rule:
Fingers → Current
Thumb → North Pole / Magnetic field direction inside the solenoid

Easy Memory Trick:

Anticlockwise current as seen from a face → That face is North.
Clockwise current as seen from a face → That face is South.

11. Why is the Field Inside a Solenoid Nearly Uniform?

The magnetic fields produced by individual turns add together inside the solenoid. For a long solenoid, the contributions combine in such a way that the field lines inside are nearly parallel and equally spaced. Therefore, the magnetic field inside is approximately uniform.

12. Magnetic Field Outside a Long Solenoid

For an ideal long solenoid, the magnetic field outside is very weak compared with the field inside. The field inside is therefore considered strong and approximately uniform, while outside it is much weaker.

लंबे आदर्श सोलनॉइड के बाहर चुंबकीय क्षेत्र अंदर की तुलना में बहुत कमजोर माना जाता है।

13. Effect of Increasing Current

From:

B = μ₀nI

If n remains constant:

B ∝ I

Therefore, doubling current doubles the magnetic field.

14. Effect of Turns per Unit Length

From:

B = μ₀nI

Therefore:

B ∝ n

If the number of turns per unit length is doubled, the magnetic field also doubles, provided current remains constant.

15. Soft Iron Core in Solenoid

When a suitable soft iron core is placed inside a current-carrying solenoid, the magnetic field becomes much stronger. The combination is called an electromagnet. Soft iron is preferred because it magnetises strongly when current flows and loses most of its magnetism when current is removed.

हिन्दी:
सोलनॉइड के अंदर नरम लोहे का कोर रखने पर चुंबकीय क्षेत्र बहुत अधिक मजबूत हो जाता है। इस व्यवस्था को विद्युतचुंबक कहा जाता है।

16. Solenoid vs Bar Magnet

Solenoid	Bar Magnet
Magnetism is produced by electric current.	Permanent magnetic behaviour is present.
Strength can be controlled by current and turns.	Strength is not normally controlled by simply switching current.
Can be switched ON/OFF.	Cannot simply be switched OFF.
Polarity can be reversed by reversing current.	Polarity does not reverse merely by changing an external current.

17. Electromagnet

An electromagnet is a temporary magnet produced when electric current flows through a coil, usually around a soft iron core.

Applications:
• Electric bells
• Relays
• Electromagnetic cranes
• Loudspeakers
• Motors and other electrical devices

18. How to Make a Strong Electromagnet

A strong electromagnet can generally be obtained by:

✔ Increasing current
✔ Increasing number of turns per unit length
✔ Using a suitable soft iron core
✔ Keeping the coil arrangement compact and appropriate

19. Numerical Example

Question: A long solenoid has 1000 turns distributed uniformly over a length of 0.5 m. It carries a current of 2 A. Calculate the magnetic field inside it in air.

Solution: N = 1000
L = 0.5 m
I = 2 A

n = N/L
n = 1000/0.5 = 2000 m⁻¹

B = μ₀nI

B = 4π × 10⁻⁷ × 2000 × 2

B = 16π × 10⁻⁴ T

Approximately: B ≈ 5.03 × 10⁻³ T

20. 30 MCQs | बहुविकल्पीय प्रश्न

1. A solenoid is:

A. A single straight wire

B. A long coil of many turns

C. A battery

D. A resistor

Answer: B

2. A current-carrying solenoid produces:

A. Magnetic field

B. Only sound

C. Only light

D. No field

Answer: A

3. The magnetic field inside a long solenoid is approximately:

A. Zero

B. Uniform

C. Random

D. Circular only

Answer: B

4. Magnetic field inside a long solenoid is:

A. B = μ₀nI

B. B = μ₀I/n

C. B = n/μ₀I

D. B = μ₀/nI

Answer: A

5. Number of turns per unit length is:

A. n = L/N

B. n = NI

C. n = N/L

D. n = N+L

Answer: C

6. Increasing current in a solenoid:

A. Decreases B

B. Increases B

C. Makes B zero

D. Has no effect

Answer: B

7. Increasing turns per unit length:

A. Increases magnetic field

B. Decreases magnetic field

C. Makes field zero

D. Has no effect

Answer: A

8. The magnetic field of a solenoid resembles that of:

A. Capacitor

B. Bar magnet

C. Resistor

D. Battery

Answer: B

9. A solenoid can be used to make:

A. Electromagnet

B. Thermometer

C. Fuse only

D. Ammeter only

Answer: A

10. The SI unit of magnetic field is:

A. Ampere

B. Tesla

C. Volt

D. Ohm

Answer: B

11. The direction of polarity of a solenoid can be determined using:

A. Right-hand grip rule

B. Ohm's law

C. Joule's law

D. Snell's law

Answer: A

12. A suitable core used in an electromagnet is:

A. Soft iron

B. Wood

C. Plastic

D. Glass

Answer: A

13. If current is doubled, magnetic field becomes:

A. Half

B. Double

C. Four times

D. Zero

Answer: B

14. If turns per unit length are doubled, B becomes:

A. Half

B. Same

C. Double

D. Zero

Answer: C

15. If current direction is reversed, magnetic field direction:

A. Reverses

B. Remains same

C. Becomes zero

D. Becomes infinite

Answer: A

16. The field lines inside a long solenoid are:

A. Nearly parallel

B. Random

C. Circular around every point

D. Absent

Answer: A

17. Outside an ideal long solenoid, magnetic field is:

A. Very strong

B. Much weaker than inside

C. Always infinite

D. Equal everywhere

Answer: B

18. An electromagnet is generally:

A. Temporary magnet

B. Permanent magnet only

C. Non-magnetic object

D. Battery

Answer: A

19. Soft iron is used in electromagnets because it:

A. Is easily magnetised

B. Cannot be magnetised

C. Is transparent

D. Produces electricity itself

Answer: A

20. The field of a solenoid becomes stronger with:

A. More turns per unit length

B. Less current

C. Fewer turns

D. No current

Answer: A

21. If N = 500 and L = 0.5 m, n is:

A. 100 turns/m

B. 500 turns/m

C. 1000 turns/m

D. 250 turns/m

Answer: C

22. For a long solenoid, B depends on:

A. n and I

B. Only length

C. Only radius

D. Only resistance

Answer: A

23. If current becomes zero, ideal electromagnet loses its magnetic field:

A. Immediately/approximately

B. It becomes stronger

C. It becomes infinite

D. It doubles

Answer: A

24. Which rule identifies the north pole of a solenoid?

A. Right-hand grip rule

B. Fleming's left-hand rule

C. Ohm's law

D. Lens rule

Answer: A

25. The field inside a long solenoid is approximately:

A. Uniform

B. Zero

C. Non-existent

D. Completely random

Answer: A

26. If n and I are both doubled, B becomes:

A. Half

B. Double

C. Four times

D. Same

Answer: C

27. A solenoid has 2000 turns in 1 m. Its n is:

A. 200 turns/m

B. 2000 turns/m

C. 20 turns/m

D. 2 turns/m

Answer: B

28. Which material is generally preferred as an electromagnet core?

A. Soft iron

B. Rubber

C. Plastic

D. Paper

Answer: A

29. A solenoid has two magnetic poles:

A. North and South

B. East and West

C. Positive and negative

D. Hot and cold

Answer: A

30. A major advantage of an electromagnet is:

A. Its magnetism can be controlled

B. It cannot be switched

C. It always remains magnetic

D. It does not require current

Answer: A

21. 30 Subjective Questions with Answers

2 Marks 1. What is a solenoid?

A solenoid is a long cylindrical coil having a large number of closely wound turns of insulated conducting wire.

2 Marks 2. What happens when current passes through a solenoid?

A magnetic field is produced around the solenoid. The solenoid behaves like a bar magnet.

2 Marks 3. What is meant by number of turns per unit length?

It is the number of turns present per unit length of the solenoid. n = N/L

2 Marks 4. Write the formula for magnetic field inside a long solenoid.

B = μ₀nI

2 Marks 5. Name the material generally used as core of an electromagnet.

Soft iron is generally used because it can be strongly magnetised and largely loses its magnetism when current is removed.

3 Marks 6. Why does a solenoid behave like a bar magnet?

A current-carrying solenoid produces a magnetic field pattern similar to a bar magnet and has two poles, North and South.

3 Marks 7. How can the polarity of a solenoid be determined?

Use the right-hand grip rule. Curl the fingers in the direction of current; the thumb points towards the North pole.

3 Marks 8. Why is the magnetic field inside a long solenoid nearly uniform?

The fields produced by the closely spaced turns add together. Inside a long solenoid, the resultant field lines are nearly parallel and equally spaced.

3 Marks 9. What happens to B when current is doubled?

Since B = μ₀nI and B ∝ I, doubling current doubles the magnetic field.

3 Marks 10. What happens to B when turns per unit length are doubled?

Since B ∝ n, doubling turns per unit length doubles the magnetic field.

4 Marks 11. Derive B = μ₀NI/L for a long solenoid.

For a long solenoid: B = μ₀nI But: n = N/L Therefore: B = μ₀(N/L)I Hence: B = μ₀NI/L

4 Marks 12. Explain the right-hand grip rule.

Hold the solenoid with your right hand so that the curled fingers point in the direction of current through its turns. The thumb points towards the North pole of the solenoid.

4 Marks 13. What is an electromagnet?

An electromagnet is a temporary magnet produced by passing current through a coil, generally wound around a soft iron core.

4 Marks 14. Give three ways of increasing the strength of an electromagnet.

1. Increase current.
2. Increase number of turns per unit length.
3. Use a suitable soft iron core.

4 Marks 15. Distinguish between solenoid and electromagnet.

A solenoid is the coil of many turns of wire. When a suitable magnetic core such as soft iron is placed inside and current flows, the arrangement acts as a strong electromagnet.

5 Marks 16. Explain the factors affecting the magnetic field of a solenoid.

B = μ₀nI Therefore:
• B increases with current I.
• B increases with turns per unit length n.
• Reversing current reverses field direction.
• A suitable soft iron core greatly strengthens the field.

5 Marks 17. Explain why soft iron is used in electromagnets.

Soft iron has high magnetic permeability and can become strongly magnetised in the presence of the magnetic field. It also loses most of its magnetism when the current is switched off, making it suitable for temporary magnets.

5 Marks 18. Compare magnetic field inside and outside a long solenoid.

Inside:
• Stronger
• Approximately uniform
• Field lines nearly parallel

Outside:
• Much weaker for an ideal long solenoid
• Field lines complete their closed paths outside

5 Marks 19. A solenoid has 500 turns and length 0.25 m. Find turns per metre.

n = N/L
n = 500/0.25
n = 2000 turns/m

5 Marks 20. A solenoid has 1000 turns, length 0.5 m and current 2 A. Calculate B in air.

B = μ₀NI/L

= (4π×10⁻⁷ ×1000×2)/0.5

B ≈ 5.03×10⁻³ T

6 Marks 21. Explain the construction and working of an electromagnet.

A coil of insulated wire is wound around a suitable soft iron core. When current passes through the coil, the magnetic field of the coil magnetises the iron core. The combination becomes a strong magnet. When current is switched off, most of the magnetism disappears.

6 Marks 22. Explain why a solenoid is similar to a bar magnet.

Both have two magnetic poles and produce similar external magnetic field patterns. Magnetic field lines emerge from the north pole and enter the south pole outside. A current-carrying solenoid can therefore behave like a bar magnet.

6 Marks 23. What happens to magnetic field if current direction is reversed?

The polarity of the solenoid reverses. The North pole becomes South and the South pole becomes North. The magnetic field direction also reverses.

6 Marks 24. Why is a solenoid useful for producing a strong magnetic field?

A solenoid contains many closely spaced turns. The magnetic fields of individual turns add together inside the solenoid. Therefore, a strong and approximately uniform magnetic field can be obtained.

6 Marks 25. Explain the relation B = μ₀nI.

For a long solenoid, magnetic field is proportional to the number of turns per unit length and current. The proportionality constant in free space is μ₀. Therefore: B = μ₀nI

6 Marks 26. If the length of a solenoid is doubled while total number of turns remains the same, what happens to B?

n = N/L If L is doubled while N remains constant, n becomes half. Since: B ∝ n the magnetic field becomes half, assuming current remains unchanged.

6 Marks 27. If both N and L are doubled, what happens to B?

n = N/L New: n' = 2N/2L = N/L Therefore n remains unchanged. Hence, for the same current, B remains unchanged.

6 Marks 28. If current is doubled and length is also doubled while N remains constant, what happens to B?

B = μ₀NI/L B'/B = (2I)/(2L) × L/I = 1. Therefore, magnetic field remains unchanged.

6 Marks 29. Explain two advantages of an electromagnet over a permanent magnet.

1. An electromagnet can be switched ON and OFF by controlling current.

2. Its strength can be changed by changing current or number of turns.

3. Its polarity can be reversed by reversing current.

6 Marks 30. Write a complete note on solenoid and its magnetic field.

A solenoid is a long cylindrical coil consisting of many closely wound turns of insulated wire. When current passes through it, a magnetic field is produced. The field inside a long solenoid is approximately uniform. The magnetic field is:

B = μ₀nI = μ₀NI/L

The field increases with current and turns per unit length. A suitable soft iron core makes the field much stronger and produces an electromagnet. The polarity can be determined using the right-hand grip rule.

22. Assertion–Reason Questions

Assertion (A): The magnetic field inside a long solenoid is approximately uniform.

Reason (R): The field lines inside a long solenoid are nearly parallel and equally spaced.

Answer: Both A and R are true, and R correctly explains A.

Assertion (A): Increasing current in a solenoid increases its magnetic field.

Reason (R): B = μ₀nI.

Answer: Both A and R are true, and R correctly explains A.

Assertion (A): A soft iron core strengthens an electromagnet.

Reason (R): Soft iron can become strongly magnetised in the magnetic field produced by the coil.

Answer: Both A and R are true, and R correctly explains A.

Assertion (A): Reversing current reverses the polarity of a solenoid.

Reason (R): The direction of the magnetic field depends on the direction of current.

Answer: Both A and R are true, and R correctly explains A.

23. HOTS Questions | उच्च स्तरीय प्रश्न

HOTS 1: Two solenoids have the same current. Solenoid A has 1000 turns/m and Solenoid B has 2000 turns/m. Compare their magnetic fields.

B ∝ n Therefore: B₂/B₁ = 2000/1000 = 2. Solenoid B produces twice the magnetic field.

HOTS 2: A solenoid has 2000 turns in 1 m. Its length is increased to 2 m without changing total turns. What happens to B?

n = N/L. When length doubles, n becomes half. Therefore B becomes half if current remains unchanged.

HOTS 3: A solenoid's current is doubled while its turns per unit length are halved. What happens to B?

B ∝ nI. The increase in I by factor 2 and decrease in n by factor 2 cancel each other. Therefore B remains unchanged.

24. One-Minute Revision | एक मिनट Revision

🧲 Solenoid = Many Closely Wound Turns

Formula: B = μ₀nI

Since: n = N/L
Therefore: B = μ₀NI/L

Remember: Current ↑ → B ↑
Turns per unit length ↑ → B ↑
Soft iron core → Strong electromagnet
Current reversed → Polarity reversed

Inside long solenoid: Nearly uniform magnetic field.

Right-Hand Grip Rule: Fingers → Current
Thumb → North Pole

25. Quick Concept Map

Electric Current ↓ Solenoid ↓ Magnetic Field ↓ B = μ₀nI ↓ Nearly Uniform Field ↓ Electromagnet

26. Exam Golden Points

✔ Solenoid = long coil of many closely wound turns.

✔ Current through solenoid produces magnetic field.

✔ Long solenoid has approximately uniform field inside.

✔ B = μ₀nI.

✔ n = N/L.

✔ Therefore B = μ₀NI/L.

✔ Increasing current increases B.

✔ Increasing turns per unit length increases B.

✔ Reversing current reverses polarity.

✔ Solenoid behaves like a bar magnet.

✔ Soft iron core makes an electromagnet stronger.

✔ Right-hand grip rule determines polarity.

27. Final Summary | अंतिम सारांश

English:
A solenoid is a long cylindrical coil containing many closely wound turns of insulated conducting wire. When current flows through it, a magnetic field is produced. The field inside a long solenoid is approximately uniform and strong. The magnetic field in an ideal long solenoid is:

B = μ₀nI

where n is the number of turns per unit length. If the solenoid has N turns and length L:

B = μ₀NI/L

Increasing current or turns per unit length increases the field. A suitable soft iron core makes the field much stronger, forming an electromagnet. Reversing current reverses the polarity.

हिन्दी:
सोलनॉइड एक लंबी बेलनाकार कुंडली है जिसमें विद्युतरोधी तार के बहुत सारे पास-पास लपेटे हुए फेरे होते हैं। इसमें धारा प्रवाहित करने पर चुंबकीय क्षेत्र उत्पन्न होता है। लंबे सोलनॉइड के अंदर चुंबकीय क्षेत्र लगभग एकसमान होता है। लंबे सोलनॉइड के लिए:

B = μ₀nI

जहाँ n प्रति इकाई लंबाई फेरों की संख्या है। यदि कुल फेरे N तथा लंबाई L हो:

B = μ₀NI/L

धारा तथा प्रति इकाई लंबाई फेरों की संख्या बढ़ाने पर चुंबकीय क्षेत्र बढ़ता है। नरम लोहे का कोर लगाने पर शक्तिशाली विद्युतचुंबक बनाया जा सकता है। धारा की दिशा बदलने पर ध्रुवता भी बदल जाती है।

Magnetic Effects of Electric Current
Solenoid and Its Magnetic Field

Class 10 Science | CBSE | Foundation | Competitive Preparation

Class 10th Science Chapter 12 – Magnetic Effects of Electric Current | Magnetic Field Due to a Current-Carrying Circular Loop

Magnetic Effects of Electric Current

Magnetic Field Due to a Current-Carrying Circular Loop

कक्षा 10 विज्ञान | Class 10 Science | CBSE + Foundation + Competitive

Magnetic Field Due to a Current-Carrying Circular Loop

1. Introduction | परिचय

English:
When electric current passes through a circular conducting loop, a magnetic field is produced around the loop. The magnetic field at the centre of the loop is directed perpendicular to the plane of the loop.

हिन्दी:
जब किसी वृत्ताकार चालक कुंडली में विद्युत धारा प्रवाहित की जाती है, तो उसके चारों ओर चुंबकीय क्षेत्र उत्पन्न होता है। कुंडली के केन्द्र पर चुंबकीय क्षेत्र की दिशा कुंडली के तल के लंबवत होती है।

2. Magnetic Field Pattern of a Circular Loop | चुंबकीय क्षेत्र की आकृति

A current-carrying circular loop produces magnetic field lines that pass through the centre of the loop. Near the centre, the field lines become nearly parallel to one another.

वृत्ताकार धारा-वाहक कुंडली के कारण उत्पन्न चुंबकीय क्षेत्र रेखाएँ कुंडली के केन्द्र से होकर गुजरती हैं। केन्द्र के पास ये रेखाएँ लगभग समानांतर दिखाई देती हैं।

Orange arrows = Current Direction | नीले तीर = Magnetic Field

3. Direction of Magnetic Field | चुंबकीय क्षेत्र की दिशा

The direction of magnetic field around a circular loop can be determined using the Right-Hand Thumb Rule. Hold the circular loop with the fingers of your right hand in the direction of current. Your thumb then points towards the magnetic field direction through the centre of the loop.

यदि उँगलियाँ धारा की दिशा में मोड़ी जाएँ, तो दायाँ अंगूठा कुंडली के केन्द्र पर चुंबकीय क्षेत्र की दिशा बताता है।

Current Direction | धारा की दिशा	Magnetic Field at Centre | केन्द्र पर क्षेत्र
Anticlockwise | वामावर्त	Out of the plane / बाहर की ओर
Clockwise | दक्षिणावर्त	Into the plane / अन्दर की ओर

Memory Trick:
Anticlockwise Current → Magnetic field comes OUT of the page.
Clockwise Current → Magnetic field goes IN to the page.

याद रखें:
वामावर्त धारा → क्षेत्र बाहर की ओर।
दक्षिणावर्त धारा → क्षेत्र अन्दर की ओर।

4. Magnetic Field at the Centre of a Circular Loop

For a circular loop of radius R carrying current I, the magnetic field at its centre is:

B = μ₀I / 2R

Where:
B = Magnetic field at centre
I = Current through the loop
R = Radius of circular loop
μ₀ = Permeability of free space

In SI units: μ₀ = 4π × 10⁻⁷ T m A⁻¹ Therefore,

B = (4π × 10⁻⁷ × I) / (2R)

5. Effect of Current on Magnetic Field | धारा का प्रभाव

From the formula:

B ∝ I

Thus, if current increases, the magnetic field at the centre also increases. Example:
If current is doubled while radius remains constant, magnetic field becomes twice.

यदि धारा दोगुनी कर दी जाए और त्रिज्या समान रहे, तो केन्द्र पर चुंबकीय क्षेत्र भी दोगुना हो जाएगा।

6. Effect of Radius | त्रिज्या का प्रभाव

From:

B ∝ 1/R

Therefore, for the same current, a smaller circular loop produces a stronger magnetic field at its centre than a larger loop.

समान धारा के लिए छोटी त्रिज्या वाली कुंडली के केन्द्र पर चुंबकीय क्षेत्र अधिक होता है।

7. Effect of Number of Turns | फेरों की संख्या का प्रभाव

If a circular coil has N turns, the magnetic field at its centre is:

B = μ₀NI / 2R

Therefore:

B ∝ N

The magnetic field increases when the number of turns increases.

यदि कुंडली में फेरों की संख्या बढ़ाई जाती है, तो केन्द्र पर चुंबकीय क्षेत्र भी बढ़ जाता है।

Important Formula:
For a single circular turn: B = μ₀I / 2R

For N circular turns: B = μ₀NI / 2R

8. Factors Affecting Magnetic Field

Factor	Effect on B
Current I increases	Magnetic field increases
Number of turns N increases	Magnetic field increases
Radius R increases	Magnetic field decreases
Current direction reverses	Magnetic field direction reverses

9. Magnetic Field Lines Around a Circular Loop

The magnetic field lines of a circular loop are similar to the magnetic field pattern of a bar magnet. One face of the loop behaves like a magnetic north pole and the other face behaves like a magnetic south pole.

वृत्ताकार कुंडली का चुंबकीय क्षेत्र पैटर्न बार मैग्नेट के क्षेत्र जैसा होता है। कुंडली का एक फलक उत्तर ध्रुव तथा दूसरा फलक दक्षिण ध्रुव जैसा व्यवहार कर सकता है।

Important:
The magnetic field is stronger where magnetic field lines are closer together.

जहाँ चुंबकीय क्षेत्र रेखाएँ अधिक पास-पास होती हैं, वहाँ चुंबकीय क्षेत्र अधिक प्रबल होता है।

10. Direction Using Clock Face Rule

When a circular coil is viewed from one side:

Anticlockwise current → that face behaves as North Pole.
Clockwise current → that face behaves as South Pole.

हिन्दी:
वामावर्त धारा दिखाई दे तो वह फलक उत्तर ध्रुव जैसा व्यवहार करता है।
दक्षिणावर्त धारा दिखाई दे तो वह फलक दक्षिण ध्रुव जैसा व्यवहार करता है।

11. Comparison with a Straight Conductor

Straight Conductor	Circular Loop
Field lines are concentric circles around conductor.	Field lines pass through and around the loop.
Field depends on current and distance.	At centre, B = μ₀NI/2R.
Direction by Right-Hand Thumb Rule.	Direction also determined using right-hand rule.

12. Numerical Example | संख्यात्मक उदाहरण

Question:
A circular coil of radius 0.1 m carries a current of 2 A. Find the magnetic field at its centre.

Solution:

B = μ₀I / 2R

μ₀ = 4π × 10⁻⁷ T m A⁻¹
I = 2 A
R = 0.1 m

B = (4π × 10⁻⁷ × 2)/(2 × 0.1)

B = 4π × 10⁻⁶ T

Approximately: B ≈ 1.26 × 10⁻⁵ T

13. Important Conceptual Points

1. A current-carrying circular loop produces a magnetic field.
2. At the centre of the loop, the magnetic field is perpendicular to the plane of the loop.
3. Magnetic field increases with current.
4. Magnetic field increases with number of turns.
5. Magnetic field decreases with radius.
6. Reversing current reverses magnetic field direction.
7. A circular loop can behave like a bar magnet.

14. 30 MCQs | बहुविकल्पीय प्रश्न

1. A current-carrying circular loop produces:
धारा-वाहक वृत्ताकार कुंडली उत्पन्न करती है:

A. Only electric field

B. Only heat

C. Magnetic field

D. Gravitational field

Answer: C
A current-carrying conductor produces a magnetic field.

2. The magnetic field at the centre of a circular loop is directed:

A. Along the radius

B. Perpendicular to the plane of loop

C. Along the circumference

D. Randomly

Answer: B

3. For a single circular loop, magnetic field at centre is:

A. μ₀I/2R

B. μ₀IR/2

C. 2μ₀R/I

D. μ₀/IR

Answer: A

4. If current is doubled, magnetic field at centre becomes:

A. Half

B. Same

C. Double

D. Four times

Answer: C

5. Magnetic field at centre is inversely proportional to:

A. Current

B. Number of turns

C. Radius

D. μ₀

Answer: C

6. For N turns, magnetic field at centre is:

A. μ₀I/2NR

B. μ₀NI/2R

C. 2μ₀R/NI

D. μ₀R/NI

Answer: B

7. If number of turns is increased, magnetic field:

A. Decreases

B. Increases

C. Becomes zero

D. Remains unchanged

Answer: B

8. Anticlockwise current in a loop makes the viewed face behave like:

A. South pole

B. Neutral face

C. North pole

D. Electric pole

Answer: C

9. Clockwise current in a loop makes the viewed face behave like:

A. North pole

B. South pole

C. Both poles

D. No pole

Answer: B

10. SI unit of magnetic field is:

A. Weber

B. Tesla

C. Ampere

D. Volt

Answer: B

11. μ₀ represents:

A. Electric charge

B. Resistance

C. Permeability of free space

D. Current

Answer: C

12. The magnetic field is stronger where field lines are:

A. Far apart

B. Closer together

C. Absent

D. Random

Answer: B

13. If radius becomes twice, keeping current same, B becomes:

A. Twice

B. Four times

C. Half

D. Same

Answer: C

14. Magnetic field is directly proportional to:

A. Radius

B. Current

C. 1/current

D. 1/turns

Answer: B

15. The direction of magnetic field can be determined by:

A. Right-hand rule

B. Ohm's law

C. Joule's law

D. Archimedes principle

Answer: A

16. If current direction is reversed, magnetic field direction:

A. Remains same

B. Reverses

C. Becomes zero

D. Doubles

Answer: B

17. For a coil with 10 turns, compared with one turn under same I and R, B is:

A. 10 times

B. 5 times

C. 1/10

D. Same

Answer: A

18. The field at the centre of circular loop is:

A. Zero always

B. Perpendicular to loop plane

C. Along circumference

D. Along radius

Answer: B

19. A circular coil behaves magnetically similar to:

A. Battery

B. Resistor

C. Bar magnet

D. Capacitor

Answer: C

20. Which change increases B most directly?

A. Decreasing current

B. Increasing radius

C. Increasing number of turns

D. Reversing current only

Answer: C

21. The magnetic field formula for N turns is:

A. B = μ₀NI/2R

B. B = 2R/μ₀NI

C. B = μ₀R/2NI

D. B = NI/μ₀R

Answer: A

22. If both I and N are doubled, B becomes:

A. Half

B. Double

C. Four times

D. Same

Answer: C

23. If I is doubled and R is doubled, B becomes:

A. Same

B. Double

C. Half

D. Four times

Answer: A

24. Which quantity has unit tesla?

A. Current

B. Magnetic field

C. Charge

D. Resistance

Answer: B

25. The field at the centre increases when radius:

A. Increases

B. Decreases

C. Becomes infinite

D. Has no effect

Answer: B

26. The magnetic field of a loop depends on:

A. Current

B. Radius

C. Number of turns

D. All of these

Answer: D

27. In the formula B = μ₀NI/2R, B is measured in:

A. Tesla

B. Volt

C. Ohm

D. Joule

Answer: A

28. A smaller loop with the same current produces at centre:

A. Smaller B

B. Greater B

C. Zero B

D. Same B always

Answer: B

29. Which rule helps determine the direction of magnetic field?

A. Right-hand thumb rule

B. Fleming's left-hand rule only

C. Ohm's law

D. Snell's law

Answer: A

30. For a circular coil, increasing turns generally makes the magnetic field:

A. Weaker

B. Stronger

C. Zero

D. Unchanged

Answer: B

15. 30 Subjective Questions with Answers

2 Marks
1. What happens when current flows through a circular loop?

A magnetic field is produced around the circular loop. At the centre, the field is perpendicular to the plane of the loop.

2 Marks
2. State the direction of magnetic field at the centre of a loop carrying anticlockwise current.

The magnetic field is directed out of the plane of the loop towards the observer.

2 Marks
3. What happens to magnetic field when current is reversed?

The direction of magnetic field reverses.

2 Marks
4. Write the SI unit of magnetic field.

The SI unit of magnetic field is tesla (T).

2 Marks
5. Write the formula for magnetic field at the centre of a circular loop.

For one turn: B = μ₀I/2R For N turns: B = μ₀NI/2R.

3 Marks
6. State three factors affecting magnetic field at the centre of a circular coil.

The magnetic field depends on: 1. Current I 2. Number of turns N 3. Radius R B increases with I and N but decreases with R.

3 Marks
7. Explain the effect of increasing current on magnetic field.

For a circular coil: B = μ₀NI/2R For constant N and R: B ∝ I Therefore, increasing current increases the magnetic field proportionally.

3 Marks
8. Explain the effect of radius on magnetic field.

B = μ₀NI/2R Therefore: B ∝ 1/R Hence, increasing radius decreases magnetic field at the centre.

3 Marks
9. Why does increasing the number of turns increase magnetic field?

Each turn produces its own magnetic field. The fields of the turns add together at the centre. Therefore, increasing the number of turns increases the resultant magnetic field.

3 Marks
10. How does a circular loop behave like a bar magnet?

A current-carrying circular loop produces a magnetic field pattern similar to that of a bar magnet. One face behaves as a north pole and the other as a south pole.

4 Marks
11. Derive the relation for magnetic field at the centre of a circular loop.

For a circular loop of radius R carrying current I, the magnetic field at its centre is: B = μ₀I/2R If there are N turns, each turn contributes equally, so: B = N(μ₀I/2R) Therefore: B = μ₀NI/2R

4 Marks
12. Explain the right-hand rule for a circular loop.

Curl the fingers of your right hand in the direction of current through the loop. The extended thumb gives the direction of magnetic field through the centre of the loop.

4 Marks
13. A coil has 20 turns. What happens to its field if turns are increased to 40?

Since B ∝ N: B₂/B₁ = 40/20 = 2 Therefore, magnetic field becomes twice the original value, if current and radius remain unchanged.

4 Marks
14. A circular coil has current 4 A. If current becomes 8 A, compare magnetic fields.

B ∝ I Therefore: B₂/B₁ = 8/4 = 2 The new magnetic field is twice the original magnetic field.

4 Marks
15. What happens if radius of a coil is reduced to half?

Since B ∝ 1/R: If R₂ = R₁/2, B₂/B₁ = R₁/R₂ = 2. Therefore, magnetic field becomes twice.

5 Marks
16. Explain the factors responsible for strength of magnetic field at the centre of a circular coil.

According to: B = μ₀NI/2R 1. B increases with current I. 2. B increases with number of turns N. 3. B decreases with radius R. 4. μ₀ is constant for free space. 5. Reversing current changes direction, not the magnitude if I remains the same.

5 Marks
17. Explain how direction of current determines polarity of a circular coil.

Viewed from one face:

Anticlockwise current → North pole.
Clockwise current → South pole. This follows from the right-hand rule.

5 Marks
18. Explain the magnetic field pattern of a current-carrying circular loop.

The field lines emerge from one face and enter the other face outside the loop, forming a pattern similar to a bar magnet. Near the centre, the field lines are nearly parallel and perpendicular to the plane of the loop.

5 Marks
19. Why is a coil with many turns used to obtain a strong magnetic field?

The magnetic field due to each turn adds to the fields produced by other turns. Since: B ∝ N increasing the number of turns increases the resultant magnetic field.

5 Marks
20. Calculate B for N = 100, I = 2 A and R = 0.1 m.

B = μ₀NI/2R = (4π×10⁻⁷ ×100×2)/(2×0.1) = 4π×10⁻⁴ T ≈ 1.26×10⁻³ T.

6 Marks
21. Explain the working principle of a current-carrying circular loop as an electromagnet.

When current flows through the loop, it produces a magnetic field. Increasing current or number of turns strengthens the field. The direction of the field depends on current direction. Thus, a current-carrying coil can act as a controllable magnet.

6 Marks
22. Compare a single loop and a coil of N turns.

Single Loop	N-turn Coil
B = μ₀I/2R	B = μ₀NI/2R
One turn contributes.	N turns contribute.
Comparatively weaker.	Comparatively stronger.

6 Marks
23. Explain how magnetic field can be increased without increasing current.

From B = μ₀NI/2R, magnetic field can be increased by:
1. Increasing number of turns N.
2. Decreasing radius R, if the arrangement permits.
3. Using an appropriate magnetic core in practical coil systems where applicable.

6 Marks
24. A coil has radius 20 cm and current 5 A. Explain how its magnetic field changes if radius is reduced to 10 cm.

Since B ∝ 1/R: R₁ = 20 cm R₂ = 10 cm B₂/B₁ = R₁/R₂ = 20/10 = 2. Therefore, the magnetic field becomes twice, provided current and number of turns remain unchanged.

6 Marks
25. Why does reversing current reverse magnetic field?

The direction of magnetic field is linked to the direction of current according to the right-hand rule. When current direction is reversed, the curling direction of the magnetic field also reverses.

6 Marks
26. Explain the importance of circular coils in electromagnets.

Circular turns concentrate and add their magnetic fields. By using many turns and suitable current, a strong magnetic field can be produced. Such coils are therefore important components of electromagnets and other electrical devices.

6 Marks
27. What happens to B if current, number of turns and radius are all doubled?

B = μ₀NI/2R New values: N' = 2N I' = 2I R' = 2R Therefore: B'/B = (2N × 2I)/(2R) ÷ (NI/R) = 2 Hence, magnetic field becomes twice.

6 Marks
28. A circular coil produces magnetic field B. What will happen if current is halved and radius is also halved?

B ∝ I/R. Both I and R are halved: B'/B = (I/2)/(R/2) × R/I = 1. Therefore, magnetic field remains unchanged.

6 Marks
29. Explain why the magnetic field near the centre of a circular loop is nearly uniform.

Near the centre, contributions from different portions of the circular loop combine symmetrically. The field direction changes very little over a small central region, so field lines are approximately parallel and the field can be considered nearly uniform there.

6 Marks
30. Write a complete summary of the magnetic field due to a circular loop.

A current-carrying circular loop produces a magnetic field. At its centre the field is perpendicular to the plane of the loop. Its magnitude for N turns is: B = μ₀NI/2R The field increases with current and number of turns and decreases with radius. Its direction is determined by the right-hand rule. A circular current-carrying coil behaves magnetically like a bar magnet.

16. Assertion–Reason Questions

Assertion (A): The magnetic field at the centre of a circular coil increases when current is increased.

Reason (R): The magnetic field at the centre is directly proportional to current.

Answer: Both A and R are true, and R correctly explains A.

Assertion (A): Increasing the radius of a circular coil increases the magnetic field at its centre.

Reason (R): B is inversely proportional to radius.

Answer: A is false, but R is true.

Assertion (A): Increasing the number of turns increases the magnetic field.

Reason (R): Magnetic fields produced by the turns add at the centre.

Answer: Both A and R are true, and R correctly explains A.

Assertion (A): Reversing current reverses the direction of magnetic field.

Reason (R): Magnetic field direction depends on current direction.

Answer: Both A and R are true, and R correctly explains A.

17. HOTS Questions | उच्च स्तरीय प्रश्न

HOTS 1:
Two coils have the same current and same number of turns. Coil A has radius 10 cm and Coil B has radius 20 cm. Which coil has stronger field at the centre?

Since B ∝ 1/R, the coil with smaller radius produces the stronger field. Therefore, Coil A has stronger magnetic field.

HOTS 2:
A coil has 50 turns. If the number of turns is increased to 150 without changing current and radius, how does B change?

B ∝ N. Therefore: B₂/B₁ = 150/50 = 3. The magnetic field becomes three times.

HOTS 3:
If both current and radius of a circular coil are reduced to half, what happens to the magnetic field?

B ∝ I/R. Both I and R are halved, so their ratio remains unchanged. Therefore, magnetic field remains unchanged.

18. One-Minute Revision | एक मिनट में Revision

🧲 Current + Circular Loop → Magnetic Field

Formula: B = μ₀NI/2R

Remember: I ↑ → B ↑
N ↑ → B ↑
R ↑ → B ↓

Direction: Anticlockwise → North face → Field outward
Clockwise → South face → Field inward

Memory Trick: “More Current + More Turns = More Magnetic Field”
“More Radius = Less Field”

19. Quick Concept Map

Electric Current ↓ Circular Loop ↓ Magnetic Field ↓ Direction by Right-Hand Rule ↓ B = μ₀NI/2R

20. Exam Golden Points

✔ Current-carrying circular loop produces magnetic field.

✔ Magnetic field at centre is perpendicular to the plane of loop.

✔ B = μ₀NI/2R

✔ B increases with current.

✔ B increases with number of turns.

✔ B decreases with radius.

✔ Reversal of current reverses magnetic field direction.

✔ Anticlockwise current → North face.

✔ Clockwise current → South face.

✔ A circular current-carrying coil behaves like a bar magnet.

21. Final Summary | अंतिम सारांश

English:
A current-carrying circular loop produces a magnetic field. The magnetic field at the centre of the loop is perpendicular to its plane. For a coil having N turns, current I and radius R, the field at the centre is given by:

B = μ₀NI/2R

The magnetic field increases with current and number of turns and decreases with radius. Its direction is determined using the right-hand rule. The magnetic field pattern of a circular coil resembles that of a bar magnet.

हिन्दी:
धारा-वाहक वृत्ताकार कुंडली चुंबकीय क्षेत्र उत्पन्न करती है। कुंडली के केन्द्र पर चुंबकीय क्षेत्र उसके तल के लंबवत होता है। N फेरों वाली कुंडली के लिए:

B = μ₀NI/2R

चुंबकीय क्षेत्र धारा और फेरों की संख्या बढ़ने पर बढ़ता है तथा त्रिज्या बढ़ने पर घटता है। चुंबकीय क्षेत्र की दिशा दाएँ हाथ के नियम से निर्धारित की जाती है। वृत्ताकार कुंडली का चुंबकीय क्षेत्र बार मैग्नेट के समान होता है।

Magnetic Effects of Electric Current
Magnetic Field Due to a Current-Carrying Circular Loop

Class 10 Science | CBSE | Foundation | Competitive Preparation

Class 10th Science Chapter 12 – Magnetic Effects of Electric Current | Right-Hand Thumb Rule

🧲 Magnetic Effects of Electric Current

Right-Hand Thumb Rule

दाहिने हाथ के अंगूठे का नियम

Class 10 Science | CBSE + Foundation + Competitive

🧲 Right-Hand Thumb Rule | दाहिने हाथ के अंगूठे का नियम

1. Introduction | परिचय

The Right-Hand Thumb Rule is used to determine the direction of the magnetic field around a straight current-carrying conductor.

दाहिने हाथ के अंगूठे का नियम सीधे धारावाही चालक के चारों ओर उत्पन्न चुंबकीय क्षेत्र की दिशा ज्ञात करने के लिए प्रयोग किया जाता है।

It gives the relationship between the direction of conventional current and the direction of the circular magnetic field around the conductor.

2. Statement of Right-Hand Thumb Rule

Rule:

If we hold a straight current-carrying conductor in our right hand with the thumb pointing in the direction of conventional current, then the curled fingers give the direction of the magnetic field around the conductor.

हिंदी में:

यदि किसी सीधे धारावाही चालक को दाहिने हाथ में इस प्रकार पकड़ें कि अंगूठा पारंपरिक धारा की दिशा में हो, तो मुड़ी हुई उँगलियाँ चालक के चारों ओर चुंबकीय क्षेत्र की दिशा बताती हैं।

3. Animated Concept Diagram | गतिशील अवधारणा चित्र

4. Three Simple Steps | तीन आसान चरण

Step 1

Right hand से conductor को पकड़ें।

Step 2

Thumb को conventional current की दिशा में रखें।

Step 3

Curled fingers magnetic field की दिशा बताएँगी।

5. When Current Flows Upward | जब धारा ऊपर की ओर हो

Suppose the conventional current flows upward through a vertical conductor. Point your right thumb upward. Your curled fingers show the direction of the magnetic field around the wire.

यदि पारंपरिक धारा ऊर्ध्व दिशा में बह रही है, तो दाहिने हाथ का अंगूठा ऊपर की ओर रखें। मुड़ी हुई उँगलियाँ चुंबकीय क्षेत्र की दिशा बताएँगी।

6. When Current Flows Downward | जब धारा नीचे की ओर हो

If the current direction is reversed, point your right thumb downward. The curled fingers now show the magnetic field in the opposite direction.

यदि धारा की दिशा नीचे कर दी जाए, तो अंगूठे को नीचे की ओर रखने पर मुड़ी हुई उँगलियाँ चुंबकीय क्षेत्र की विपरीत दिशा बताएँगी।

Current ↑
↓
Field Direction A

Current ↓
↓
Field Direction B
(Opposite)

7. Clockwise and Anticlockwise Field

For a conductor perpendicular to the plane of the paper:

• If current comes out of the page, the magnetic field is anticlockwise.
• If current goes into the page, the magnetic field is clockwise.

8. Dot and Cross Convention | Dot और Cross चिन्ह

Symbol	Meaning	Magnetic Field Direction
⊙	Current coming out of the page	Anticlockwise
⊗	Current going into the page	Clockwise

Memory Trick:

Dot (•) = arrowhead coming towards you → current out of page.

Cross (×) = tail/feathers of an arrow going away from you → current into page.

9. Current Direction vs Magnetic Field Direction

Current	Magnetic Field
Along the conductor	Forms circular paths around conductor
Shown by thumb	Shown by curled fingers
Reversal changes current direction	Field direction also reverses

10. Relation with Magnetic Field Magnitude

The right-hand thumb rule gives the direction of the magnetic field. The magnitude for a long straight conductor in free space is given by:

B = μ₀I / 2πr

Therefore:

B ∝ I

B ∝ 1/r

11. Direction-Based Examples

Example 1: A straight conductor carries conventional current upward. What is the direction of the magnetic field when viewed from above?

Using the right-hand thumb rule, point the right thumb upward. The curled fingers give the magnetic field direction.

Answer: Anticlockwise when viewed from above.

Example 2: Current is directed downward through a vertical conductor. What happens to the magnetic field direction?

Reverse the thumb direction. The curled fingers also reverse.

Answer: Magnetic field direction reverses.

Example 3: A current is coming out of the page. Determine the field direction.

Use the thumb pointing toward you.

The fingers curl anticlockwise.

Answer: Anticlockwise.

12. Common Mistakes | सामान्य गलतियाँ

❌ Thumb shows magnetic field direction.
✅ Thumb shows conventional current direction.

❌ Fingers show current direction.
✅ Curled fingers show magnetic field direction.

❌ Reversing current keeps field direction unchanged.
✅ Reversing current reverses field direction.

❌ Dot means current going into the page.
✅ Dot means current coming out of the page.

❌ Cross means current coming out of the page.
✅ Cross means current going into the page.

13. 30 MCQs | बहुविकल्पीय प्रश्न

1. The right-hand thumb rule is used to determine:

A. Direction of magnetic field around a straight current-carrying conductor

B. Resistance

C. Voltage

D. Electric power

Answer: A

2. In the right-hand thumb rule, the thumb represents:

A. Magnetic field

B. Conventional current

C. Resistance

D. Voltage

Answer: B

3. The curled fingers represent:

A. Current

B. Voltage

C. Magnetic field direction

D. Resistance

Answer: C

4. If current flows upward, the right thumb should point:

A. Downward

B. Sideways

C. Horizontally

D. Upward

Answer: D

5. If the current direction is reversed, magnetic field direction:

A. Reverses

B. Doubles

C. Becomes zero

D. Remains unchanged

Answer: A

6. Magnetic field lines around a straight conductor are:

A. Radial

B. Concentric circles

C. Straight parallel lines

D. Squares

Answer: B

7. If current comes out of the page, the magnetic field is:

A. Clockwise

B. Straight

C. Anticlockwise

D. Zero

Answer: C

8. If current goes into the page, the magnetic field is:

A. Anticlockwise

B. Random

C. Zero

D. Clockwise

Answer: D

9. The dot symbol generally represents current:

A. Out of the page

B. Into the page

C. Along the page

D. Zero

Answer: A

10. The cross symbol generally represents current:

A. Out of the page

B. Into the page

C. Upward

D. Downward

Answer: B

11. The right-hand thumb rule is mainly related to:

A. Magnetic effect of current

B. Heating effect

C. Chemical effect

D. Photoelectric effect

Answer: A

12. The magnetic field direction depends on:

A. Current direction

B. Wire colour

C. Battery size only

D. Room temperature

Answer: A

13. Which hand is used in the standard right-hand thumb rule?

A. Left hand

B. Both hands

C. Right hand

D. No hand

Answer: C

14. A straight conductor carries current upward. The field around it is:

A. Circular

B. Rectangular

C. Radial

D. None

Answer: A

15. The right-hand thumb rule does not directly give:

A. Field direction

B. Current direction relation

C. Field magnitude

D. Circular sense of field

Answer: C

16. For a long straight conductor, B is proportional to:

A. I

B. r

C. 1/I

D. r²

Answer: A

17. For a long straight conductor, B is inversely proportional to:

A. Current

B. Distance

C. Charge

D. Time

Answer: B

18. If current is doubled, field magnitude at the same distance becomes:

A. Half

B. Same

C. Double

D. One-fourth

Answer: C

19. If distance is doubled, field magnitude becomes:

A. Double

B. Four times

C. Same

D. Half

Answer: D

20. Which statement is correct?

A. Thumb gives current direction

B. Thumb gives resistance

C. Fingers give current

D. Fingers give voltage

Answer: A

21. The magnetic field around a current-carrying wire is:

A. Independent of current

B. Related to current

C. Always zero

D. Only gravitational

Answer: B

22. The direction of field around a wire can be detected using:

A. Compass

B. Ammeter only

C. Voltmeter only

D. Thermometer

Answer: A

23. A current coming out of the page is represented by:

A. ×

B. +

C. •

D. −

Answer: C

24. A current entering the page is represented by:

A. •

B. ×

C. →

D. ↑

Answer: B

25. Current out of the page produces a field that is:

A. Clockwise

B. Anticlockwise

C. Zero

D. Straight

Answer: B

26. Current into the page produces a field that is:

A. Clockwise

B. Anticlockwise

C. Radial

D. Zero

Answer: A

27. If the current is reversed twice, the field direction:

A. Reverses once

B. Remains opposite

C. Returns to original direction

D. Becomes zero

Answer: C

28. Which is the correct memory rule?

A. Thumb → Field, fingers → Current

B. Thumb → Current, fingers → Field

C. Thumb → Resistance, fingers → Voltage

D. Thumb → Voltage, fingers → Current

Answer: B

29. The magnetic field around a straight conductor is:

A. Circular

B. Always vertical

C. Always horizontal

D. Random

Answer: A

30. The right-hand thumb rule connects:

A. Current direction and magnetic field direction

B. Voltage and resistance only

C. Heat and mass

D. Power and energy only

Answer: A

14. 30 Subjective Questions | वर्णनात्मक प्रश्न

2 Marks 1. What is the Right-Hand Thumb Rule?

Answer: It determines the direction of magnetic field around a straight current-carrying conductor.

2 Marks 2. What does the thumb represent?

Answer: The thumb represents the direction of conventional current.

2 Marks 3. What do the curled fingers represent?

Answer: They represent the direction of magnetic field.

2 Marks 4. What happens to magnetic field direction when current is reversed?

Answer: The magnetic field direction is reversed.

2 Marks 5. What is represented by the dot symbol?

Answer: Current coming out of the plane of the page.

3 Marks 6. State and explain the right-hand thumb rule.

Answer: Hold the conductor in the right hand with the thumb pointing along conventional current. The curled fingers show the magnetic field direction.

3 Marks 7. What is represented by the cross symbol?

Answer: It represents current going into the plane of the page.

3 Marks 8. What is the direction of magnetic field for current coming out of the page?

Answer: Anticlockwise.

3 Marks 9. What is the direction of magnetic field for current going into the page?

Answer: Clockwise.

3 Marks 10. Why does reversing current reverse the magnetic field?

Answer: The magnetic field direction is determined by current direction. Hence reversing current reverses the field direction.

4 Marks 11. Explain the rule with a suitable example.

Answer: If current flows upward in a straight wire, point the right thumb upward. The curled fingers show the circular magnetic field direction around the wire.

4 Marks 12. Differentiate between dot and cross symbols.

Dot (⊙)	Cross (⊗)
Current out of page	Current into page
Field anticlockwise	Field clockwise

4 Marks 13. Explain the relationship between current and magnetic field direction.

Answer: Current direction determines the sense of circular magnetic field around the conductor. Reversing current reverses the magnetic field direction.

4 Marks 14. Why is the right-hand thumb rule important?

Answer: It provides a simple method for determining the direction of magnetic field around a current-carrying straight conductor.

5 Marks 15. Explain the right-hand thumb rule with a labelled diagram.

Answer: The thumb points in conventional current direction and the curled fingers show magnetic field direction. The field lines are concentric circles around the conductor.

5 Marks 16. Explain current coming out of the page using the rule.

Answer: Point the right thumb towards yourself. The fingers curl anticlockwise. Therefore the magnetic field is anticlockwise.

5 Marks 17. Explain current going into the page using the rule.

Answer: Point the right thumb away from yourself. The fingers curl clockwise. Therefore the magnetic field is clockwise.

5 Marks 18. Explain how the direction of current can be inferred from the magnetic field direction.

Answer: The right-hand thumb rule can be used in reverse. If the circular field direction is known, curl the fingers in that direction; the thumb then gives the conventional current direction.

6 Marks 19. Describe the right-hand thumb rule and explain its applications.

Answer: The rule determines magnetic field direction around a straight current-carrying conductor. It is useful in analysing magnetic field patterns, determining clockwise/anticlockwise direction and understanding electromagnetic devices.

6 Marks 20. Explain the effect of reversing current using the right-hand thumb rule.

Answer: When the current direction is reversed, the thumb direction must also be reversed. Consequently, the curled fingers reverse their direction. Hence the magnetic field direction reverses.

6 Marks 21. Explain dot and cross notation in magnetic field diagrams.

Answer: Dot represents a vector coming out of the page, while cross represents a vector going into the page. For a current-carrying straight conductor, dot corresponds to anticlockwise field and cross corresponds to clockwise field.

3 Marks 22. What is conventional current?

Answer: It is the assumed direction of current from the positive terminal to the negative terminal through the external circuit.

4 Marks 23. Does the right-hand thumb rule give the magnitude of magnetic field?

Answer: No. It gives the direction. For a long straight conductor, magnitude is given by B = μ₀I/2πr.

4 Marks 24. If current direction is known, how can magnetic field direction be found?

Answer: Point the right thumb in the current direction. The curled fingers give the magnetic field direction.

5 Marks 25. If the magnetic field is anticlockwise, what is the current direction?

Answer: By the right-hand thumb rule, the thumb points out of the page. Therefore the conventional current is coming out of the page.

5 Marks 26. If the magnetic field is clockwise, what is the current direction?

Answer: The current is directed into the page.

6 Marks 27. Explain the right-hand thumb rule using a straight vertical conductor.

Answer: Hold the conductor with the right hand. When thumb points upward, fingers curl in one circular direction around the conductor. If thumb points downward, the fingers curl in the opposite direction.

6 Marks 28. Explain why the magnetic field lines are circular around a straight conductor.

Answer: The field has circular symmetry around the straight conductor. Points at equal perpendicular distances have the same field magnitude, producing concentric circular field lines.

5 Marks 29. What happens if the current direction is changed twice?

Answer: The first reversal reverses the field direction. The second reversal brings it back to its original direction, provided the current magnitude and geometry are unchanged.

6 Marks 30. Explain the complete concept of the Right-Hand Thumb Rule.

Answer: A current-carrying straight conductor produces circular magnetic field lines. The right-hand thumb rule determines their direction: thumb indicates conventional current and curled fingers indicate magnetic field. Reversing current reverses the field direction. Dot and cross notation can represent current out of and into the page respectively.

15. Assertion–Reason Questions

1. Assertion: The right-hand thumb rule determines the direction of magnetic field around a straight conductor.
Reason: The magnetic field direction is related to the direction of conventional current.

Answer: Both Assertion and Reason are true, and the Reason explains the Assertion.

2. Assertion: Reversing current reverses magnetic field direction.
Reason: The curled fingers must reverse when the thumb direction reverses.

Answer: Both are true, and the Reason correctly explains the Assertion.

3. Assertion: The thumb represents magnetic field direction.
Reason: Curled fingers represent current direction.

Answer: Both Assertion and Reason are false.

4. Assertion: Current coming out of the page produces an anticlockwise magnetic field.
Reason: The right thumb points towards the observer.

Answer: Both statements are true, and the Reason explains the Assertion.

5. Assertion: Current entering the page produces an anticlockwise field.
Reason: Cross denotes current entering the page.

Answer: Assertion is false, Reason is true.

16. HOTS / Competency-Based Questions

1. A current is coming out of a page. Without drawing the conductor, state the field direction.

Answer: Anticlockwise.

2. The magnetic field is clockwise around a point in a diagram. What is the current direction?

Answer: Into the page.

3. A student uses the left hand instead of the right hand. What is the problem?

Answer: The standard rule being asked here is the right-hand thumb rule. Using the wrong hand can lead to an incorrect direction.

4. Current is reversed but its magnitude remains unchanged. What changes?

Answer: The direction of magnetic field changes; its magnitude at the same distance remains unchanged under the same conditions.

5. A current is doubled. Does the right-hand thumb rule predict the field magnitude?

Answer: No. The rule gives direction only. Magnitude is obtained from the appropriate magnetic-field relation.

17. CBSE Golden Points | परीक्षा के लिए महत्वपूर्ण तथ्य

✔ Right-Hand Thumb Rule determines magnetic field direction.

✔ Thumb → Conventional current.

✔ Curled fingers → Magnetic field.

✔ Straight conductor → Circular magnetic field lines.

✔ Current out of page → Anticlockwise field.

✔ Current into page → Clockwise field.

✔ Reverse current → Reverse magnetic field direction.

✔ Dot (⊙) → Out of page.

✔ Cross (⊗) → Into page.

✔ Right-Hand Thumb Rule gives direction, not directly the magnitude.

18. Super Memory Trick | सबसे आसान याद करने की ट्रिक

🖐️ RIGHT HAND = RIGHT DIRECTION

👍 Thumb = Current
🌀 Fingers = Field

OUT → ANTI
Current OUT of page → Anticlockwise

IN → CLOCK
Current IN to page → Clockwise

I ↑ → B ↑
r ↑ → B ↓

19. One-Minute Revision | एक मिनट में पूरा Topic

1. Current-carrying conductor produces magnetic field.

2. Straight conductor has circular magnetic field lines.

3. Right thumb gives conventional current direction.

4. Curled fingers give magnetic field direction.

5. Current out of page → anticlockwise.

6. Current into page → clockwise.

7. Reverse current → reverse field.

8. B ∝ I.

9. B ∝ 1/r.

20. Concept Map | अवधारणा मानचित्र

Electric Current
↓
Magnetic Field

Straight Conductor
↓
Circular Field

Right Thumb
↓
Current

Curled Fingers
↓
Magnetic Field

Current OUT
↓
Anticlockwise

Current IN
↓
Clockwise

21. Final Summary | अंतिम सारांश

The Right-Hand Thumb Rule is a simple method for determining the direction of magnetic field around a straight current-carrying conductor.

When the right-hand thumb points in the direction of conventional current, the curled fingers show the direction of the magnetic field.

For current coming out of the page, the magnetic field is anticlockwise. For current going into the page, the magnetic field is clockwise.

For a long straight conductor, the magnitude of magnetic field is:

B = μ₀I / 2πr

Thus, the direction is determined by the right-hand thumb rule, while the magnitude depends on current and distance.

Class 10 Science | Chapter 13
Magnetic Effects of Electric Current

Right-Hand Thumb Rule
दाहिने हाथ के अंगूठे का नियम

CBSE • Foundation • Competitive Preparation