CUET UG Physics Booster Test 2-Faraday’s Laws and Lenz’s Law -

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CUET UG Physics Booster Test 2-Faraday’s Laws and Lenz’s Law

⏱️ 30 Minutes

❓ 20 Questions

📌 Answers are locked once submitted — results and explanations appear at the end.

CUET UG Physics Booster Test 2-Faraday’s Laws and Lenz’s Law

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QUESTION 1 OF 20

If a coil of 500 turns and resistance 2 Ω generates an estimated induced emf of 3.8 × 10⁻³ V, what is the estimated induced current?

Answer Review

1 If a coil of 500 turns and resistance 2 Ω generates an estimated induced emf of 3.8 × 10⁻³ V, what is the estimated induced current?

A 1.9 × 10⁻³ A

B 7.6 × 10⁻³ A

C 3.8 × 10⁻³ A

D 1.5 × 10⁻² A

💡 Short Explanation

�� Apply Ohm's law. �� Current depends on emf and resistance. �� Direct substitution.

✅ Correct Answer Explanation

Given: ε = 3.8 × 10⁻³ V R = 2 Ω → Using Ohm's law: I = ε/R = (3.8 × 10⁻³)/2 = 1.9 × 10⁻³ A → Therefore Option A is correct. → Option B is four times larger than the calculated value. → Option C incorrectly treats current equal to emf. → Option D is much larger than the actual value.

❓ Why Other Options Are Incorrect

�� Option B. 7.6 × 10⁻³ A → Incorrect calculation.
�� Option C. 3.8 × 10⁻³ A → Ignores resistance.
�� Option D. 1.5 × 10⁻² A → Significantly larger than actual value.

🎯 Strategy

Used

�� Substitution

Application:

�� Substitute given values into Ohm's law.

Final Logic:

�� I = ε/R = 1.9 × 10⁻³ A.

🧠 Memory Trick

Current = EMF ÷ Resistance

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Induced Current Calculation

2 Choose the correct regarding quantitative experimental results of Faraday
Statements
1. A stationary magnet inside a coil induces steady current.
2. Faster relative motion results in a larger induced current.
3. The direction of deflection is independent of the magnet's pole.
4. A soft iron rod decreases the deflection dramatically.

A (1)

B (2)

C (3)

D (4)

💡 Short Explanation

�� Faster motion changes flux rapidly. �� Larger emf is induced. �� Deflection increases.

✅ Correct Answer Explanation

Faraday observed that induced current depends on the rate of change of magnetic flux. → Faster relative motion between magnet and coil increases dΦ/dt. → Larger induced emf and current are produced. → Hence Statement 2 is correct.

❓ Why Other Options Are Incorrect

�� Option A. (1) → Stationary magnet produces no change in flux.
�� Option C. (3) → Deflection direction depends on pole and motion.
�� Option D. (4) → Soft iron increases flux and deflection.

🎯 Strategy

Used

�� Elimination

Application:

�� Identify which statement agrees with Faraday's observations.

Final Logic:

�� Faster flux change produces larger induced current.

🧠 Memory Trick

Faster Motion = Bigger Current

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Faraday's Experimental Observations

3 Effect on induced emf magnitude if the rate of change of flux is doubled, and if number of turns is halved:

A Doubled, Halved

B Halved, Doubled

C Doubled, Doubled

D Halved, Halved

💡 Short Explanation

�� Faraday's law contains N and dΦ/dt. �� EMF is directly proportional to both. �� Analyze each change separately.

✅ Correct Answer Explanation

Faraday's law: ε = N(dΦ/dt) → If dΦ/dt doubles: ε doubles. → If N is halved: ε becomes half. → Therefore the correct pair is: Doubled, Halved.

❓ Why Other Options Are Incorrect

�� Option B. Halved, Doubled → Exactly opposite.
�� Option C. Doubled, Doubled → Incorrect effect of N.
�� Option D. Halved, Halved → Incorrect effect of flux change.

🎯 Strategy

Used

�� Conceptual Recall

Application:

�� Recall direct proportionality in Faraday's law.

Final Logic:

�� EMF ∝ N(dΦ/dt).

🧠 Memory Trick

Double Flux Rate → Double EMF

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Faraday's Law

4 Match List I with List II regarding flux derivative

List I	List II
(1) ΔΦB = 0	(a) Constant non-zero induced EMF magnitude
(2) dΦB/dt = constant > 0	(b) Mathematical expression for induced EMF
(3) −dΦB/dt	(c) Zero average induced EMF over the interval
(4) dΦB/dt = 0	(d) Stationary coil in uniform steady field

A 1-c, 2-a, 3-b, 4-d

B 1-a, 2-b, 3-c, 4-d

C 1-d, 2-c, 3-b, 4-a

D 1-b, 2-d, 3-a, 4-c

💡 Short Explanation

Zero net change in flux gives zero average induced emf. A constant rate of change of flux produces a constant induced emf. The quantity −dΦB/dt represents Faraday's law. A stationary coil in a steady magnetic field has no change in flux.

✅ Correct Answer Explanation

(1) ΔΦB = 0 When the net change in magnetic flux over a time interval is zero, the average induced emf over that interval is also zero. → 1 → c → (2) dΦB/dt = constant > 0 A constant positive rate of change of magnetic flux produces a constant non-zero induced emf magnitude. → 2 → a → (3) −dΦB/dt According to Faraday's law, ε = −dΦB/dt Thus, it is the mathematical expression for induced emf. → 3 → b → (4) dΦB/dt = 0 For a stationary coil in a uniform steady magnetic field, the magnetic flux remains constant. Therefore, dΦB/dt = 0 → 4 → d Therefore: 1 → c 2 → a 3 → b 4 → d Hence, Option A is correct.

❓ Why Other Options Are Incorrect

Option B: Incorrectly matches ΔΦB = 0 with constant induced emf and misidentifies Faraday's law.
Option C: Incorrectly associates a constant rate of flux change with zero average emf.
Option D: Incorrectly matches the mathematical expression of emf and steady-field condition.

🎯 Strategy

Used

Matching / Formula Interpretation

Application:

Use Faraday's law:
ε = −dΦB/dt
Interpret each mathematical expression physically.

Final Logic:

ΔΦB = 0 → Zero average emf
Constant dΦB/dt → Constant emf
��dΦB/dt → Faraday's law expression
dΦB/dt = 0 → Stationary coil in steady field

🧠 Memory Trick

"Rate of Flux Change → EMF"

📚 Topic: Mathematical Form of Faraday's Law.

5 Multiple turn coils statements
Statements
1. The total induced emf is equal to the sum of emfs of individual turns.
2. Change of flux is assumed same for each turn in a closely wound coil.
3. Increasing N directly decreases the total induced emf.
4. The expression for total induced emf is −N(dΦB/dt).

A (1), (2), (4)

B (1), (2), (3)

C (2), (3), (4)

D (1), (3), (4)

💡 Short Explanation

�� Emfs add in a coil. �� Same flux change per turn. �� EMF increases with N.

✅ Correct Answer Explanation

Total emf equals the sum of induced emfs in all turns. → In a closely wound coil, each turn experiences nearly identical flux change. → Therefore: ε = −N(dΦB/dt) → Statement 3 is incorrect because increasing N increases emf.

❓ Why Other Options Are Incorrect

�� Option B → Includes false Statement 3.
�� Option C → Includes false Statement 3.
�� Option D → Includes false Statement 3.

🎯 Strategy

Used

�� Elimination

Application:

�� Identify the incorrect statement regarding N.

Final Logic:

�� EMF is directly proportional to number of turns.

🧠 Memory Trick

More Turns = More EMF

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Multiple Turn Coils

6 Incorrect statement about flux linkage factors

A Flux linkage depends on the magnitude of the magnetic field.

B Flux linkage depends on the resistance of the wire.

C Flux linkage can be altered by changing the shape of the coil.

D Flux linkage depends on the orientation angle between the coil and the field.

💡 Short Explanation

�� Flux depends on B, A and θ. �� Resistance does not affect flux. �� Flux is geometric and magnetic.

✅ Correct Answer Explanation

Magnetic flux: Φ = BA cosθ → Flux depends on magnetic field, area and orientation. → Resistance affects current, not magnetic flux. → Therefore Option B is incorrect and hence the correct answer.

❓ Why Other Options Are Incorrect

�� Option A → Magnetic field directly affects flux.
�� Option C → Shape change changes area.
�� Option D → Orientation changes flux.

🎯 Strategy

Used

�� Elimination

Application:

�� Compare variables appearing in Φ = BA cosθ.

Final Logic:

�� Resistance is absent from the flux equation.

🧠 Memory Trick

Flux = B, A, Angle

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Magnetic Flux Factors

7 Correct statements about varying field magnitude
Statements
1. A changing current in a neighboring coil can vary the field magnitude.
2. Inserting an iron rod into a coil changes the resulting magnetic field magnitude.
3. Only permanent magnets can provide a varying magnetic field magnitude.
4. The steady Earth's magnetic field easily induces an emf in a stationary loop.

A (1), (2)

B (3), (4)

C (1), (3)

D (2), (4)

💡 Short Explanation

�� Changing current changes magnetic field. �� Iron core strengthens magnetic field. �� Flux variation induces emf.

✅ Correct Answer Explanation

A changing current in a nearby coil produces a changing magnetic field, thereby changing magnetic flux. → Inserting an iron rod increases magnetic permeability and strengthens the magnetic field. → Both methods vary field magnitude and can induce emf. → Therefore Statements 1 and 2 are correct. → Statement 3 is incorrect because electromagnets and varying currents can also produce changing magnetic fields. → Statement 4 is incorrect because a steady magnetic field acting on a stationary loop does not produce changing flux.

❓ Why Other Options Are Incorrect

�� Option B. (3), (4) → Both statements are incorrect.
�� Option C. (1), (3) → Statement 3 is incorrect.
�� Option D. (2), (4) → Statement 4 is incorrect.

🎯 Strategy

Used

�� Elimination

Application:

�� Identify which statements actually produce a change in magnetic flux.

Final Logic:

�� Only changing field conditions can induce emf.

🧠 Memory Trick

Change Current or Core → Change Flux

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Methods of Changing Magnetic Flux

8 If the area of a rectangular loop changes from A₁ to A₂ in time Δt within a uniform perpendicular magnetic field B, the magnitude of the average induced emf is

A B|A₂ − A₁| / Δt

B B(A₁ + A₂) / Δt

C B(A₂ / A₁) / Δt

D BΔt / |A₂ − A₁|

💡 Short Explanation

�� Flux depends on area. �� Apply Faraday's law. �� Use change in area.

✅ Correct Answer Explanation

Magnetic flux: Φ = BA (since field is perpendicular to the loop) → Change in flux: ΔΦ = B(A₂ − A₁) → Average induced emf: ε = |ΔΦ|/Δt = B|A₂ − A₁|/Δt → Therefore, Option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Uses sum of areas instead of change in area.
�� Option C → Uses ratio of areas, which is not part of Faraday's law.
�� Option D → Inverse relation is incorrect.

🎯 Strategy

Used

�� Substitution

Application:

�� Apply Φ = BA and Faraday's law.

Final Logic:

�� EMF depends on rate of area change.

🧠 Memory Trick

Change Area ÷ Time = EMF

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Flux Change by Area Variation

9 When a circular coil is rotated about its vertical diameter by 180° in a uniform horizontal magnetic field,

A the magnetic flux through the coil remains strictly constant

B the final magnetic flux becomes the exact negative of the initial magnetic flux

C the induced emf is zero at every instant during the rotation

D the area vector remains perpendicular to the magnetic field throughout

💡 Short Explanation

�� Rotation changes orientation. �� Flux depends on angle. �� Area vector reverses after 180° rotation.

✅ Correct Answer Explanation

Magnetic flux: Φ = BA cosθ → Rotating the coil through 180° reverses the direction of the area vector. → Initial flux = +BA cosθ → Final flux = −BA cosθ → Thus the final flux is exactly the negative of the initial flux. → Therefore Option B is correct.

❓ Why Other Options Are Incorrect

�� Option A → Flux changes continuously during rotation.
�� Option C → Changing flux induces emf during rotation.
�� Option D → Area vector orientation changes continuously.

🎯 Strategy

Used

�� Conceptual Recall

Application:

�� Analyze effect of rotating the area vector through 180°.

Final Logic:

�� Reversed area vector gives reversed flux.

🧠 Memory Trick

180° Rotation → Flux Sign Reverses

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Rotation-Induced Flux Change

10 A uniform magnetic field of 0.10 T is set up across a loop of area 100 cm² at an angle where cos(θ) = 1. The field is decreased to zero in 0.5 s. The magnitude of the induced emf is:

A 2.0 × 10⁻³ V

B 2.0 × 10⁻⁴ V

C 1.0 × 10⁻³ V

D 5.0 × 10⁻⁴ V

💡 Short Explanation

�� Use Faraday's law. �� Calculate flux change. �� Divide by time.

✅ Correct Answer Explanation

Given: B = 0.10 T A = 100 cm² = 1.0 × 10⁻² m² cosθ = 1 Δt = 0.5 s → Initial flux: Φ₁ = BA = (0.10)(1.0 × 10⁻²) = 1.0 × 10⁻³ Wb → Final flux: Φ₂ = 0 → Change in flux: ΔΦ = 1.0 × 10⁻³ Wb → Induced emf: ε = ΔΦ/Δt = (1.0 × 10⁻³)/(0.5) = 2.0 × 10⁻³ V → Therefore Option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Ten times smaller than actual value.
�� Option C → Uses incorrect division.
�� Option D → Half of the correct value.

🎯 Strategy

Used

�� Substitution

Application:

�� Substitute values into Φ = BA and ε = ΔΦ/Δt.

Final Logic:

�� ε = 2.0 × 10⁻³ V.

🧠 Memory Trick

Flux Change ÷ Time = EMF

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Magnitude of Induced EMF from Changing Magnetic Field

11 The negative sign in Faraday's law equation ε = -dΦB/dt
Statements
1. Indicates that flux always decreases with time.
2. Represents Lenz's law and the opposition to flux change.
3. Implies that induced current is always counter-clockwise.
4. Means that the magnetic field must be negative.

A (1)

B (2)

C (3)

D (4)

💡 Short Explanation

�� Negative sign represents opposition. �� It expresses Lenz's law. �� Ensures energy conservation.

✅ Correct Answer Explanation

The negative sign in Faraday's law indicates that the induced emf produces a current whose magnetic effect opposes the change in magnetic flux responsible for its production. → This is Lenz's law. → The sign does not indicate that flux must decrease, nor does it specify current direction. → Therefore Statement 2 is correct.

❓ Why Other Options Are Incorrect

�� Option A. (1) → Flux may increase or decrease.
�� Option C. (3) → Direction depends on the situation.
�� Option D. (4) → Magnetic field need not be negative.

🎯 Strategy

Used

�� Conceptual Recall

Application:

�� Recall the physical meaning of the negative sign.

Final Logic:

�� Negative sign = opposition to flux change.

🧠 Memory Trick

Minus Sign = Oppose Change

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Lenz's Law

12 Induced current direction to oppose increasing flux; induced current direction to oppose decreasing flux:

A Creates opposing flux, Creates supporting flux

B Creates supporting flux, Creates opposing flux

C Creates opposing flux, Creates opposing flux

D Creates supporting flux, Creates supporting flux

💡 Short Explanation

�� Oppose the change, not the flux. �� Increasing flux is opposed. �� Decreasing flux is supported.

✅ Correct Answer Explanation

If magnetic flux is increasing, induced current produces magnetic flux opposite to the increase. → If magnetic flux is decreasing, induced current produces magnetic flux in the same direction as the original field to oppose the decrease. → Thus: Increasing flux → opposing flux. Decreasing flux → supporting flux. → Therefore, Option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Reverses Lenz's law.
�� Option C → Incorrect for decreasing flux.
�� Option D → Incorrect for increasing flux.

🎯 Strategy

Used

�� Conceptual Recall

Application:

�� Apply Lenz's law separately to increasing and decreasing flux.

Final Logic:

�� Induced current opposes the change in flux.

🧠 Memory Trick

Increase? Oppose. Decrease? Support.

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Direction of Induced Current

13 Match List I with List II for a Bar Magnet Approaching a Coil

List I	List II
(1) N-pole approaches coil	(a) Coil face becomes N-pole
(2) S-pole approaches coil	(b) Coil face becomes S-pole
(3) N-pole recedes from coil	(c) Opposes recession by becoming S-pole
(4) S-pole recedes from coil	(d) Opposes recession by becoming N-pole

A 1-a, 2-b, 3-c, 4-d

B 1-a, 2-b, 3-b, 4-a

C 1-b, 2-a, 3-d, 4-c

D 1-a, 2-c, 3-b, 4-d

💡 Short Explanation

Approaching poles are repelled. Receding poles are attracted. The induced current creates a magnetic field that opposes the motion. This is the essence of Lenz's law.

✅ Correct Answer Explanation

(1) N-pole approaches coil The coil opposes the approach by becoming a N-pole on the near face, causing repulsion. → 1 → a → (2) S-pole approaches coil The coil opposes the approach by becoming a S-pole on the near face, causing repulsion. → 2 → b → (3) N-pole recedes from coil The coil opposes the recession by attracting the N-pole. Therefore, the near face becomes a S-pole. → 3 → c → (4) S-pole recedes from coil The coil opposes the recession by attracting the S-pole. Therefore, the near face becomes a N-pole. → 4 → d Therefore: 1 → a 2 → b 3 → c 4 → d Hence, Option A is correct.

❓ Why Other Options Are Incorrect

Option B: Incorrectly assigns the coil polarity for the recession cases.
Option C: Reverses the induced polarity for approaching poles.
Option D: Incorrectly matches the S-pole approach and N-pole recession cases.

🎯 Strategy

Used

Option Grouping

Application:

Apply Lenz's law to determine whether the coil must repel or attract the magnet.

Final Logic:

Approach → Repel
Recede → Attract
Induced polarity always opposes the change in magnetic flux.

🧠 Memory Trick

"Approach = Repel, Recede = Attract"

📚 Topic: Polarity of Induced Current and Lenz's Law.

14 Magnetic polarity of coils statements
Statements
1. A counter-clockwise induced current creates a North polarity.
2. A clockwise induced current creates a South polarity.
3. An open circuit loop develops magnetic polarity and current.
4. The polarity acts to accelerate an approaching magnet.

A (1), (2)

B (1), (3), (4)

C (2), (3)

D (1), (2), (3)

💡 Short Explanation

�� Right-hand rule determines polarity. �� Closed circuit needed for current. �� Induced polarity opposes motion.

✅ Correct Answer Explanation

Viewed from a face of the coil: Counter-clockwise current → North pole. Clockwise current → South pole. → Therefore Statements 1 and 2 are correct. → Statement 3 is incorrect because an open circuit does not allow sustained induced current. → Statement 4 is incorrect because induced polarity opposes motion.

❓ Why Other Options Are Incorrect

�� Option B → Includes false Statements 3 and 4.
�� Option C → Includes false Statement 3.
�� Option D → Includes false Statement 3.

🎯 Strategy

Used

�� Elimination

Application:

�� Apply right-hand grip rule and Lenz's law.

Final Logic:

�� Induced polarity opposes flux change.

🧠 Memory Trick

CCW = North, CW = South

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Magnetic Polarity of Coils

15 Incorrect statement about perpetual motion in the context of induction

A A perpetual motion machine violates the law of conservation of energy.

B If induced current supported flux change, it would lead to a perpetual-motion machine.

C A gentle push would continuously accelerate the magnet without energy expenditure in that hypothetical case.

D Lenz's law enables the construction of a perpetual-motion machine.

💡 Short Explanation

�� Lenz's law prevents perpetual motion. �� Energy conservation is preserved. �� Continuous free acceleration is impossible.

✅ Correct Answer Explanation

Lenz's law opposes the change causing induction. → This opposition prevents energy from being created without external work. → Therefore Lenz's law actually prevents perpetual-motion machines. → Hence Option D is the incorrect statement.

❓ Why Other Options Are Incorrect

�� Option A → Correct statement.
�� Option B → Correct consequence.
�� Option C → Correct hypothetical outcome if Lenz's law were absent.

🎯 Strategy

Used

�� Elimination

Application:

�� Identify which statement contradicts conservation of energy.

Final Logic:

�� Lenz's law prevents perpetual motion.

🧠 Memory Trick

Lenz Stops Free Energy

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Conservation of Energy

16 Correct statements about work done against forces
Statements
1. The repulsive force due to induced current necessitates work to move the magnet.
2. This work is entirely stored as potential energy in the magnet.
3. The work done by a person is dissipated as Joule heating.
4. No work is required if an open circuit is used in place of a closed loop.

A (1), (3)

B (2), (4)

C (1), (3), (4)

D (1), (2), (3)

💡 Short Explanation

�� Induced forces oppose motion. �� External work is required. �� Energy becomes heat.

✅ Correct Answer Explanation

The induced magnetic force opposes motion, requiring external work. → The mechanical work supplied is converted into electrical energy and finally dissipated as Joule heat. → Statement 2 is incorrect because the energy is not stored entirely as potential energy. → Statement 4 is incorrect because "no work is required" is not generally true.

❓ Why Other Options Are Incorrect

�� Option B → Both statements are incorrect.
�� Option C → Includes incorrect Statement 4.
�� Option D → Includes incorrect Statement 2.

🎯 Strategy

Used

�� Elimination

Application:

�� Apply energy conservation in electromagnetic induction.

Final Logic:

�� Mechanical work ultimately appears as electrical energy and heat.

🧠 Memory Trick

Work Against Force = Heat Produced

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Work Done Against Induced Forces

17 If the energy spent by a person moving a magnet towards a coil is E, and the energy dissipated by Joule heating due to the induced current is H, conservation of energy dictates that

A E + H = 0

B E = H

C E = H²

D H = E/2

💡 Short Explanation

�� Energy is conserved. �� Mechanical energy becomes heat. �� Input equals output.

✅ Correct Answer Explanation

The work done by the person moving the magnet is converted into electrical energy. → In a resistive circuit, this electrical energy is dissipated as Joule heat. → Therefore: E = H → Option B is correct.

❓ Why Other Options Are Incorrect

�� Option A → Violates energy conservation.
�� Option C → Dimensionally incorrect.
�� Option D → No basis in conservation law.

🎯 Strategy

Used

�� Energy Conservation

Application:

�� Equate mechanical energy input to thermal energy output.

Final Logic:

�� Input energy = dissipated energy.

🧠 Memory Trick

Work In = Heat Out

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Joule Heating and Energy Conservation

18 In the context of energy transformation during electromagnetic induction, the mechanical work done in moving a magnet towards a coil...

A destroys the magnetic field of the bar magnet

B is converted into thermal energy due to Joule heating by the induced current

C violates the law of conservation of energy

D generates a static electric field that stops the magnet

💡 Short Explanation

�� Mechanical energy is transformed. �� Induced current carries energy. �� Heat is produced in resistance.

✅ Correct Answer Explanation

Work done against induced magnetic forces generates electrical energy. → The induced current flowing through the resistance dissipates this energy as Joule heat. → Thus mechanical energy is converted into thermal energy.

❓ Why Other Options Are Incorrect

�� Option A → Magnetic field is not destroyed.
�� Option C → Energy conservation is obeyed.
�� Option D → Static electric field is not the mechanism.

🎯 Strategy

Used

�� Conceptual Recall

Application:

�� Trace energy conversion pathway.

Final Logic:

�� Mechanical → Electrical → Thermal.

🧠 Memory Trick

Motion Makes Heat

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Energy Transformation

19 If a loop moves into a magnetic field, and the work done against the opposing force generates 5.0 J of Joule heating over 2.5 s, what is the average mechanical power expended?

A 2.0 × 10⁰ W

B 1.25 × 10¹ W

C 5.0 × 10⁰ W

D 2.5 × 10⁰ W

💡 Short Explanation

�� Power = Work ÷ Time. �� Joule heat equals work done. �� Direct calculation.

✅ Correct Answer Explanation

Given: Work = 5.0 J Time = 2.5 s → Average power: P = W/t = 5.0/2.5 = 2.0 W → Therefore, Option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Incorrect calculation.
�� Option C → Assumes power equals work.
�� Option D → Incorrect division.

🎯 Strategy

Used

�� Substitution

Application:

�� Substitute values into power formula.

Final Logic:

�� P = 2.0 W.

🧠 Memory Trick

Power = Energy ÷ Time

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Power and Energy Conversion

20 When a planar loop of irregular shape is pulled completely out of a uniform magnetic field
Statements
1. The induced current is zero once the loop is entirely outside the field region.
2. The induced emf remains constant throughout the exit process.
3. The induced current flows to further decrease the magnetic flux.
4. No emf is induced at any point during its motion.

A (1)

B (2)

C (3)

D (4)

💡 Short Explanation

�� Flux changes during exit. �� EMF exists while flux changes. �� No flux means no emf.

✅ Correct Answer Explanation

While the loop is leaving the magnetic field, magnetic flux decreases and induced emf is produced. → Once the loop is completely outside the field, magnetic flux becomes constant (zero). → Therefore induced emf and current become zero. → Hence Statement 1 is correct.

❓ Why Other Options Are Incorrect

�� Option B. (2) → EMF is generally not constant.
�� Option C. (3) → Induced current opposes the decrease in flux.
�� Option D. (4) → EMF is induced during motion through the boundary.

🎯 Strategy

Used

�� Elimination

Application:

�� Track flux changes during and after motion.

Final Logic:

�� No flux change ⇒ No induced current.

🧠 Memory Trick

Outside Field = Zero Current

📚 Class XII – Physics Part I – Chapter: Electromagnetic Induction – Topic: Moving Loops in Magnetic Fields

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