Faraday's law, induced emf, Lenz's law and eddy currents — practice questions

Question 1 · difficulty L1 · recall

Faraday's law states that induced emf equals

• A. the negative time rate of change of magnetic flux linkage
• B. magnetic flux itself
• C. current divided by resistance only
• D. the rate of change of electric charge regardless of flux

Question 2 · difficulty L1 · understanding

Lenz's law determines the direction of induced current by requiring that it

• A. always reinforces the external flux
• B. opposes the change in magnetic flux that produces it
• C. always flows clockwise
• D. depends only on wire resistance

Question 3 · difficulty L2 · understanding

A single loop experiences its magnetic flux changing uniformly from 0.20 Wb to 0.50 Wb in 0.10 s. The emf magnitude is

• A. 0.3 V
• B. 7.0 V
• C. 3.0 V
• D. 0.03 V

Question 4 · difficulty L2 · application

A 100-turn coil has flux per turn changing at 2×10^-4 Wb s^-1. The induced emf magnitude is

• A. 2×10^-4 V
• B. 2.0 V
• C. 50 V
• D. 0.020 V

Question 5 · difficulty L2 · understanding

A square loop of side 2 cm is placed in a time varying magnetic field with magnitude as $B=0.4 \sin (300 t)$ Tesla. The normal to the plane of loop makes an angle of $60^{\circ}$ with the field. The maximum induced emf produced in the loop is $\_\_\_\_$ mV .

• A. 12
• B. 18
• C. 21
• D. 24

Question 6 · difficulty L2 · understanding

A circular loop of radius 7 cm is placed in uniform magnetic field of 0.2 T directed perpendicular to plane of loop. The loop is converted into a square loop in 0.5 s . The EMF induced in the loop is $\_\_\_\_$ mV.

• A. 13.2
• B. 6.6
• C. 1.32
• D. 8.25

Question 7 · difficulty L2 · understanding

A 20 m long uniform copper wire held horizontally is allowed to fall under the gravity $\left(g=10 \mathrm{~m} / \mathrm{s}^2\right)$ through a uniform horizontal magnetic field of 0.5 Gauss perpendicular to the length of the wire. The induced EMF across the wire when it travells a vertical distance of 200 m is $\_\_\_\_$ mV .

• A. $200 \sqrt{10}$
• B. $0.2 \sqrt{10}$
• C. $20 \sqrt{10}$
• D. $2 \sqrt{10}$

Question 8 · difficulty L2 · understanding

A 1 m long metal rod AB completes the circuit as shown in figure. The area of circuit is perpendicular to the magnetic field of 0.10 T . If the resistance of the total circuit is $2 \Omega$ then the force needed to move the rod towards right with constant speed $(v)$ of $1.5 \mathrm{~m} / \mathrm{s}$ is $\_\_\_\_$ N.

• A. $5.7 \times 10^{-2}$
• B. $7.5 \times 10^{-3}$
• C. $5.7 \times 10^{-3}$
• D. $7.5 \times 10^{-2}$

Question 9 · difficulty L2 · understanding

A uniform magnetic field of 0.4 T acts perpendicular to a circular copper disc 20 cm in radius. The disc is having a uniform angular velocity of 10 $\pi$ rad s -1 about an axis through its centre and perpendicular to the disc. What is the potential difference developed between the axis of the disc and the rim? $(\pi=3.14)$

• A. 0.5024 V
• B. 0.0628 V
• C. 0.2512 V
• D. 0.1256 V

Question 10 · difficulty L2 · understanding

A square loop of side $$15 \mathrm{~cm}$$ being moved towards right at a constant speed of $$2\mathrm{~cm} / \mathrm{s}$$ as shown in figure. The front edge enters the $$50 \mathrm{~cm}$$ wide magnetic field at $$t=0$$. The value of induced emf in the loop at $$t=10 \mathrm{~s}$$ will be :

• A. zero
• B. 4.5 mV
• C. 0.3 mV
• D. 3 mV