Question 1

Which of the following best describes the concept of magnetic flux through a surface?

Accepted Answer

The number of magnetic field lines passing through a given surface area, mathematically defined as $$\phi_m = \int \vec{B} \cdot d\vec{A}.$$

Question 2

According to Faraday's law, what is required to induce an emf in a closed loop?

Accepted Answer

A change in the magnetic flux through the loop over time.

Question 3

A circular coil of wire with $$N$$ turns is placed in a time-varying magnetic field. Which equation correctly gives the induced emf?

Accepted Answer

$$\varepsilon = -N \frac{d\phi_m}{dt}$$

Question 4

In the context of electromagnetic induction, what does Lenz's law state?

Accepted Answer

The induced current in a loop will flow in a direction that opposes the change in magnetic flux that produced it.

Question 5

A bar magnet is pushed into a coil connected to a galvanometer. The galvanometer shows a deflection. What is the cause of this deflection?

Accepted Answer

A changing magnetic flux through the coil induces an emf, causing current to flow.

Question 6

Which of the following scenarios will NOT induce an emf in a wire loop?

Accepted Answer

A constant magnetic field with a stationary loop.

Question 7

A rectangular loop of wire is pulled at constant velocity into a region with a uniform magnetic field perpendicular to the plane of the loop. What is the direction of the induced current as the loop enters the field?

Accepted Answer

Such that it creates a magnetic field opposing the increase in flux (Lenz's law).

Question 8

The magnetic flux through a loop is given by $$\phi_m = B A \cos 	heta. If $$the area $$A is $$doubled while $$B$$ and $$	heta$$ remain constant, what happens to the flux?

Accepted Answer

It doubles.

Question 9

In the jumping ring experiment, a conducting ring is placed over an electromagnet and the current is suddenly switched on. Why does the ring jump?

Accepted Answer

A rapidly changing magnetic field induces a current in the ring, which creates a magnetic field opposing the original field, resulting in a repulsive force.

Question 10

A loop of wire rotates with constant angular velocity $$\omega in a $$uniform magnetic field $$B. $$The magnetic flux through the loop at time $$t is $$given by $$\phi_m = B A \cos(\omega t). $$What is the expression for the induced emf?

Accepted Answer

$$\varepsilon = B A \omega \sin(\omega t)$$

Question 11

A credit card reader works by detecting the changing magnetic field as the card is swiped. Which principle does this rely on?

Accepted Answer

Faraday's law of electromagnetic induction

Question 12

If the magnetic field through a coil is decreasing, what is the direction of the induced current according to Lenz's law?

Accepted Answer

It will flow so as to create a magnetic field that tries to maintain the original flux (oppose the decrease).

Question 13

A conducting loop is placed in a region where the magnetic field is perpendicular to the plane of the loop and increases linearly with time. What is the induced emf in the loop if the area is $$A$$ and the field increases at a rate $$k$$ ($$B = k t$$)?

Accepted Answer

$$\varepsilon = -A k$$

Question 14

Which of the following best explains why a current is induced in a secondary coil when the current in a nearby primary coil is switched on or off?

Accepted Answer

The changing current in the primary coil creates a changing magnetic field, which induces an emf in the secondary coil.

Question 15

A wire loop is oriented perpendicular to a uniform magnetic field. If the loop is suddenly rotated by 90 degrees, what happens to the magnetic flux through the loop?

Accepted Answer

It changes from a maximum value to zero.

Question 16

Which of the following is a correct unit for magnetic flux?

Accepted Answer

Weber (Wb)

Question 17

A magnet is moved toward a stationary conducting loop. What determines the magnitude of the induced emf?

Accepted Answer

The rate at which the magnetic flux through the loop changes.

Question 18

In a generator, a coil rotates in a magnetic field. If the angular velocity of the coil is doubled, what happens to the maximum induced emf?

Accepted Answer

It doubles.

Question 19

Which of the following real-world phenomena is best explained by electromagnetic induction?

Accepted Answer

The generation of electricity in power plants using rotating turbines.

LEC 4: Electromagnetic Induction: Faraday’s Law, Lenz’s Law, and Motional EMF

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