Question 1

A credit card reader at a gas station requires you to quickly swipe your card rather than hold it still. Which physical principle explains why the card must be moved for the reader to work?

Accepted Answer

Faraday's law of electromagnetic induction

Question 2

Which of the following best describes the condition for maximum magnetic flux through a flat surface of area $$A in a $$uniform magnetic field $$B$$?

Accepted Answer

The surface is perpendicular to the magnetic field

Question 3

A coil of wire is placed in a region with a constant magnetic field. Which of the following will NOT induce an emf in the coil?

Accepted Answer

Keeping the coil stationary and the magnetic field constant

Question 4

Which equation correctly expresses Faraday's law for a single loop?

Accepted Answer

$$\mathcal{E} = -\frac{d\Phi_B}{dt}$$

Question 5

A coil with $$N$$ turns is placed in a changing magnetic field. What is the total induced emf in the coil?

Accepted Answer

$$\mathcal{E} = -N \frac{d\Phi_B}{dt}$$

Question 6

According to Lenz's law, the direction of an induced current in a loop is always such that:

Accepted Answer

It opposes the change in magnetic flux that produced it

Question 7

A metal rod of length $$L$$ moves with velocity $$v$$ perpendicular to a uniform magnetic field $$B. $$What is the induced emf across the ends of the rod?

Accepted Answer

$$\mathcal{E} = vBL$$

Question 8

Which of the following is a real-world application of eddy currents?

Accepted Answer

Airport metal detectors

Question 9

Which Maxwell equation states that the net magnetic flux through any closed surface is zero?

Accepted Answer

$$\oint \vec{B} \cdot d\vec{A} = 0$$

Question 10

A superconductor is cooled below its critical temperature $$T_c in a $$uniform magnetic field. What happens to the magnetic flux inside the superconductor?

Accepted Answer

All magnetic flux is expelled (Meissner effect)

Question 11

Which of the following best describes the symmetry in Maxwell's equations in empty space?

Accepted Answer

A changing electric field creates a magnetic field, and a changing magnetic field creates an electric field

Question 12

A circular loop of wire is placed in a region where the magnetic field is increasing upward. According to Lenz's law, what is the direction of the induced current as viewed from above?

Accepted Answer

Counterclockwise

Question 13

Which of the following is the correct integral form of Faraday's law?

Accepted Answer

$$\oint \vec{E} \cdot d\vec{l} = -\frac{d\Phi_B}{dt}$$

Question 14

A paramagnetic material is placed near a permanent magnet. What happens?

Accepted Answer

It is attracted to the magnet due to alignment of magnetic dipoles

Question 15

Which of the following is NOT a consequence of electromagnetic induction?

Accepted Answer

A stationary magnet near a stationary coil produces a current

Question 16

A coil is placed in a region where the magnetic field is decreasing. What is the sign of the induced emf according to Faraday's law?

Accepted Answer

Positive

Question 17

Which of the following is the correct expression for the magnetic flux $$\Phi_B$$ through a flat surface of area $$A at an $$angle $$	heta to a $$uniform magnetic field $$B$$?

Accepted Answer

$$\Phi_B = BA \cos 	heta$$

Question 18

Which of the following statements about superconductors is true?

Accepted Answer

They expel all magnetic flux below their critical temperature

Question 19

Which of the following best describes the displacement current in Maxwell's equations?

Accepted Answer

It accounts for the changing electric field between capacitor plates

Electromagnetic Induction and Maxwell's Equations

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