Question 1

Which of the following best describes the source of the magnetic field around a long, straight wire carrying a current?

Accepted Answer

The macroscopic movement of charges as a current.

Question 2

A proton moves with velocity $$\vec{v}$$ perpendicular to a uniform magnetic field $$\vec{B}. $$What is the direction of the magnetic force $$\vec{F}$$ acting on the proton?

Accepted Answer

Perpendicular to both $$\vec{v}$$ and $$\vec{B}$$, as given by the right-hand rule.

Question 3

Two long, straight, parallel wires carry currents in the same direction. What is the nature of the force between them?

Accepted Answer

They attract each other.

Question 4

The magnetic field at the center of a circular loop of radius $$R$$ carrying current $$I is $$given by which of the following equations?

Accepted Answer

$$B = \frac{\mu_0$ I}{2R}$$

Question 5

A charged particle moves with velocity $$\vec{v} at an $$angle $$\alpha to a $$magnetic field $$\vec{B}. $$What is the magnitude of the magnetic force on the particle?

Accepted Answer

$$F = qvB \sin \alpha$$

Question 6

A beam of electrons moves to the right and enters a region with a uniform magnetic field pointing out of the page. Which path will the electrons follow?

Accepted Answer

A circular path curving downward (toward the bottom of the page).

Question 7

What is the direction of the magnetic field at a point above a long, straight wire carrying current to the right (along the +x-axis)?

Accepted Answer

Into the plane of the page.

Question 8

If the radius of a current-carrying circular loop is doubled while keeping the current the same, how does the magnetic field at the center change?

Accepted Answer

It becomes half as strong.

Question 9

A proton with kinetic energy $$K = 11$$ MeV moves in a cyclotron with a magnetic field $$B = 1.2 T. $$What is the radius $$r of $$its orbit just before it exits? (Proton mass $$m_p = 1.67 	imes 10$$^{-27}$$ kg, charge e = 1.60$$ 	imes $$10^{-19} C)$$

Accepted Answer

$$r = \frac{\sqrt{2m_p K}}{eB}$$

Question 10

A long, straight wire carries a current $$I. At $$what distance $$r$$ from the wire is the magnetic field equal to $$B$$? (Use $$\mu_0$ for the permeability of free space.)

Accepted Answer

$$r = \frac{\mu_0$ I}{2\pi B}$$

Question 11

A small particle of charge $$-1.9 	imes 10$$^{-6}$$ C and mass 3.1$$ 	imes $$10^{-12} kg $$enters a region of uniform magnetic field at $$8.1 	imes 10^3$ m/s and travels in a semicircular path of radius R = 5.0$ cm. What is the magnitude of the magnetic field?

Accepted Answer

$$B = \frac{mv}{|q|R}$$

Question 12

Which of the following statements about the force between two parallel current-carrying wires is correct?

Accepted Answer

The force per unit length is $$F/L = \frac{\mu_0$ I_1$ I_2$}{2\pi d}$$, where $$d is $$the distance between the wires.

Question 13

A magnetic dipole is stable when:

Accepted Answer

It is aligned with the external magnetic field.

Question 14

Which of the following best explains why a refrigerator magnet can hold a paper to a refrigerator door?

Accepted Answer

The magnetic field exerts a force on the magnetic domains in the steel door, attracting the magnet.

Question 15

A solenoid has $$N$$ turns, length $$L$$, and carries current $$I. $$What is the magnetic field inside the solenoid?

Accepted Answer

$$B = \frac{\mu_0$ N I}{L}$$

Question 16

A charged particle moves in a circle of radius $$r in a $$uniform magnetic field $$B. $$What is the relationship between the particle's speed $$v$$, charge $$q$$, and mass $$m$$?

Accepted Answer

$$r = \frac{mv}{|q|B}$$

Question 17

Which of the following is a correct statement about the direction of the magnetic field lines around a bar magnet?

Accepted Answer

They point away from the north pole and toward the south pole.

Question 18

A current-carrying wire is placed in a uniform magnetic field. The force on the wire is maximum when:

Accepted Answer

The current is perpendicular to the magnetic field.

Question 19

Which of the following is the SI unit of magnetic field strength?

Accepted Answer

Tesla (T)

Magnetic Fields and Forces: Sources, Properties, and Applications

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