Question 1

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

Accepted Answer

$$F = qvB$$

Question 2

Which of the following statements about the direction of the magnetic force on a moving charge is correct?

Accepted Answer

The force is always perpendicular to both the velocity and the magnetic field.

Question 3

An electron moves to the right in a region where the magnetic field points upward. What is the direction of the magnetic force on the electron?

Accepted Answer

Out of the page

Question 4

A charged particle moves in a circle of radius $$R in a $$uniform magnetic field $$B. $$Which equation correctly relates the radius to the particle's mass $$m$$, charge $$q$$, and speed $$v$$?

Accepted Answer

$$R = \frac{mv}{qB}$$

Question 5

A current-carrying wire of length $$L is $$placed perpendicular to a uniform magnetic field $$B. $$What is the magnitude of the force on the wire?

Accepted Answer

$$F = ILB$$

Question 6

Two long parallel wires carry currents $$I_1$$ and $$I_2 in $$opposite directions and are separated by a distance $$r. $$What is the force per unit length between them?

Accepted Answer

$$F/L = \frac{\mu_0$ I_1$ I_2$}{2\pi r} ($$repulsive)

Question 7

A positive ion enters a region with both electric field $$\vec{E}$$ and magnetic field $$\vec{B}$$ perpendicular to each other and to the ion's velocity. If the ion passes through undeflected, what is the relationship between $$E$$, $$B$$, and the ion's speed $$v$$?

Accepted Answer

$$v = E/B$$

Question 8

A small metal ball with charge $$q = +0.05\,\mathrm{C}$$ and mass $$m = 0.025\,\mathrm{kg}$$ enters a region of magnetic field $$B = 0.5\,\mathrm{T}$$ perpendicular to its velocity $$v = 200\,\mathrm{m/s}. $$What is the radius of its circular path?

Accepted Answer

$$R = 250\,\mathrm{m}$$

Question 9

Which of the following best describes the motion of a charged particle entering a uniform magnetic field at an angle (not perpendicular or parallel) to the field?

Accepted Answer

The particle moves in a helical (spiral) path.

Question 10

In a mass spectrometer, ions are separated based on which property?

Accepted Answer

Their mass-to-charge ratio ($$m/q)$$

Question 11

A wire of length $$0.5\,\mathrm{m}$$ carries a current of $$2\,\mathrm{A}$$ perpendicular to a $$0.3\,\mathrm{T}$$ magnetic field. What is the magnitude of the force on the wire?

Accepted Answer

$$0.15\,\mathrm{N}$$

Question 12

Which of the following is NOT a factor that affects the magnitude of the magnetic force on a moving charge?

Accepted Answer

The charge's mass

Question 13

A proton and an electron move with the same velocity perpendicular to the same magnetic field. How do the directions of the forces on them compare?

Accepted Answer

The forces are in opposite directions.

Question 14

A current-carrying wire is placed in a uniform magnetic field. If the current is reversed, what happens to the direction of the force on the wire?

Accepted Answer

The force direction reverses.

Question 15

Which rule is used to determine the direction of the magnetic force on a positive moving charge?

Accepted Answer

Right-hand rule

Question 16

A wire carries a current $$I in $$the $$+x$$ direction and is placed in a uniform magnetic field $$\vec{B} in $$the $$+y$$ direction. What is the direction of the force on the wire?

Accepted Answer

$$+z$$ direction

Question 17

If the velocity of a charged particle is parallel to the magnetic field, what is the magnetic force acting on it?

Accepted Answer

Zero

Question 18

A cyclotron accelerates charged particles using which combination of fields?

Accepted Answer

Both electric and magnetic fields

Question 19

Which of the following best explains why the aurora (northern lights) occur near the Earth's poles?

Accepted Answer

Charged particles spiral along Earth's magnetic field lines and collide with the atmosphere near the poles.

Magnetic Force on Moving Charges and Current-Carrying Wires

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