SAT Physics Subject Test

Chapter 11 Magnetic Forces and Fields

Chapter 11 Review Questions

See Chapter 17 for solutions.

  1. Which of the following is/are true concerning magnetic forces and fields?

    I. The magnetic field lines due to a current-carrying wire radiate away from the wire.

   II. The kinetic energy of a charged particle can be increased by a magnetic force.

  III. A charged particle can move through a magnetic field without feeling a magnetic force.

(A) I only

(B) II and III only

(C) I and II only

(D) III only

(E) I and III only

  2. The velocity of a particle of charge +4.0 × 10–9 C and mass 2 × 10–4 kg is perpendicular to a 0.1-tesla magnetic field. If the particle’s speed is 3 × 104 m/s, what is the acceleration of this particle due to the magnetic force?

(A) 0.0006 m/s2

(B) 0.006 m/s2

(C) 0.06 m/s2

(D) 0.6 m/s2

(E) None of the above

  3. In the figure below, what is the direction of the magnetic force FB ?

(A) To the right

(B) Downward, in the plane of the page

(C) Upward, in the plane of the page

(D) Out of the plane of the page

(E) Into the plane of the page

  4. In the figure below, what must be the direction of the particle’s velocity, v ?

(A) To the right

(B) Downward, in the plane of the page

(C) Upward, in the plane of the page

(D) Out of the plane of the page

(E) Into the plane of the page

  5. Due to the magnetic force, a positively charged particle executes uniform circular motion within a uniform magnetic field, B. If the charge is q and the radius of its path is r, which of the following expressions gives the magnitude of the particle’s linear momentum?

(A) qBr

(B) 

(C) 

(D) 

(E) 

  6. A straight wire of length 2 m carries a 10-amp current. How much stronger is the magnetic field at a distance of 2 cm from the wire than it is at 4 cm from the wire?

(A) 2

(B) 2

(C) 4

(D) 4

(E) 8

  7. Due to the magnetic force, a positively charged particle undergoes uniform circular motion in a uniform magnetic field. Which of the following changes could cause the radius of the circular path to decrease?

(A) Increase the mass of the particle

(B) Increase the speed of the particle

(C) Decrease the charge of the particle

(D) Decrease the strength of the magnetic field

(E) None of the above

  8. In the figure below, what must be the direction of the magnetic field?

(A) To the left, in the plane of the page

(B) Upward, in the plane of the page

(C) Downward, in the plane of the page

(D) Out of the plane of the page

(E) Into the plane of the page

  9. A particle of charge –0.04 C is projected with speed 2 × 104 m/s into a uniform magnetic field, B, of strength 0.5 T. If the particle’s velocity as it enters the field is perpendicular to B, what is the magnitude of the magnetic force on this particle?

(A)    4 N

(B)    8 N

(C)  40 N

(D)  80 N

(E) 400 N

10. A charge of mass m and charge q is moving in a circle of radius r and speed v due to a uniform magnetic field B. If the speed is doubled to 2v, what happens to the period, T ?

(A) T increases by a factor of 2

(B) T increases by a factor of 4

(C) T stays the same

(D) T decreases by a factor of 2

(E) T decreases by a factor of 4

Keywords

magnetic fields

tesla (T)

gauss (G)

magnetic force

right-hand rules

Summary

·        When a particle with a charge (q) moves through a magnetic field (B), it experiences a magnetic force (FB). The direction of FB perpendicular to both v and B and is given by the right-hand rule.

·        Magnetic forces affect moving charges, and a current-carrying wire contains charges that move. The magnetic force that affects a wire that carries a current is represented by the equation FB = IℓB sin θ.

·        Magnetic forces never change the speed of a charge, they only turn it.

·        Magnetic forces do no work.

·        Magnetic fields are created by current-carrying wires because of the motion of the electric charges that flow down a wire. The current (I) generates a magnetic field (B) in the surrounding space that is proportional to the current and inversely proportional to the distance from the wire (r). Use the equation B ∞ .