Lesson 3.7.5.2

3.7.5.2 Moving charges in a magnetic field Quiz: AQA Physics, Unit 7

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.7.5.2, Moving charges in a magnetic field: 20 multiple choice questions for the AQA Physics (7408), Unit 7: Fields and their consequences (A-level only), written with Revision Ninja.

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The 20 questions

  1. Which equation gives the magnetic force on a charge Q moving at speed v perpendicular to a magnetic field B?

    • F = BQ/v
    • F = BQv
    • F = Bv/Q
    • F = B Q^2 v

  2. A positive charge moves to the right in a magnetic field directed into the page. In which direction is the force on it?

    • upward, in the plane of the page
    • downward, in the plane of the page
    • out of the page
    • to the left, opposite to its velocity

  3. An electron replaces the positive charge in the previous set-up, moving the same way in the same field. In which direction is the force on it?

    • into the page
    • upward, in the plane of the page
    • out of the page
    • downward, in the plane of the page

  4. Why does a magnetic force not change the speed of a charged particle?

    • The force is always perpendicular to the velocity, so no work is done on the particle
    • The force is parallel to the velocity and cancels out the motion
    • The magnetic field is too weak to act on fast-moving particles
    • The particle's mass increases to balance the force exactly

  5. What is the purpose of the magnetic field in a cyclotron?

    • to make charged particles follow a circular path so they can be repeatedly accelerated by an alternating electric field
    • to cool the particles to low temperatures
    • to focus the particles into a narrow beam at the exit
    • to slow the particles down before they hit the target

  6. A proton of mass 1.67 x 10^-27 kg moves at 3.0 x 10^6 m/s perpendicular to a field of 0.50 T. What is the radius of its path? Use Q = 1.6 x 10^-19 C.

    • 0.63 cm
    • 2.5 cm
    • 6.3 cm
    • 63 cm

  7. An electron moves at 2.0 x 10^6 m/s perpendicular to a field of 0.10 T. What is the magnetic force on it?

    • 2.0 x 10^-13 N
    • 3.2 x 10^-15 N
    • 3.2 x 10^-14 N
    • 1.6 x 10^-14 N

  8. A charge of 1.6 x 10^-19 C enters a field of 0.30 T at right angles and experiences a force of 2.4 x 10^-15 N. What is its speed?

    • 2.0 x 10^4 m/s
    • 5.0 x 10^3 m/s
    • 5.0 x 10^4 m/s
    • 5.0 x 10^5 m/s

  9. A proton's speed doubles in a fixed magnetic field, perpendicular to its motion. What happens to the radius of its circular path?

    • It quadruples
    • It is unchanged
    • It halves
    • It doubles

  10. For a particle moving at fixed speed perpendicular to a field, the field strength is doubled. What happens to the radius of its path?

    • It doubles
    • It quadruples
    • It is unchanged
    • It halves

  11. An electron and a proton enter the same uniform field at the same speed, perpendicular to the field. Which has the larger circular radius?

    • the proton, because its mass is about 1800 times larger
    • the electron, because its charge is smaller
    • they have equal radii, since speed and field are equal
    • the electron, because its mass is larger

  12. Why is the period of the circular motion of a charged particle in a uniform magnetic field independent of its speed?

    • Because the radius is proportional to speed, so the circumference divided by speed is constant
    • Because the field strength varies with the particle's speed
    • Because the particle's mass falls as its speed rises
    • Because the magnetic force speeds the particle up as fast as it bends the path

  13. A charged particle moves in a circle of radius 0.10 m in a field of 0.20 T. Its charge is 1.6 x 10^-19 C and its mass is 9.1 x 10^-31 kg. What is its speed?

    • 3.5 x 10^5 m/s
    • 3.5 x 10^6 m/s
    • 1.8 x 10^7 m/s
    • 7.0 x 10^6 m/s

  14. Which vector relationship gives the direction of the magnetic force on a moving positive charge?

    • F is proportional to v divided by B
    • F is proportional to v dot B
    • F is proportional to v cross B
    • F is proportional to B cross I

  15. An alpha particle of charge 3.2 x 10^-19 C moves at 1.0 x 10^6 m/s perpendicular to a 0.50 T field. What is the force on it?

    • 3.2 x 10^-13 N
    • 1.6 x 10^-19 N
    • 1.6 x 10^-13 N
    • 8.0 x 10^-14 N

  16. For a charge moving in a uniform magnetic field, which direction is the magnetic force relative to its velocity and the field?

    • parallel to the velocity
    • perpendicular to both the velocity and the field
    • at 45 degrees to the velocity
    • parallel to the field

  17. A charged particle moves parallel to a uniform magnetic field. What is the magnetic force on it?

    • BQv multiplied by the square root of two
    • BQv divided by two
    • zero
    • BQv, the maximum value

  18. An electron moves at 4.0 x 10^6 m/s perpendicular to a 0.020 T field. What is the radius of its path? Use m = 9.1 x 10^-31 kg.

    • 0.11 mm
    • 5.7 mm
    • 1.1 m
    • 1.1 mm

  19. A particle's charge is doubled while its mass and speed are unchanged, in a fixed field perpendicular to its motion. What happens to its radius?

    • It quadruples
    • It doubles
    • It halves
    • It is unchanged

  20. What determines the maximum kinetic energy of particles leaving a cyclotron?

    • the number of turns in the coil that makes the field
    • the frequency of the alternating voltage only
    • the mass of the target material placed at the exit
    • the radius of the dees and the magnetic flux density, since v_max = BQr/m

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