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Electric and Magnetic Fields

Forces on currents and moving charges

  • 2 things to know
  • Papers 1 and 3

Overview

Recall questions on F = BIl sinθ and F = BQv, Fleming's left-hand rule, why a magnetic field does no work, and why a charge entering at right angles follows a circle.

What you need to know

Answer each one before you reveal it.

Your recallNot started
Key term

How is Fleming's left-hand rule applied to an electron, and what provides the centripetal force on a charge circling in a magnetic field?

Process

What happens to the direction of the force on a current-carrying wire in a magnetic field if the current is reversed?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the equation for the force on a charged particle moving through a magnetic field:

    Model answer

    F = Bqv sin θ, where q is the charge, v its speed and θ the angle between the velocity and the field.

  2. 2

    What do the thumb and first two fingers represent in Fleming's left-hand rule?

    Model answer

    The first finger represents the field, the second finger the conventional current and the thumb the direction of the force or motion.

  3. 3

    How is Fleming's left-hand rule applied to an electron moving through a magnetic field?

    Model answer

    The second finger must point opposite to the electron's velocity, because conventional current is in the opposite direction to the flow of negative charge.

  4. 4

    Why does a charged particle entering a uniform magnetic field at right angles follow a circular path?

    Model answer

    The magnetic force is always perpendicular to the velocity and constant in magnitude, so it acts as a centripetal force and continually changes the direction of motion without changing the speed.

  5. 5

    Why does a magnetic field do no work on a moving charged particle?

    Model answer

    Because the force is always at right angles to the velocity, so there is no component of force along the direction of motion and the kinetic energy is unchanged.

  6. 6

    What happens to a charged particle that moves parallel to a magnetic field?

    Model answer

    It experiences no magnetic force, because sin θ is zero, so it continues in a straight line at constant speed.

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