20.4 - Motor Effect

A current in a magnetic field experiences a force

• When a current-carrying wire is placed in a magnetic field, the magnetic field around the wire interacts with the magnetic field between the poles.
• The two fields exert a force on each other, causing the wire to move. This is called the motor effect.
• By Newton's third law, the magnet and the wire exert an equal and opposite force on each other.

Factors affecting the force

• Angle:
  • The full force acts when the wire is at 90° (right angles) to the magnetic field.
  • A wire running parallel to the field experiences no force.
  • The force always acts at right angles to both the magnetic field and the current.
• Magnitude - The force increases if the current increases, a stronger magnet is used, or a longer length of wire lies in the field.
• Direction - Reversing either the current or the magnetic field reverses the direction of the force.

Demonstrating the motor effect

• Place a light metal rod across a pair of parallel rails inside a horseshoe magnet to complete a circuit.
• When current flows, the magnetic force pushes the rod so it rolls along the rails.
• The rails and rod are made from a non-magnetic metal (such as aluminium or brass) so they aren't simply attracted to the magnet.

Fleming's left-hand rule

Fleming's left-hand rule finds the direction of the force on a current-carrying conductor in a magnetic field. Hold the thumb, first finger, and second finger of the left hand at right angles to each other:

• The magnetic field always runs from north to south.
• The current is conventional current, which flows from positive to negative.
• Reversing either the current or the field reverses the direction of the force.

The force on charged particles

• Since an electric current is a flow of charge, a beam of charged particles moving through a magnetic field experiences a force in the same way as a current in a wire.
• The force only acts if the particles are not moving parallel to the magnetic field lines.
• Fleming's left-hand rule gives the direction of the force, using conventional current for the second finger:
  • Positive particles - Conventional current is in the same direction as the particles' motion.
  • Negative particles (e.g. electrons) - Conventional current is in the opposite direction to the particles' motion.

Finding the field from beam deflection

A beam of electrons travels horizontally from right to left and curves upwards as it enters a magnetic field, as shown below:

1. The beam deflects upwards, so the force on the electrons is upwards (thumb points up).
2. Electrons are negatively charged, so the conventional current is opposite to their motion (second finger points opposite to electron flow).
3. Aligning the fingers shows that the first finger points into the page, so the magnetic field acts into the page.