Direction of Induced Current (WJEC GCSE Physics) : Revision Note

Direction of Induced Current

• A generator uses the generator effect to induce (create) alternating current in a conductor

The A.C. Generator

A generator consists of a rotating coil in a magnetic field connected to slip rings

Explaining the Direction of Induced Alternating Current

• A rectangular coil is forced to spin in a uniform magnetic field

  • The coil is connected to an ammeter

  • The slip rings and brushes provide a connection between the coil and the meter by metal brushes that press on two metal slip rings

• When the coil turns in one direction:

  • The pointer deflects first one way, then the opposite way, and then back again

  • This is because the coil cuts through the magnetic field lines, so a voltage and therefore current is induced in the coil

• The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins

  • This continues as long as the coil keeps turning in the same direction

• The induced voltage and the current alternate because they repeatedly change direction

• The graph of alternating current against time as the coil makes one rotation in a magnetic field follows an oscillating curve

  • The starting point depends on the starting position of the coil

A Graph of Current Against Time for a Rotating Coil

Diagram showing how the current varies with the position of the rotating coil

Flemings Right-Hand Rule

• The direction of the rotating current in the coil can be determined by Fleming's right-hand rule

  • It is important to realise that in a motor where motion is induced then Fleming's left-hand rule is used to determine the direction of the current

• To use the rule:

  First Finger = Field:

  • Start by pointing the first finger (on the right hand) in the direction of the field

  ThuMb = Motion:

  • Next, point the thumb in the direction that the wire is moving in

  SeCond = Current:

  • The Second finger will now be pointing in the direction of the current

Fleming's Right Hand Rule

When a coil is spun in a magnetic field, a current is induced between the ends of the coil. Fleming's right-hand rule can determine the direction of the current