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
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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
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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
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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
Worked Example
A simple d.c. electric motor is shown below.
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Explain why motors like this produce an electric current but unlike a.c. generators they generate a d.c. output.
Answer:
d.c. motors contain a split-ring commutator whilst a.c. motors do not
This reverses the direction of the current in the coil every half turn / every 180°
Examiner Tips and Tricks
Don't get confused between the properties and workings of a.c. and d.c motors. Learn this information here carefully and make sure you understand all the nuances of the language used. e.g. "induced" means created
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