Inducing an EMF in a Conductor
- When a conductor (such as a metal wire) is moved through a magnetic field, it cuts through the field lines, inducing an EMF in the wire
As the wire moves downwards, it cuts through field lines, inducing an EMF in the wire
- A similar effect occurs if a magnet is pushed into, or taken out of a coil:
- As the magnet moved through the coil, the field lines cut through the turns on the coil
- This induces an emf in the coil
When the magnet enters the coil, the field lines cut through the turns, inducing an EMF
- More generally, whenever the magnetic field passing through a loop of wire changes, an EMF is induced
Factors Affecting EM Induction
- The size of the induced EMF is proportional to the rate at which the field lines are cut:
- If the field lines are cut at a faster rate, the EMF will increase
- This can be achieved by:
- Moving the wire (or magnet) faster
- Using a stronger magnet (increasing the number of field lines)
- Adding more turns to a coil (assuming a coil is being used, and not just a wire)
Exam Tip
When discussing factors affecting EM Induction:
- Make sure you state:
“Add more turns to the coil” - And not just:
“Add more coils”
(This second one means something slightly different)
- Likewise, when referring to the magnet, use the phrase:
“A stronger magnet” - And not:
“A bigger magnet”
(Large magnets are not necessarily stronger)
The Right-Hand Rule
- When moving a wire through a magnetic field, the direction of the induced EMF can be worked out by using the Right-Hand Dynamo rule:
The Right-Hand Dynamo rule can be used to deduce the direction of the induced EMF
- To use the rule:
- Start by pointing the first finger (on your right hand) in the direction of the field
(First Finger Field) - Next, rotate your hand so that the thumb point in the direction that the wire is moving in
(ThuMb Motion) - Your Second finger will now be pointing in the direction of the current (or, strictly speaking, the EMF)
(SeCond Current)
- Start by pointing the first finger (on your right hand) in the direction of the field
- The direction of the induced EMF always opposes the change that produces it
- This means that any magnetic field created by the EMF will act so that it tried to stop the wire or magnet from moving
