Up Learn – A Level physics (AQA) – MAGNETIC FIELDS/ELECTROMAGNETIC INDUCTION
How to use Lenz’s law to identify the direction of induced EMF (electromotive force) in a wire, also known as the generator effect.
Magnetic Fields and Electromagnetic Induction
Last time, we saw that if a wire cuts through magnetic field lines, we induce an emf in the wire.
And if the wire is connected to a circuit, a current will flow.
Now, to predict the direction of the current we can use something called Lenz’s Law.
We’ve seen before that, in a magnetic field, a current-carrying wire experiences a force, as long as the current isn’t parallel to the field.
So, when we move a wire through a magnetic field, and induce a current along the wire…then the wire will experience a force!
For example, if we move a wire through a magnetic field like this, it experiences a force in this direction.
And if we move the wire like this, it experiences a force in this direction.
So the direction of the force is always opposite to the wire’s velocity.
And now, since we know the direction of the field lines, and the force we end up with… we can use Fleming’s left hand rule to work out the direction of the current.
So, if we move the wire in this direction, and it experiences a force in this direction, what’s the direction of the current?
The current flows in this direction.
This phenomenon was first observed by the Russian physicist Emil Lenz, who wrote a law to describe it…
Lenz’s law states that: “the direction of the induced emf opposes the change that caused it”.
So, for our wire, it’s moving in this direction – that’s the change…
Which causes an induced emf in the wire…
And the emf creates a current in this direction, so the wire experiences a force that opposes its motion
Finally, so far we’ve focused on a single wire, but the current through the wire also depends on the circuit it’s part of…
And that’s why the formal definition of Lenz’s law talks about emf, not current.
So, to sum up…
If a wire cuts through magnetic field lines, it experiences a force that is in the opposite direction to its velocity.
And this is an example of Lenz’s Law, which states that the direction of the induced emf opposes the change that caused it.