When a circuit is exposed to a magnetic field, magnetic field lines may pass through the circuit, as shown below in the case of a loop of wire. If the loop is pushed into the magnetic field, the amount of flux through it will change and because of this there will be an induced emf in the loop.
The size and direction of the induced EMF is governed by the equations,
where
is the angle between the normal to the surface and the magnetic field. Lenz's Law accounts for the minus sign – The direction of the Induced EMF opposes the change causing it, and Faraday's Law for the magnitude.
Induced EMF's are induced whenever there are conductors moving in a magnetic field,
For example:
The Earth has a magnetic field, and when a plane flies throigh it there is a voltage induced between the wing tips. Because the Earth's magnetic field is so weak, the induced field is never very large.
The strength of magnetic fields may br measured wuth a Hall probe. This is a device in which a current is passed through a conductor. If this happens in a magnetic field, there will be voltage induced between the sides of the conductor and this voltage can be measured, hence the magnetic field deduced.
Some devices can be braked magnetically. A conductor enters the region of a large field produced by an electromagnet. A voltage will be induced which will oppose the change caused in it according to Lenz's Law. This change is the motion into the region of the field and this will slow the device. The braking effect can be varied precisely and because there is no physical contact, this method is very safe.