When a conducting rod moves in a field, the electrons experience a force so accumulate at one end. This induces an emf across the ends of the rod.
An emf can be induced in a flat coil or solenoid by moving he coil towards or away from the magnet’s poles, or moving a magnet towards or away from the coil. In either case, the magnetic flux through the coil is changing.
Magnetic flux density is the strength of the magnetic field (analogous to the number of field lines per unit area), so the total magnetic flux passing through an area A is given by:
Using cos(), where is the angle between the normal of the plane and the magnetic flux, provides the component of the flux perpendicular to the coil’s area. The unit of magnetic flux is the weber. This is defined as: a change in flux linkage of one weber in one second will induce an emf of 1 volt in a loop of wire.
The magnetic flux linkage is given by:
Where N is the number of turns in the coil.
According to Faraday’s Law, the induced emf Is directly proportional to the rate of change of flux linkage. This yields:
Lenz’s Law states that the induced emf is always in such a direction as to oppose the change that caused it. That’s why there’s a minus sign in Faraday’s law.
- If the original magnetic field is getting stronger (e.g. the coil is moving towards the permanent magnet), then the induced magnetic field will be in the opposite direction to “try to weaken” it
- If the original magnetic field is getting weaker (e.g. the coil is moving away from the permanent magnet), then the induced magnetic field will be in the same direction to “try to maintain” it. You can just use the right hand rule in place of left.
Search coils are used to investigate magnetic flux.
- Place two bar magnets a small distance apart with opposite poles facing each other
- Place a search coil (small loop with known area and number of turns) between the magnets. Connect it to a data recorder.
- Start the data recorder. Keeping it at the same orientation, quickly move it out of the coil.
- An emf will be induced due to magnetic flux density through the coil changing from maximum to zero as the coil is removed from the field
- Use the data logger to plot a graph of induced emf against time. Find the area underneath it to find the change in magnetic flux linkage.
- so, using the known values of N and A, B can be derived. A simple a.c. generator induces electric current by rotating a coil in a magnetic field.
- Has slip rings and brushes to connect it to an external circuit
- Output voltage and current change direction with every half rotation, producing a.c.
- Converts KE to EPE
A simple laminated iron-cored transformer allows you to change the size of a voltage for an alternating current.
- Two coils of wire wrapped around an iron core
- Alternating current flows through the primary coil, creating a changing magnetic field in the iron core
- The changing magnetic field passes through the iron core to a secondary coil, where it induces an alternating voltage of the same frequency. For an ideal transformer:
Transformers are used in power stations (increase V to reduce power loss) and at homes (decrease V in the interests of safety).