Topic 12: Electromagnetic Induction Flashcards
Electromagnetic induction
An emf is induced when the magnetic field passing through an electric conductor changes
Magnetic flux density
The strength of the magnetic field. The force acting on a conductor per unit current length. Unit: 1 tesla (T) = 1 NA-1m-1
Magnetic field lines
The density of the lines in an area is proportional to the strength of the field. The lines ‘thread’ or ‘link’ the coil. When the coil moves relatively to the field, the lines may ‘cut’ the coil.
Magnetic flux
The magnetic flux through a small plane surface is the product of the flux density normal to the surface and the area of the surface.
Φ=BA where B is the flux density acting at right angles to the surface and A is the area of the surface. Unit: 1T/m2 = 1 weber (Wb)
Magnetic flux linkage
The flux linkage through a coil of N turns is NΦ
Faraday’s Law
The induced emf is directly proportional to the rate of change of flux linkage or rate of flux cutting
Lenz’s Law
The direction of the induced emf is such that it tends to oppose the flux change causing it, and does oppose it if the induced current flows.
AC generator
A machine which rotates a coil relative to a magnetic field which induces a varying emf.
RMS value
Equal to the value of a direct component, dissipating same power in a fixed resistor as RMS value.
E.g. RMS current is equal to direct current, dissipating same power in a fixed resistor (as RMS current)
Peak value
The maximum value (eg Vo, Io) of a sinusoidally varying quantity.
Ideal transformer
A device which induces in a secondary coil an emf by variation of an emf in the primary coil. It is ‘ideal’ if the efficiency is 100%.
Real transformer
Energy losses are due to heating of the wires; magnetic flux linkage less than 100%; heating due to changing magnetic field (hysteresis); heating due to eddy currents.
AC transmission
Since energy losses are proportional to current2, it is important to reduce current which is achieved by increasing PD. Step-up transformers do this and therefore, transmission uses AC with a frequency of 50Hz.
Health risks
Alternating currents produce changing electric and magnetic fields which will induce emfs in conductors. There is a hypothesis that this may cause genetic damage. The effect could depend on current, frequency and exposure time.
