Magnetic fields and induction definitions Flashcards
Back emf
emf induced in the spinning coil of an electric motor or in any coil in which the current is changing (e.g. the primary coil of a transformer). A back emf acts against the change of applied p.d.
Eddy currents
induced (small loop) currents in the metal parts of ac machines due to a changing flux linkage
Faraday’s law of electromagnetic induction
the induced emf in a circuit is equal to the rate of change of magnetic flux linkage through the circuit
Fleming’s left-hand rule
rule that relates to the directions of the force, magnetic field and current on a current-carrying conductor in a magnetic field (used for the motor effect)
Fleming’s right-hand rule
rule that relates the directions of the induced current, magnetic field and velocity of the conductor when the conductor cuts across magnetic field lines and an emf is induced in it (used for EM induction)
Hall probe
a device used to measure magnetic flux density.
Lenz’s law
when a current is induced by electromagnetic induction, the direction of the induced emf and current is always such as to oppose the change that causes it.
Magnetic flux
Φ = BA, for a uniform magnetic field of flux density B that is perpendicular to an area A. The unit of magnetic flux is the weber (Wb).
Magnetic flux density B
the magnetic force per unit length per unit current on a current carrying conductor at right angles to the field lines. The unit of magnetic flux density is the tesla (T).
Magnetic flux linkage N
through a coil of N turns, flux linkage = NBA, where B is the magnetic flux density perpendicular to area A. The unit of magnetic flux linkage is Wb turns.
Motor effect
the force on a current-carrying conductor due to a magnetic field.
Simple electric motor
an electric motor with an armature consisting of a single coil of insulated wire.
1 Tesla
the magnetic flux density that produces a force of 1 Newton on a wire of length 1 metre that is carrying a current of 1 Ampere, perpendicular to the magnetic field lines.
Grid system
the network of transformers and cables that is used to distribute electrical power from power station to users.
Transformer
converts the amplitude of an alternating p.d. to a different value. It consists of two insulated coils, the primary coil and the secondary coil, wound round a soft iron laminated core; step-down transformer: a transformer in which the rms p.d. across the secondary coil is less than the rms p.d. applied to the primary coil; step-up transformer: a transformer in which the rms p.d. across the secondary coil is greater than the rms p.d. applied to the primary coil.
