# Electromagnetism - Equations and Definitions

force on a conductor (at right angles to a magnetic field) carrying a current (N) = magnetic field strength (T) × current (A) × length (m)

potential difference across primary coil (V)/potential difference across secondary coil (V) = number of turns in primary coil/number of turns in secondary coil

potential difference across primary coil (V) × current in primary coil (A) = potential difference across secondary coil (V) × current in secondary coil (A)

Magnetic Field / Magnetic Flux - an area of space in which a magnetic object would have a force exerted on it.

Magnetics Field Lines - a representation of the field as arrows which point from magnetic north poles to south.  The closer the lines are together the stronger the field at that point.

Permanent Magnet - a ferromagnetic object, (i.e. iron, cobalt or nickel), which always has a magnetic field around it.

Induced Magnet - a temporary magnetic object due to a magnetic material being in a magnetic field.

Domain Theory - a way of visualising magnetic materials as being many smaller magnets.  Normally these “domains” are randomly orientated, when magnetised they line up with one another.

Compass - a freely rotating magnet which lines up with Earth’s magnetic field.

Magnetic North Pole - the point on the earth towards which a north seeking (south) pole of a compass would point.

Magnetic Field Strength / Magnetic Flux Density - a measure of the magnitude of force that a magnet can exert on a magnetic object.  Has units Tesla (T).

Solenoid - a coil of conducting wire.  When a current is passed through it has a similar shaped field to a bar magnet.

Electromagnet - a temporary magnet caused by a current being switched on or off.

The motor effect - a force being produced on a current carrying conductor when its magnetic field interacts with a permanent magnetic field at right angles.

Fleming’s Left Hand Rule - a rule used to determine the direction of a force on a current carrying conductor in a magnetic field.  The thumb represents the force, the first finger represents the magnetic field and the second finger represents the current.

Motor - a device which rotates as the direction of the force is always in the same rotating direction.  Consists of a rotating coil of wire, a permanent magnetic field and a split-ring commutator.

Split-ring Commutator - a device which allows a DC input to change connectors every half rotation of the coil of wire every half rotation, this keeps the force in the same rotating direction.

Electromagnetic Induction - a voltage is induced across a conductor within a changing magnetic field.  Can also be caused by relative motion between conductor and magnetic field.

Induced Potential Difference

Alternating Potential Difference - a potential difference which constantly switches from a positive direction to a negative direction.

Alternating Current - a current which constantly switches from a positive direction to a negative direction.

Alternator / Generator - a device which induces an alternating voltage due to a coil of wire spinning in magnetic field.  Consists of a rotating coil of wire, a permanent magnetic (or stator coils) field and a slip ring and brushes apparatus.

Slip-rings - two rings which allow the brush connectors to the coil of wire to slip around them as the generator rotates.

Dynamo - a generator which produces DC voltage by using a split-ring commutator instead of slip-rings and brushes.  Is essentially a motor in reverse.

Transformer -  a device which uses an alternating current  in a primary coil to create a changing magnetic field in a laminated soft iron core which then induces a changing voltage in a secondary coil.  Can be used to isolate circuits and to change (transform) voltages.

Step-up transformer - a transformer with more coils on the secondary (output) coil than on the primary (input) coil.  The voltage output is greater than the voltage input.

Step-down transformer - a transformer with fewer coils on the secondary (output) coil than on the primary (input) coil.  The voltage output is less than the voltage input.

Microphone - a device which turns a sound wave into an electrical signal by means of the wave causing diaphragm to oscillate, the diaphragm is attached to a magnet which moves within a coil of wire.

Loud Speaker - a device which turns an electrical signal into a sound wave by means of the signal causing a magnet to oscillate within a coil of wire, the magnet in turn oscillates a speaker cone.