Back Emf, An emf generated by the coil in an AC generator that acts against the potential difference that has been supplied to the motor, Capacitance, The charge stored per unit pd in a capacitor, Coulomb’s Law, The size of the force that acts between two point charges is proportional to the product of their charges and inversely proportional to the square of their separation, Dielectric, An insulating material placed between the two plates of a capacitor in order to increase the amount of charge it can store, Electric Potential, the work done per unit charge on a positive test charge in bringing it from infinity to that point in the field, Equipotential, A surface of constant potential. No work is done by the field when an object moves along an equipotential, Lenz’s Law, An induced current is always in a direction so as to oppose the change that caused it, Magnetic Field, A region surrounding a magnet or current-carrying wire that will exert a force on any other magnet or current-carrying wire placed within it., Magnetic Flux Density, B, The force per unit current per unit length on a current-carrying wire placed at 90º to the field lines. Sometimes also referred to as the magnetic field strength., Permittivity of free space, ε, A measure of the ability of a vacuum to allow an electric field to pass through it., Polarised, An atom/molecule becomes polarised when an external electric field causes the negative electron cloud to be shifted in the opposite direction to the positive nucleus – the charges are pulled in opposite directions., Potential Gradient:, The change of potential per metre at a point in the field, Synchronous Orbit, An orbit in which the period of the orbit is equal to the rotational period of the object that it is orbiting, Magnetic Flux, ϕ, A value which describes the magnetic field or field lines passing through an area. It is the product of magnetic flux density and the perpendicular area it passes through., Gravitational Potential, V (at a point in the field), The work done per unit mass required to move a small test mass from infinity to that point., Electromagnetic Induction, when an emf is induced in a wire/conducting rod when it is moved relative to a magnetic field.

Topic 7: Fields and their Consequences

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