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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