Definitions 6.1+ Flashcards
Capacitance
The quantity of charge which can be stored per unit potential difference across the plates of the capacitor.
Farad
Equivalent to one coulomb per volt
Exponential Decay
The quantity decreases at a rate that is proportional to the magnitude of the quantity at that time.
Time constant
The time it takes for the charge remaining on a capacitor to decrease to 37% (1/e) of its initial value.
Electric field
The region around a body in which other charged bodies will feel a force due to the electric charge of the body.
Electric field lines
Show the shape of the field and the direction of the field line at a point in the direction in which a small positive charge would move when placed at that point.
Electric field strength
Force per unit positive charge (N/C) or (V/m)
Coulombs law
The electrical force between two points is directly proportional to the product of their charges and inversely proportional to the square of their separation.
Permittivity of free space
Physical constant related to the size of the force between electrical charges in free space (a vacuum). It has the value 8.85 x 10^-12 (C^2 m^-2 N^-1)
Permittivity
A constant related to the size of the capacitance of a capacitor.
Relative permittivity
Is the factor by which the capacitance of a capacitor with that material between its plates is increased relative to the capacitance of a capacitor with air or vacuum between its plates.
Electric potential
Work done per unit positive charge to move that charge from infinity to a point in an electric field.
Electrical potential energy
Work done to move that charge from infinity to a point in an electric field.
Magnetic field
Region around a permanent magnet or moving charge in which another body with magnetic properties will feel a force.
Magnetic field lines
Show the shape and the direction. The direction of the field line at a point shows the direction in which a plotting compass would point.
Magnetic flux
Product of magnetic flux density and the area at right angles to the flux.Φ=BAcosθ
Magnetic flux density
A measure of strength of the magnetic field.
One weber (Wb)
Is the magnetic flux when a magnetic field of magnetic flux density one tesla passes at right angles through an area of one square metre.
Fleming’s left hand rule
Shows the direction of the force on a conductor carrying a current in a magnetic field.
Velocity selector
A device using perpendicular magnetic and electric fields to select charged particles travelling at a specific velocity which leave the region of crossed fields undeflected.
Electromagnetic induction
Process of inducing an e.m.f in a conductor when there is a change in magnetic flux linkage across the conductor.
Induced e.m.f
Is the e.m.f produced by electromagnetic induction
Magnetic flux linkage
The product of the magnetic flux through the coil and the number of turns on the coil. Magnetic flux linkage = NΦ
Faraday’s Law
The magnitude of the induced e.m.f is equal to the rate of change of flux linkage.
Lenz’s law
States that the direction of any induced e.m.f or induced current is in a direction that opposes the flux change that causes it.
Search coil
A small, flat coil used to determine the strength of a magnetic field.
Generator
Device used to generate electricity, in which the work done to turn the coil with the magnetic field is transferred to electrical energy. The rotation of a coil within a magnetic field produces a constantly changing flux linkage through the coil. This in turn produces a constantly changing e.m.f. in the coil.
Alternating current (a.c.)
Electrical current that reverses its direction with a constant frequency.
e.g. UK mains supply is a.c. with a frequency of 50Hz
Transformer
Device that can either increase or decrease the size of an alternating voltage with little loss of power.
Efficiency
The ratio of useful output energy to total input energy, useful energy expressed as a percentage.
(useful output energy ÷ total input energy) x 100%
