Page 560 - Physics Coursebook 2015 (A level)

Cambridge International AS and A Level Physics
548
efficiency The ratio of useful output energy to the total input energy for a device, expressed as a percentage:
efficiency = useful output energy × 100% total input energy
Einstein relation This refers to the equation for the energy of a photon:
E=hf or E=hc λ
elastic limit The value of stress beyond which an object will not return to its original dimensions.
elastic potential energy Energy stored in a stretched or compressed material.
electric charge A property of a body that gives rise to a force on the body when it is within an electric field.
electric field A region in which a charged body experiences a force.
electric field strength The force per unit positive charge at a point. Unit: V m−1 or N C−1.
electric potential The energy per unit charge due to a charged body’s position in an electric field.
electrical resistance The ratio of potential difference to current. Unit: ohm (Ω).
electrolyte An electrically conducting solution. The conduction is due to positive and negative ions in the solution.
electromagnetic spectrum The family of waves that travel through a vacuum at a speed of 3.00 × 108 m s−1.
electronvolt The energy gained by an electron travelling through a p.d. of 1 volt. 1 eV = 1.60 × 10−19 J.
elementary charge The smallest unit of charge that a particle or an object can have. It has a magnitude of 1.60 × 10−19 C.
emission line spectrum A sharp and bright line of a unique wavelength seen in a spectrum.
energy A calculated quantity which is conserved during any change; that which is transferred when a force does work.
energy band A range of permitted electron energies in a solid.
energy level A quantised energy state of an electron in an atom.
equation of state for an ideal gas:
pV=nRT or pV=NkT
(Also known as the ideal gas equation.)
equations of motion Four interrelated equations that can be used to determine the displacement, initial velocity, final velocity and acceleration of a body moving with constant acceleration.
equilibrium An object in equilibrium is either at rest or travelling with a constant velocity because the resultant force on it is zero.
errors Inaccuracies when taking measurements. evaporation The process by which a liquid becomes a gas
at a temperature below its boiling point.
exponential decay Describes the decrease of a quantity where the rate of decrease is proportional to the value of the quantity.
extension The change in the length of a material from its original length.
farad The unit of capacitance (abbreviated F). 1 F = 1 C V−1.
Faraday’s law of electromagnetic induction The induced e.m.f. is proportional to the rate of change of magnetic flux linkage.
field lines Lines drawn to represent the strength and direction of a field of force.
field of force A region of space where an object feels a force; the force may be gravitational, electric, magnetic, etc.
First law of thermodynamics The increase in internal energy of a body is equal to the thermal energy transferred to it by heating plus the mechanical work done on it.
Fleming’s left-hand (motor) rule This rule is used to predict the force experienced by a current-carrying conductor placed in an external magnetic field: thumb → motion, first finger → magnetic field and second finger → conventional current.
Fleming’s right-hand (generator) rule This rule is used to predict the direction of the induced current or e.m.f. in a conductor moved at right angles to a magnetic field: thumb → motion, first finger → magnetic field and second finger → induced conventional current.
forbidden gap A range of energy values which an electron in a solid cannot have.
force constant The ratio of force to extension for a spring or a wire. Unit: N m−1.
forced oscillation An oscillation caused by an external driving force; the frequency is determined by the driving force, and is not the natural frequency of the oscillator.
free-body force diagram A diagram showing all the forces acting on an object (but not the forces it exerts on other objects).