A2 Physics Definition List

Period, T

Time taken to complete one complete cycle.

Radian

Angle subtended at the centre of a circle by an arc of length equal to the radius of the circle.

Angular acceleration

Rate of change of angular velocity.

Angular displacement, θ

The angle through which the object has moved.

Angular velocity, ω

Rate of change of angular displacement (ω = 2πf = 2π/T).

Tangential speed, v

Speed of object that is tangent to the circular path (v = rω).

Gravitational field

A region of space where a force per unit mass acts on a particle with mass.

Gravitational field lines

Lines with arrows that show the direction of gravitational force on a mass.

Newton's Law of Gravitation

Gravitational force is directly proportional to the product of masses and inversely proportional to the square of the separation.

Gravitational field strength, g

Gravitational force per unit mass on the object.

Gravitational potential, Φ

Work done per unit mass in bringing the mass from infinity to a point.

Gravitational potential energy

Work done in bringing a mass from infinity to a point.

Geostationary orbit

Orbit in which a satellite is positioned so that it orbits the Earth at the same rate as the Earth's rotation. The satellite remains above a fixed point on the Earth's surface.

Absolute zero

Temperature at which atoms have minimum or zero energy.

Heat / Thermal Energy, Q

Energy transferred from one object to another because of a temperature difference. Increases internal energy.

Temperature

A measure of the average kinetic energy of particles; shows the direction of net heat flow between two bodies in contact.

Thermal Equilibrium

When two or more objects in contact have the same temperature so that there is no net flow of thermal energy.

Calibration

Uses fixed points as upper/lower reference points and assumes a linear change of property with temperature.

Fixed points

Standard reference temperatures used when calibrating thermometers.

Thermocouple

Device consisting of wires of two different metals across which an e.m.f. is produced when the two junctions are at different temperatures.

Thermodynamic / Absolute Scale

A temperature scale that does not depend on the property of any particular substance.

Boiling

The process by which a liquid changes into its gaseous state at a constant specific temperature. Heat energy goes towards overcoming intermolecular forces so that interatomic forces and potential energy become negligible.

Evaporation

The process by which molecules on the surface of a liquid with sufficient kinetic energy break free from attractive intermolecular forces and escape as gas particles. Occurs below the boiling point.

Melting

The process by which a solid changes into its liquid state at a constant specific temperature. Heat energy overcomes rigid forces between atoms; potential energy increases, kinetic energy stays constant.

Specific heat capacity, c

Energy required per unit mass per unit temperature rise of 1 K or 1°C.

Specific latent heat of fusion

Energy required per unit mass of a substance to change it from solid to liquid without a change in temperature.

Specific latent heat of vaporisation

Energy required per unit mass of a substance to change it from liquid to gas without a change in temperature.

Specific latent heat, L

Energy required per unit mass of a substance to change its state without any change in temperature.

Ideal gas

A gas that obeys the ideal gas law PV = NkT and has no intermolecular forces.

Mole

The amount of a substance which contains the same number of particles as there are atoms in 0.012 kg of carbon-12.

Brownian Motion

The random movement of tiny suspended particles (such as smoke or pollen) in a fluid, caused by collisions with surrounding molecules.

Kinetic Model of Gas Pressure

Molecules collide with container walls, causing a change in momentum and hence a force (F = Δp/Δt). Many collisions over the area produce pressure (P = F/A).

Kinetic Theory

Molecules are in rapid, random motion. Collisions with walls are elastic. Intermolecular forces act only during collisions. Duration of collisions and volume of molecules are negligible.

Internal energy, U

Sum of the random distribution of kinetic and potential energies of all particles/molecules in a system.

First Law of Thermodynamics

The increase in internal energy of a body equals the thermal energy transferred to it by heating plus the mechanical work done on it (ΔU = q + w).

Amplitude, a

Maximum displacement from the equilibrium position.

Simple Harmonic Motion

Acceleration is directly proportional to displacement from the equilibrium position, and always directed towards it. Acceleration and displacement are in opposite directions (a = −ω²x).

Damping

Oscillations/amplitude/energy decrease over time due to friction or resistive forces from the surroundings.

Forced oscillation

Oscillation caused by an external driving force; the frequency equals that of the driving force.

Free oscillation

Oscillation whose frequency is the natural frequency of the oscillator.

Natural frequency

The unforced frequency of oscillation of a freely oscillating object.

Resonance

When a system is forced to vibrate at or near its natural frequency, amplitude increases rapidly. Maximum amplitude occurs when driving frequency equals natural frequency.

Coulomb

A charge of 1 C passes a point when a current of 1 A flows for 1 s.

Electric field

A region of space where a force per unit charge acts on a particle with charge.

Electric field lines

Line spacing represents electric field strength. Arrows show the direction of force on a positive test charge.

Electronvolt

The energy gained by an electron travelling through a p.d. of 1 V.

Elementary charge

The smallest unit of charge that a particle can have.

Coulomb's Law

Electric force is proportional to the product of the charges and inversely proportional to the square of the separation.

Electric field strength, E

Electric force per unit positive test charge.

Electric potential, V

Work done per unit charge (by an external force) in bringing a unit positive charge from infinity to a point.

Electric potential energy

Work done (by an external force) in bringing a unit positive charge from infinity to a point.

Potential gradient

Electric field strength is the negative potential gradient.

Capacitance, C

Charge (on one plate) per unit potential difference across the plates.

Exponential decrease (graph)

A graph where the rate of decrease (gradient) is proportional to the quantity itself.

Time constant

The time taken for the charge on a capacitor to decrease to 37% of its original value. Equal to the product of R and C.

Magnetic field

A region of space where a moving charge experiences a magnetic force.

Magnetic field lines

Smooth curves that point from North to South pole outside a magnet.

Magnetic flux density, B

Force per unit length per unit current in a straight conductor placed at right angles to the field.

Magnetic flux linkage, NΦ

Product of magnetic flux and the number of turns.

Magnetic flux, Φ

Product of the magnetic flux density normal to a circuit and the cross-sectional area of the circuit.

Tesla

The unit of magnetic flux density; one newton per ampere per metre for a current-carrying wire at right angles to the field.

Right-hand grip rule

Direction of magnetic field (curl fingers) is perpendicular to current (thumb). B is proportional to current.

Fleming's Left-hand rule

When current/charge moves perpendicular to a magnetic field, the magnetic force is perpendicular to both the magnetic field (B) and the current/velocity of the charge.

Velocity selector

A setup where electric and magnetic fields are perpendicular so that particles of a specific speed pass through undeviated, as electric and magnetic forces are equal and opposite.

Hall voltage

p.d. between opposing sides of a conductor/semiconductor due to a current in a magnetic field, where electric and magnetic forces on moving charges are equal.

Semiconductor

A material with fewer free electrons per unit volume compared to conductors, resulting in a larger Hall voltage.

Faraday's Law

The induced e.m.f. is proportional to the rate of change of magnetic flux linkage.

Lenz's Law

The induced current is in a direction so as to produce effects which oppose the change producing it.

Eddy current

Induced currents in large conductors (e.g. metal plates and iron cores) that dissipate electrical energy as thermal energy.

Soft iron core

Easily magnetised and demagnetised material used to concentrate magnetic flux and increase flux linkage. Can be laminated to reduce eddy current losses.

r.m.s. current

Value of direct current that produces the same mean power or heating effect as the alternating current in a resistor.

Rectification

Conversion from alternating to direct current using diodes.

Rectification (Full wave)

Output produced by a bridge rectifier, giving a higher mean power.

Rectification (Half wave)

Output produced by diodes in a circuit to ensure current only flows in one direction.

Smoothing

Reduction in the variation of output voltage/current.

Photon

A quantised packet of electromagnetic energy (E = hf).

Photoelectric effect

Interaction between a photon and an electron in which the electron is removed from the atom.

Stopping potential

The potential difference required to bring a moving electron (with kinetic energy) to rest.

Threshold frequency

Minimum frequency required to release electrons from the surface of a metal.

Work function, Φ

Minimum amount of energy required by a surface electron to escape the metal.

de Broglie wavelength

Wavelength associated with a moving particle (λ = h/p).

Absorption line spectrum

A dark line of a unique wavelength seen in a continuous spectrum.

Emission line spectrum

A sharp and bright line of a unique wavelength seen in a spectrum.

Discrete Energy Levels

A change in electron energy level emits or absorbs a single photon, where the difference in energy levels equals the photon energy at its frequency. Discrete frequencies imply discrete energy gaps, and therefore discrete energy levels.

Isotopes

Same atomic number but different nucleon number. Same number of protons but different number of neutrons.

Nucleus

The tiny central region of an atom that contains most of the mass and all of its positive charge.

Nuclide

One type of nucleus characterised by a particular nucleon number and a particular proton number.

Binding energy

Minimum energy needed to separate the nucleons in a nucleus to infinity.

Einstein's Equation

The mass of a system increases when energy is supplied to it (E = mc²).

Mass defect

Difference between the total mass of the individual separate nucleons and the mass of the nucleus. The difference in mass is converted to energy.

Rest mass

The mass of a particle when it is stationary.

Nuclear fission

The splitting of a heavy nucleus into two large fragments and a small number of neutrons.

Nuclear fusion

A nuclear reaction where two light nuclei join together to form a heavier but more stable nucleus.

Decay constant, λ

Probability of a nucleus decaying per unit time interval.

Half-life

Mean time taken for half the number of active nuclei in a radioactive sample to decay.

Radioactive Decay

Spontaneous emission of ionising radiation by an unstable nucleus.

Random

Cannot predict when or which nucleus will decay next, though the probability of decay of any given nucleus is constant.

Spontaneous

Nuclear decay is not affected by any external/environmental factors such as temperature and pressure.

Attenuation

Exponential decrease of wave intensity/power/amplitude as it travels through a medium, due to energy absorption by the medium.

X-Ray Scan

X-rays pass through a structure and the transmitted waves are detected. Differences in transmitted intensities form an image.