EDUQAS Physics A level definitions

Scalar

A scalar is a quantity that has magnitude only.

Vector

A vector is a quantity that has magnitude and direction.

Density of a material, ρ

density = mass/volume, unit: kg m-3 or g cm-3, in which mass and volume apply to any sample of the material.

Moment (or torque) of a force

The moment (or torque) of a force about a point is defined as the force x the perpendicular distance from the point to the line of action of the force, i.e. moment = F x d, unit: Nm

The principle of moments

For a system to be in equilibrium, Σ anticlockwise moments around a point = Σ clockwise moments about the same point.

Centre of gravity

The centre of gravity is the single point within a body at which the entire weight of the body may be considered to act.

Displacement

The displacement of a point B from a point A is the shortest distance from A to B, together with the direction. Unit: m

Mean speed

Mean speed = total distance travelled/total time taken = Δx/Δt. Unit: ms-1

Instantaneous speed

Instantaneous speed = rate of change of distance. Unit: ms-1

Mean velocity

Mean velocity = total displacement/total time taken. Unit: ms-1

Instantaneous velocity

The velocity of a body is the rate of change of displacement

Mean acceleration

Mean acceleration = change in velocity = Δv/Δt. Unit: ms-2

Terminal velocity

The terminal velocity is the constant maximum velocity of an object when the resistive forces on it are equal and opposite to the 'accelerating' force (e.g. pull of gravity).

Force, F

A force on a body is a push or a pull acting on the body from some external body. Unit: N

Newton's 3rd Law

If a body A exerts a force on a body B, then B exerts an equal and opposite force on A.

ΣF = ma

The mass of a body x its acceleration is equal to the vector sum of the forces acting on the body. This vector sum is called the resultant force.

Momentum

The momentum of an object is its mass multiplied by its velocity (p=mv). It is a vector. Unit: kgms-1 or Ns

Newton's 2nd Law

The rate of change of momentum of an object is proportional to the resultant force acting on it and takes place in the direction of that force.

The principle of conservation of momentum

The vector sum of the momenta of bodies in a system stays constant even if forces act between the bodies provided there is no external resultant force.

Elastic collision

A collision in which there is no change in total kinetic energy.

Inelastic collision

A collision in which kinetic energy is lost.

Work, W

Work done by a force is the product of the magnitude of the force and the distance moved in the direction of the force (WD = Fxcosθ)

Unit: J

Principle of conservation of energy

Energy cannot be created or destroyed, only transferred form one form to another. Energy is a scalar.

Potential energy, Ep

This is energy possessed by an object by virtue of its position. Ep = mgh

Unit: J

Kinetic energy, Ek

This is energy possessed by an object by virtue of its motion. Ek = 1/2mv2

Unit: J

Elastic potential energy

This is the energy possessed by an object when it has been deformed due to forces acting on it. Eelastic = 1/2kx2

Unit: J

Energy

The energy of a body or system is the amount of work it can do.

Unit: J

Power, P

This is the work done per second, or energy transferred per second.

Unit: W

Period, T for a point describing a circle

Time taken for one complete circuit

Frequency, f

The number of circuits or cycles per second.

Radian

A unit of measurement of angles equal to about 57.3deg, equivalent to the angle subtended at the centre of a circle by an arc equal in length to the radius.

Unit: rad

Angular velocity, ω

For an object describing a circle at uniform speed, the angular velocity ω is equal to the angle θ swept out by the radius in time Δt divided by t (ω = Δθ/Δt).

Unit: rads-1

Simple harmonic motion (shm)

Shm occurs when an object moves such that its acceleration is always directed toward a fixed point and is proportional to its distance from the fixed point.

(a = -ω2x)

Period, T for an oscillating body

The time taken for one complete cycle

Amplitude, A of an oscillating object

The maximum value of the object's displacement (from its equilibrium position).

Phase

The phase of an oscillation is the angle (ωt + ε) in the equation x = Acos(ωt + ε).

Unit: rad

Frequency, f

The number of oscillations per second.

Unit: Hz

Free oscillations

Free oscillations occur when an oscillatory system (such as a mass on a spring, or a pendulum) is displaced and released. The frequency of the free oscillations is called the system's natural frequency.

Damping

Damping is the dying away, due to resistive forces of the amplitude of free oscillations.

Critical damping

Critical damping is the case when the resistive forces on the system are just large enough to prevent oscillations occurring at all when the system is displaced and released.

Forced oscillations

These occur when a sinusoidally varying 'driving' force is applied to an oscillatory system, causing it to oscillate with the frequency of the applied force.

Resonance

If, in forced vibrations, the frequency of the applied force is equal to the natural frequency of the system (e.g. mass on spring), the amplitude of the resulting oscillations is large. This is resonance.

Ideal gas

An ideal gas strictly obeys the equation of state pV = nRT, in which n is the number of moles, T is the kelvin temperature and R is the molar gas constant (8.31 J mol-1 K-1). With the exception of very high densities a real gas approximates well to an ideal gas.

The mole

The mole is the S.I. unit of an 'amount of substance'. It is the amount containing as many particles (e.g. molecules) as there are atoms in 12g of carbon-12

Avogadro constant, Na

This is the number of particles per mole (6.02x10^23 mol-1).

Internal energy, U, of a system

This is the sum of the kinetic and potential energies of the particles of a system.

Heat, Q

This is the energy flow from a region at higher temperature to a region at lower temperature, due to the temperature difference. In thermodynamics we deal with heat going into or out of a system. It makes no sense to speak of heat in a system.

Work, W

If the system is a gas, in a cylinder fitted with a piston, the gas does work of amount pΔV when it exerts a pressure p and pushes the piston out a small way, so the gas volume increases by ΔV. Work, like heat, is energy in transit from (or to) the system.

First law of thermodynamics

The increase, ΔU, in internal energy of a system is ΔU = Q - W in which Q is the heat entering the system and W is the work done by the system. Any of the terms in the equation can be positive or negative, e.g. if 100J of heat is lost from a system Q = -100J

Specific heat capacity, c

The heat required, per kilogram, per degree celsius or kelvin, to raise the temperature of a substance.

Unit: J kg-1 K-1

Electric current, I

This is the rate of flow or electric charge. I = ΔQ/Δt.

Unit: A

Efficiency of a system

% Efficiency = 100x (useful work out/work in)

Potential difference (pd), V

The pd between two points is the energy converted from electrical potential energy to some other form per coulomb of charge flowing from one point to the other.

Unit: V (=JC-1)

Ohm's law

The current in a metal wire at constant temperature is proportional to the pd across it.

Electrical resistance, R

The resistance of a conductor is the pd (V) placed across it divided by the resulting current (I) through it. R = V/I.

Unit: Ω

Resistivity, ρ

The resistance, R, of a metal wire of length L and cross-sectional area A is given by R = ρL/A, in which ρ the resistivity, is a constant (at constant temperature) for the material of the wire.

Unit: Ωm

Superconducting transition temperature, Tc

The temperature at which a material, when cooled, loses all its electrical resistance, and becomes super-conducting. Some materials (e.g. copper) never become superconducting however low the temperature becomes.

The law of conservation of charge

Electric charge cannot be created or destroyed, (though positive and negative charges can neutralise each other). Charge cannot pile up at a point in a circuit.

Emf, E

The emf of a source is the energy converted from some other form (e.g. chemical) to electrical potential energy per coulomb of charge flowing through the source.

Unit: V

Capacitor

A capacitor is a pair of conducting plates separated by an insulator. If a potential difference is placed across the plates, they acquire equal and opposite charges.

Capacitance, C, of a capacitor

capacitance = charge on either plate/pd between plates

Unit: F (=CV-1)

Dielectic

Insulator between the plates of a capacitor, also serving to make the capacitance larger than if there were just empty space.

Hooke's law

The tension in a spring or wire is proportional to its extension from its natural length, provided the extension is not too great.

Spring constant, k

The spring constant is the force per unit extension.

Unit: Nm-1

Stress, σ

Stress is the force per unit cross-sectional area when equal opposing forces act on a body.

Unit: Pa or Nm-2

Strain, ε

Strain is defined as the extension per unit length due to an applied stress.

Unit: none

Young modulus, E

Young modulus E = tensile stress/tensile strain

Unless otherwise indicated this is defined for the Hooke's law region.

Unit: Pa or Nm-2

Crystal

Solid in which atoms are arranged in a regular array. There is a long range order within crystal structures.

Crystalline solid

Solid consisting of a crystal, or of many crystals, usually arranged randomly. The latter is strictly a polycrystalline solid. Metals are polycrystalline.

Amorphous solid

A truly amorphous solid would have atoms arranged quite randomly. Examples are rare. In practice we include solids such as glass or brick in which there is no long range order int he way atoms are arranged, though there may be ordered clusters of atoms.

Polymeric solid

A solid which is made up of chain-like molecules.

Ductile material

A material which can be drawn out into a wire. This implies that plastic strain occurs under enough stress.

Elastic strain

This is the strain that disappears when the stress is removed, that is the specimen returns to its original size and shape.

Plastic (or inelastic) strain

This is strain that decreases only slightly when the stress is removed. In a metal it arises from the movement of dislocations within the crystal structure.

Elastic limit

This the point at which deformation ceases to be elastic. For a specimen it is usually measured by the maximum force, and for a material, by the maximum stress, before the strain ceases to be elastic.

Dislocations in crystals

Certain faults in crystals which (if there are not too many) reduce the stress needed for planes of atoms to slide. The easiest dislocation to picture is an edge dislocation: the edge of an intrusive, incomplete plane of atoms.

Grain boundaries

The boundaries between crystals (grains) in a polycrystalline material.

Ductile fracture (necking)

The characteristic fracture process in a ductile material. The fracture of a rod or wire is preceded by local thinning which increases the stress.

Brittle material

Material with no region of plastic flow, which, under tension, fails by brittle fracture.

Brittle fracture

This is the fracture under tension of brittle materials by means of crack propagation.

Elastic hysteresis

When a material such as rubber is put under stress and the stress is then relaxed, the stress-strain graphs for increasing and decreasing stress do not coincide, but form a loop. This is hysteresis.

Newton's law of gravitation

The gravitational force between two particles is proportional to the product of their masses, m1 and m2, and inversely proportional to their separation squared, r^2.

F = Gm1m2/r^2 in which G is the gravitational constant (6.67 x 10^-11 N m2 kg-2).

Coulomb's law

The electrostatic force, F, between two small bodies is proportional to the product of their charges, Q1 and Q2, and inversely proportional to their separation squared, r^2.

F = Q1Q1/4πε0r^2 in which ε0 is the permittivity of free space (8.85x10^-12 Fm-1)

Electric field strength, E

The force experienced per unit charge by a small positive charge placed in the field. Unit: Vm-1 or NC-1

Gravitational field strength, g

The force experienced per unit mass by a mass placed in the field.

Unit: ms-2 or Nkg-1

Electric potential, Ve

Electric potential at a point is the work done per unit charge in bringing a positive charge from infinity to that point.

Unit: V or JC-1

Gravitational potential, Vg

Gravitational potential at a point is the work done per unit mass in bringing a mass from infinity to that point.

Unit: Jkg-1

Black body

A black body is a body (or surface) which absorbs all the electromagnetic radiation that falls upon it. No body is a better emitter of radiation at any wavelength than a black body at the same temperature.

Wien's displacement law

The wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body.

λmax = W/T where W = the Wien constant = 2.90x10^-3 m K

Absolute or kelvin temperature

The temperature, T, in kelvin (K) is related to the temperature, θ, in celsius by: T(K) = θ(degC) + 273.15.

At 0K the energy of partcles in a body is the lowest it can possibly be.

Stefan's law (the Stefan-Boltzmann law)

The total electromagnetic radiation energy emitted per unit time by a black body is given by a power = AσT^4 in which A is the body's surface area and σ is a constant called the Stefan constant ( 5.67x10^-8 W m-2 K-4).

Luminosity of a star

The luminosity of a star is the total energy it emits per unit time in the form of electromagnetic radiation.

Unit: W

Intensity

The intensity of radiation at a distance R from a source is given by I = P/4πR^2.

Unit: Wm-2

Kepler's laws of planetary motion: 1

Each planet moves in an ellipse with the Sun at one focus.

Kepler's laws of planetary motion: 2

The line joining a planet to the centre of the Sun sweeps out equal areas in equal times.

Kepler's laws of planetary motion: 3

T^2, the square of the period of the planet's motion, is proportional to r^3, in which r is the semi-major axis of its ellipse.

Dark matter

Matter which we can't see, or detect by any sort of radiation, but whose existence we infer from its gravitational effects.

Radial velocity of a star (in the context of Doppler shift)

This is the component of a star's velocity along the line joining it and an observer on the Earth.

Galactic radial velocity

This is the mean component of a galaxy's velocity along the line joining it and an observer on Earth.

Progressive wave

A pattern of disturbances travelling through a medium and carrying energy with it, involving the particles of the medium oscillating about their equilibrium positions.