Physics Paper 1 Definitions

Nucleon

A proton or neutron in the nucleus of an atom

Nucleon number/ Mass number

The number of protons and neutrons in the nucleus of an atom

Proton number/ Atomic number

The number of protons in the nucleus of an atom

Isotope

An atom of the same element (i.e. same number of protons) but with a different number of neutrons

Specific charge

The ratio: charge of particle / mass of particle

Alpha particle

Two protons and two neutrons. Alpha decay results in proton number decreasing by 2, and nucleon number decreasing by 4.

Beta (minus) particle

An electron. In beta (minus) decay, a fast moving electron and an antineutrino are emitted from the nucleus. Beta minus decay results in proton number increasing by 1 and nucleon number remaining the same.

Beta (plus) particle

A position (anti electron). In beta (plus) decay, a fast moving positron and a neutrino are emitted from the nucleus. Beta plus decay results in proton number decreasing by 1 and nucleon number remaining the same.

Gamma radiation

High frequency e-m radiation emitted by an unstable nucleus.

Photon

A packet of light energy. Photon energy = hf

Antimatter

Each particle of normal matter has a corresponding antiparticle with the same mass but opposite charge.

Annihilation

When a particle and its corresponding antiparticle meet and annihilate, with their mass converting to the energy of two emitted photons with the same frequency.

Pair production

The opposite of annihilation. The energy of a photon can be used to create a particle and its corresponding antiparticle.

An electron volt (eV)

Kinetic energy gained by an electron when it is accelerated by a potential difference of one volt.

The strong nuclear force

The nuclear force that holds nucleons together. Attractive at ranges of between 0.5 fm and 3-4 fm. Repulsive at ranges of less than 0.5 fm, which prevents nucleus from collapsing.

The weak nuclear force

The nuclear force responsible for Beta decay, Mediated by exchange of W bosons.

The electromagnetic force

The force acting between objects due to their charge. Mediated by exchange of virtual photons.

Hadrons

One of the two families of matter particles. Hadrons contain quarks and interact through the strong interaction.

Baryons

Baryons are hadrons that consist of three quarks/ anti quarks. All baryons eventually decay into protons.

Mesons

Mesons are hadrons consisting of a quark, anti quark pair. Includes pions and kaons.

Quarks

Fundamental particles consisting of three groups: up and down, strange and charm, top and bottom.

Leptons

The other of the two families of matter particles. Leptons do not interact through the strong interaction. Leptons consist of three groups - electron and electron neutrino, muon and muon neutrino, tao and tao neutrino.

Photon

A packet of light energy. Photon energy = hf = he / A

Photoelectric effect

When light above a threshold frequency is incident on a metal, electrons absorb photons (one to one relationship) and the electrons gain enough energy to be released from the metal.

Threshold frequency, f0

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

Work function

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

Photoelectric effect equation

hf= work function + K.E. max

Stopping potential

The minimum negative voltage required to stop electrons with the maximum kinetic energy. Equal to K.E. max/ e (= K.E. max in eV)

Ground state

The lowest energy state of an atom.

Excited state

When one or more of its electrons moves to a higher energy level, the atom is in an excited state.

Excitation

An electron jumps up to a higher energy level of an atom when it absorbs a photon of the right frequency.

Ionisation

When there is sufficient energy for an electron to become excited enough to leave the atom. Ionisation energy is (photon) energy required for an electron to be freed i.e. to reach n=infinity.

De-excitation

An electron goes down one or more energy levels after a certain amount of time and emits a photon (with the same energy as the difference between the energy levels).

de Broglie wavelength

Wavelike behaviour of a matter particle characterised by a wavelength, A = h/(mv)

Longitudinal wave

A wave with oscillations that are parallel to the direction of energy propagation. Sound waves are an example of a longitudinal wave.

Transverse wave

A wave with oscillations that are perpendicular to the direction of energy propagation.

Displacement

The displacement of a vibrating particle is its distance and direction from the equilibrium position.

Amplitude

The maximum displacement of a vibrating particle.

Wavelength

The distance between two adjacent positions with identical phase. It is commonly measured from peak to peak or trough to trough.

Time period

The time for one complete wave cycle to pass a fixed point.

Frequency

The number of waves that pass a point in a second. It is the reciprocal of the time period.

Wavespeed

Speed of the wave

Plane Polarisation

The restriction of a wave so that it can only oscillate only in a single plane. Only transverse waves can be plane polarised.

Phase

A measure of how far through the wave's cycle a given point on the wave is.

Wavefront

A line of constant phase

Phase difference

The difference in phase between two points on a wave. It is usually expressed in fractions of a cycle or radians.

Reflection

Waves bounce off a reflective surface. The angle of incident = the angle of reflection.

Refraction

When waves pass across a boundary between two media, the speed changes. The wavelength also changes, but the frequency remains constant. If the waves approach the boundary at an angle, the wave will change direction.

Diffraction

When waves spread out after passing through a gap or around an obstacle.

Superposition

When two or more waves meet, the displacement at a point is equal to the sum of the individual displacements of all the waves

Interference

When two waves of constant frequency and constant phase difference meet, cancellation and reinforcement occurs at fixed positions. This effect is known as interference.

Coherent waves

Waves with the same frequency and a constant phase difference (usually zero but not always). Result in an interference pattern when they overlap.

Stationary Wave

A wave that stores but does not transfer energy. The opposite of a progressive wave.

Node

A point of minimum/ zero displacement on a stationary wave because of destructive interference.

Antinode

The point of maximum vibration/ displacement from the equilibrium position of a stationary wave because of constructive interference.

Fundamental mode of vibration

The simplest mode of vibration of a stationary wave, with a node at each end, and one antinode in the middle. Also known as the first harmonic.

Overtone (1 more than harmonic)

Any frequency produced by a stationary wave which is greater than the fundamental frequency

Phase difference in stationary waves

Difference between the same point on two different waves; equal to m*pi where m is the number of nodes between the two points

Refraction

When waves pass across a boundary between two media, the speed changes. The wavelength also changes, but the frequency remains constant. If the waves approach the boundary at an angle, the wave will change direction.

Diffraction

When waves spread out after passing through a gap or around an obstacle.

Normal line

An imaginary line drawn at 90 degrees to the boundary

Angle of incidence

Angle of the incident ray measured to the normal

Angle of refraction

Angle of the refracted ray measured to the normal

Refractive index, n

A material property that is equal to the ratio between the speed of light in a vacuum, and the speed of light in a given material. Must be >= 1. For a vacuum, n=1. n of air is approximately 1.

Snell's law for refraction between two media

n1sinA = n2sinA

Critical angle

The angle of incidence at which the light ray refracts directly along the boundary i.e. angle of refraction = 90 degrees

Total Internal Reflection (TIR)

When the angle of incidence is larger than the critical angle, and the light is passing from a more to less dense medium, no light is refracted and the light ray is totally internally reflected.

Monochromatic light

Light of a single wavelength

Polychromatic light

The light has multiple wavelengths

Dispersion

Different wavelengths of light are refracted by different amounts, causing polychromatic light to spread out

Chromatic aberration

the blurring of an image due to dispersion

Vector

Any physical quantity that has both magnitude and direction e.g. force

Scalar

Any physical quantity that has only magnitude e.g. mass

Resultant force

The net force on a body, calculated by adding up the contribution from each force acting on the body

Resolving a force into perpendicular components

The process of breaking down a force into two components which act perpendicular to one another e.g. along x and y, or perpendicular to and parallel to an inclined plane.

Moment

Product of force and perpendicular distance from pivot to the line of action of the force

Centre of mass

Point through which weight acts on an object. Any force applied at the centre of mass will not have a turning effect.

Principle of moments

Sum of clockwise moments = sum of anticlockwise moments when a body is in equilibrium.

Couple

Pair of equal and opposite forces acting on a body, but not along the same line, to give the same direction of rotation

Equilibrium

Zero resultant force, and zero resultant moment

Displacement

Distance in a given direction

Speed

Change of distance per unit time.

Velocity

Change of displacement per unit time.

Average speed

Total distance travelled / total time taken

Acceleration

Change of velocity per unit time.

Acceleration due to gravity

9.81ms-2 or N/kg

Constant acceleration

Acceleration that is a constant value i.e. does not change with time. SUVAT can only be used when acceleration is constant, ignoring air resistance

Freefall

An object is in freefall if it is acted on only by the force of gravity i.e. its acceleration = 9.81ms-2 downwards.

Projectile

An object acted upon only by the force of gravity

Newton's First Law

An object will remain at a constant velocity or stationary, unless acted on by a resultant force. (An object requires a resultant force to be able to accelerate.)

Newton's Second Law

Resultant force acting on an object is equal to the rate of change of momentum of the object.

Newton's Third Law

If an object exerts a force on another object, then the other object must exert a force back, that is opposite in direction and equal in magnitude.

Weight

The force on an object due to its mass and the gravitational field strength.

Inertia

An object's resistance to a change in motion, as measured by its mass

Terminal velocity

The velocity reached when the resultant force on an object equals zero, because the weight and the drag force are equal and opposite.

Thinking distance

The distance travelled by a vehicle in the time it takes the driver to react

Braking distance

The distance travelled by a vehicle in the time it takes to stop from when the brakes are first applied

Stopping distance

The sum of the thinking distance and the braking distance.

Momentum

The product of an object's mass and velocity

Impulse

The change of momentum of an object when a force acts on it. It is equal to the product of the force acting on the object and the length of time over which it acts.