Physics definitions

Specific charge

the charge in coulombs divided by the mass in kilograms or charge to mass ratio.

Isotope

an atom with a nucleus with the same number of protons but different number of neutrons.

Photoelectric effect

the emission of electrons from metal surfaces by incident light of an appropriate frequency.

Work function

the minimum energy required for an electron to escape from the surface of the metal.

Threshold frequency

the minimum frequency of a photon to produce photoelectrons.

Electron volt

the energy given to an electron when it passes through a potential difference of 1 V.

Ionisation energy of an atom

the minimum energy required to remove an electron from an atom in its ground state.

Excitation energy

the energy required to move an electron from a lower energy level to a higher energy level.

Line spectra

the characteristic wavelengths of light produced by individual excited atoms.

Electric current

the number of coulombs of charge passing a point every second.

Potential difference

the work done per unit charge in moving charges from one point in the circuit to another.

Resistance

the ratio of the potential difference across a component to the current through it.

Ohmic conductor or resistor

the ratio of potential difference to current remains constant.

Ohm’s law

the current through component is proportional to the potential difference across it.

Critical temperature

the temperature at or below which the resistivity of a superconductor becomes zero.

Kirchhoff’s first law

the sum of the currents into the junction is zero.

Kirchhoff’s second law

in any closed loop the sum of the EMF equals the sum of the potential difference.

EMF

the total energy supplied per coulomb given to charges as they pass through the battery or cell (before losing energy to internal resistance).

Internal resistance

the resistance inside a battery, cell or power supply.

Useful volts

the potential difference across the terminals of the power supply (terminal p.d.).

Lost volts

the potential difference across the internal resistance of the power supply. This must be found by calculation.

Root mean square

the square root of the mean of all the squares of the values.

Time base

the control on an oscilloscope which changes the time it takes for the beam to cross the screen horizontally.

Y

Gain

Scalar

a physical quantity which has magnitude only.

Vector

a physical quantity which has magnitude and direction.

Equilibrium

an object is in equilibrium when the resultant force on it is zero and the resultant torque is zero.

Couple

two equal and opposite forces separated by a distance D between the lines of action of the forces (this produces rotational motion only).

Moment/torque

the moment/torque/turning force about a point is the force multiplied by the perpendicular distance from the point to the line of action of the force.

Principle of moments

for an object in equilibrium the sum of the clockwise moments equals the sum of the anticlockwise moments. This applies about any point which is on or outside the object.

Centre of mass

the point at which the whole mass of the object appears to act.

Displacement

the distance an object has moved in a particular direction.

Speed

distance travelled divided by the time taken.

Velocity

the displacement of the object divided by the time taken.

Acceleration

the change in velocity divided by the time taken to change.

Parabolic path

the shape of the path of a particle moving with a component of motion at right angles to a constant resultant force (constant acceleration).

Newton's first law

an object remains at rest or moves with a constant velocity providing no resultant external force is applied.

Newton's second law

for a constant mass the acceleration of the object is proportional to the resultant force applied. A2 definition: Force is equal to rate of change of momentum.

Newton's third law

if object A applies a force F on object B, object B applies a force of F in the opposite direction on object A.

Work done

the force multiplied by the distance moved in the direction of the force.

Power

power equals work done divided by time.

Principle of conservation of energy

energy cannot be created or destroyed, it can only be changed from one form to another.

Hooke's law

the extension of a spring or material is proportional to the tensile force applied (this applies up to the limit of proportionality of the material).

Elastic limit

the point beyond which a material or spring stretched will not return to its original length.

Tensile stress

tensile force divided by cross

Tensile strain

extension divided by original length.

Plastic behaviour

a material undergoing plastic behaviour will not return to its original shape after deforming forces are removed.

Fracture

this is when the material breaks. Materials are compared by their ultimate tensile stress, this is the tensile stress needed to break the material (strength of material).

Brittleness

a brittle material will not undergo plastic deformation before fracture.

Young's modulus

tensile stress divided by tensile strain.

Amplitude

the maximum displacement from the equilibrium position.

Frequency

the number of oscillations per second.

Wavelength

the distance between two consecutive particles in a wave which are in phase.

Wave speed

the speed of energy transfer through the medium.

Path difference

the extra distance one of the waves coming from two sources has to travel to reach a point.

Electromagnetic wave

a transverse wave, consisting of a changing magnetic field at right angles to a changing electric field.

Polarised waves

electromagnetic waves with an electric field vector in one plane only.

Unpolarised waves

electromagnetic waves with an electric field vector in many directions.

Refractive index

the absolute refractive index of the substance is the speed of light in a vacuum divided by the speed of light in the substance.

Critical angle

the angle of incidence on a boundary between two media when the angle of refraction is 90°.

Total internal reflection

total internal reflection occurs when the angle of incidence is greater than the critical angle and all the light is reflected within the substance.

Step index fibre

there is a sudden change in refractive index between the core and the cladding.

Stationary wave

a stationary wave is formed by two waves of the same frequency and similar amplitude travelling in opposite directions.

Node

a point on a stationary wave with zero amplitude.

Antinode

a point on a stationary wave with maximum amplitude.

Fundamental frequency

the lowest frequency of a wave which will produce a stationary wave.

Coherence

coherent waves have the same frequency and a constant phase relationship.

Fringe spacing

the distance between two consecutive maxima or minima in an interference pattern.

Impulse

force multiplied by time.

Principle of conservation of momentum

momentum in a collision is always conserved providing no resultant external forces are acting.

Elastic collision

kinetic energy is conserved.

Inelastic collision

kinetic energy is not conserved.

Explosion (explosive collision)

kinetic energy is increased due to transfer of energy from the explosion.

Centripetal force

this is the resultant force applied to an object to keep it moving in a circle. It is directed towards the centre of the circle and at right angles to the direction of motion of the object.

Angular speed

this is the number of radians per second swept by a rotating object.

Simple harmonic motion

the condition for SHM is the acceleration is proportional, and in the opposite direction, to the displacement from the equilibrium position. The period of oscillation is independent of the amplitude.

Energy in SHM

the energy of the oscillator is proportional to the amplitude squared. For an undamped oscillator the total energy remains constant.

Free vibrations

the oscillator vibrates at its natural frequency.

Forced vibrations

the oscillator vibrates at the frequency of the driver.

Resonance

the frequency of oscillation of the driver equals the natural frequency of the driven and maximum energy transfer occurs.

Phase relationship in resonance

the driver leads the driven by a phase angle of π/2 radians or a time of T/4 seconds.

Newton’s law of gravitation

the gravitational force between two objects is proportional to the product of their masses, and the inverse square of the separation of their centres of mass. The constant of proportionality is the universal gravitational constant (G). The force is always attractive.

Field

an area in space that explains why one object will apply a force on another object without touching.

Gravitational field strength (g)

the force per unit mass placed at that point in the field.

Electric field strength (E)

the force per unit positive charge placed at that point in the field.

Gravitational potential at a point

the work done per unit mass in bringing a small test mass from infinity (zero potential) to that point in the field (all potentials are negative as the force is attractive).

Electric potential at a point

the work done per unit positive charge in bringing a small test charge from infinity (or Earth) to that point in the field.

Inverse square law

this applies to the strength of radial fields, if the distance from the centre of the field is doubled the strength of the field is reduced by a factor of four.

Parabolic path

the path taken by a particle moving with a component of its motion at right angles to a constant force applied in the same direction, e.g. a charged particle in a uniform electric field.

Circular path

the path taken by a particle when force of constant magnitude is applied at right angles to the motion of the particle, e.g. the path of a charged particle in a uniform magnetic field or a satellite in a radial gravitational field.

Capacitance

the charge stored per volt.

Time constant

the resistance times the capacitance measured in seconds.

Magnetic flux density (B)

this is a measure of the magnetic field strength, units Tesla.

Tesla

1 Tesla is 1 Weber per metre squared.

Magnetic flux (Φ)

the component of the magnetic flux density at right angles to a surface multiplied by the area of the surface through which it passes. Units Weber (Wb).

Magnetic flux linkage (NΦ)

this is the amount of magnetic flux that passes through a coil or complete loop of wire multiplied by the number of turns of wire.

Fleming’s left

hand rule

Cyclotron

a device which uses an alternating electric field and a uniform magnetic field to accelerate charged particles.

Faraday’s law

the size of the induced EMF in a conductor is proportional to the rate of change of flux linkage.

Lenz’s law

the direction of induced current flows to oppose the change that produces it (conservation of energy).