A Level Physics Definitions

Front 1

Absolute scale

Back 1

Temperature scale in kelvins (K), defined in terms of absolute zero, 0K, and the triple point of water, 273.16K, which is the temperature at which ice, water and water vapour are thermal equilibrium.

Front 2

Absolute zero

Back 2

The lowest possible temperature, the temperature at which an object has minimum internal energy.

Front 3

Activity, A

Back 3

Of a radioactive isotope, the number of nuclei of the isotope that disintegrate per second. The unit of activity is the becquerel (Bq), equal to one disintegration per second.

Front 4

Alpha (a) decay

Back 4

Change in unstable nucleus when it emits an alpha particle consisting of two protons and two neutrons.

Front 5

Alpha radiation

Back 5

Particles that are each composed of two protons and two neutrons. An alpha particle is emitted by a heavy unstable nucleus which is then less unstable as a result. Alpha radiation is easily absorbed by paper, has a range in air of no more than a few cm and is more ionising than beta or gamma radiation.

Front 6

Angular displacement

Back 6

The angle an object in circular motion turns through. Time period T and frequency f. Angular displacement in time t, in radians = 2(pi)ft = 2(pi)t/T

Front 7

Angular speed

Back 7

The rate of change of angular displacement of an object in circular (or orbital or spinning) motion.

Front 8

Angular frequency

Back 8

For an object oscillating at frequency f in SHM, it’s angular frequency = 2(pi)f

Front 9

Atomic mass unit, u

Back 9

The unified atomic mass constant, 1/12th of the mass of an atom of carbon isotope 12 6 C, equal to 1.661x10^-27kg

Front 10

Back emf

Back 10

Emf induced in the spinning coil of an electric motor or in any coil in which the current is changing (the primary coil is a transformer). A back emf acts against the applied pd.

Front 11

Background radiation

Back 11

Radiation due to naturally occurring radioactive substances in the environment. Background radiation is also caused by cosmic radiation.

Front 12

Beta - decay

Back 12

Change in a nucleus when a neutron changes into a proton and a Beta- particle and an anti neutrino are emitted if the nucleus is neutron-rich or a proton changes to a neutron and a Beta+ particle and a neutrino are emitted if the nucleus is proton rich.

Front 13

Beta - radiation

Back 13

Electrons emitted by unstable neutron-rich nuclei. Beta minus radiation is easily absorbed by paper, has a range of no more than a few centimetres in air and is less ionising than alpha radiation and more ionising than gamma radiation.

Front 14

Beta + radiation

Back 14

Positrons emitted by unstable proton- rich nuclei. Positrons emitted in solids or liquids travel no further than about 2mm before they are annihilated.

Front 15

Binding energy of a nucleus

Back 15

The work done to separate a nucleus into its constituent neutron and protons. Binding energy = mass defect x c^2. Binding energy in MeV = mass defect in u x 931.3.

Front 16

Binding energy per nucleon

Back 16

The average work done per nucleon to separate a nucleus into its constituent parts. The binding energy per nucleon of a nucleus = the binding energy of a nucleus/ mass number A. The binding energy per nucleon is greatest for iron nuclei of mass number about 56. The binding energy curve is a graph of binding energy per nucleon against mass number A.

Front 17

Boyle’s law

Back 17

For a fixed mass of gas at constant temperature, its pressure x its volume is constant. A gas that obeys Boyle’s law is said to be an ideal gas.

Front 18

Brownian motion

Back 18

The random and unpredictable motion of a particle such as a smoke particle caused by molecules of the surrounding substance colliding at random with the particle.

Front 19

Capacitance

Back 19

The charge stored per unit pd of a capacitor. The unit of capacitance is the farad (F), equal to one coulomb per volt. For a capacitor of capacitance C at pd V, the charge stored, Q = CV.

Front 20

Capacitor energy

Back 20

Energy stored by the capacitor, E = 1/2QV = 1/2CV^2 = 1/2Q^2/C

Front 21

de Broglie wavelength

Back 21

A particle of matter has a wave-like nature which means that it can behave as a wave. For example, electrons directed at a thin crystal are diffracted by the crystal. The de Broglie wavelength of a matter particle depends on momentum, p, in accordance with de Broglie’s equation = h/p = h/mv.

Front 22

Capacitor discharge

Back 22

Through a fixed resistor of resistance R; time constant = RC; exponential decrease equation for current or charge or pd; x = x0e^-t/RC

Front 23

Centripetal acceleration

Back 23

1) For an object moving at speed v (or angular speed w) in uniform circular motion, it’s centripetal acceleration a = v^2/r = w^2/r towards the centre of the circle.

2) For a satellite in circular orbit, its centripetal acceleration v^2/r = g

Front 24

Centripetal force

Back 24

The resultant force on an object that moves along a circular path. For an object of mass m moving at speed v along a circular path of radius r, the centripetal force = mv^2/r towards the centre of the circle.

Front 25

Charles’ law

Back 25

For an ideal gas at constant pressure, its volume is directly proportional to its absolute temperature.

Front 26

Control rods

Back 26

Rods made of a neutron-absorbing substance such as cadmium or boron that are moved in or out of the core of a nuclear reactor to control the rate of fission events in the reactor.

Front 27

Coulomb’s law of force

Back 27

For two point charges Q1 and Q2 at distance apart r, the force F between the two charges is given by the equation F = Q1Q2/4(pi)EpsilonOr^2

Front 28

Count rate

Back 28

The number of counts per unit time detected by a Geiger Müller tube. Count rates should always be corrected by measuring and subtracting the background count rate.

Front 29

Critical mass

Back 29

The minimum mass of the fissile isotope (e.g. the uranium isotope 235/92 U) in a nuclear reactor necessary to produce a chain reaction. If the mass of the fissile isotope in the reactor is less than the critical mass, a chain reaction does not occur because too many fission neutrons escape from the reactor or absorbed without fission.

Front 30

Damped oscillations

Back 30

Oscillations that reduce in amplitude due to the presence of resistive forces such as friction and drag.

1) For lightly damped system, the amplitude of oscillations decreases gradually.

2) For a heavily damped system displaced from equilibrium then released, the system slowly returns to equilibrium without oscillating.

3) For a critically damped system, the system returns to equilibrium in the least possible time without oscillating.

Front 31

Decay constant

Back 31

The probability of an individual nucleus decaying per second.

Front 32

Decay curve

Back 32

An exponential decrease curve showing how the mass or activity of a radioactive isotope decreases with time.

Front 33

Diffraction

Back 33

The spreading of waves when they pass through a gap or round obstacle. X-ray diffraction is used to determine the structure of crystals, metals and long molecules. Electron diffraction is used to probe the structure of materials. High-energy electron scattering is used to determine the diameter of the nucleus.

Front 34

Eddy currents

Back 34

Unwanted induced currents in the metal parts of ac machines.

Front 35

Electric field strength, E

Back 35

At a point in an electric field, is the force per unit charge on a small positively charged object at that point in the field.

Front 36

Electric potential, V

Back 36

At a point in an electric field is the work done per unit charge on a small positively charged object to move it from infinity to that point in the field.

Front 37

Electromagnetic induction

Back 37

The generation of emf when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines.

Front 38

Electron

Back 38

A lepton of rest mass 9.11x10^-31kg and an electric charge of -1.60x10^-19C.

Front 39

Electron capture

Back 39

A proton-rich nucleus captures an inner-shell electron to cause a proton in the nucleus to change into a neutron. An electron-neutrino is emitted by the nucleus. An X-Ray photon is subsequently emitted by the atom when the inner shell vacancy is filled.

Front 40

Equipotential

Back 40

A line of surface in a field which the electric or gravitational potential is constant.

Front 41

Excited state

Back 41

An atom which is not in its ground state (lowest energy state)

Front 42

Exponential change

Back 42

Exponential change happens when the change of a quantity is proportional to the quantity itself. For an exponential decrease of a quantity x, dx/dt = -lambda(x), where lambda is referred to as the decay constant. The solution of this equation is x = x0e^-t/RC where x0 is the initial value.

Front 43

Faradays law (of electromagnetic induction)

Back 43

The induced emf in a circuit is equal to the rate of change of magnetic flux linkage through the circuit. For a changing magnetic field in a fixed coil of area A and N turns, the induced emf = -NA🔼B/🔼t.

Front 44

Fission

Back 44

The splitting of a 235/92 U nucleus or a 235/24 Pu nucleus into two approximately equal fragments. Induced fission is fission caused by an incoming neutron colliding with a 235/92 U nucleus or a 235/94 Pu nucleus.

Front 45

Fission

Back 45

The splitting of a 235/92 U nucleus or a 235/24 Pu nucleus into two approximately equal fragments. Induced fission is fission caused by an incoming neutron colliding with a 235/92 U nucleus or a 235/94 Pu nucleus.

Front 46

Fission neutrons

Back 46

Neutrons released when a nucleus undergoes fission and which may collide with nuclei to cause further fission.

Front 47

Fission

Back 47

The splitting of a 235/92 U nucleus or a 235/24 Pu nucleus into two approximately equal fragments. Induced fission is fission caused by an incoming neutron colliding with a 235/92 U nucleus or a 235/94 Pu nucleus.

Front 48

Fission neutrons

Back 48

Neutrons released when a nucleus undergoes fission and which may collide with nuclei to cause further fission.

Front 49

Fleming’s left hand rule

Back 49

Rule that relates the directions of the force, magnetic field and current on a current-carrying conductor in a magnetic field.

Front 50

Fleming’s right-hand rule

Back 50

Rule that relates the directions of the induced current, magnetic field and velocity of the conductor when the conductor cuts across magnetic field lines and an emf is induced in it.

Front 51

Force

Back 51

= rate of change of momentum

=change of momentum/ time taken

(= mass x acceleration for fixed mass)

Front 52

Free electrons

Back 52

Electrons in a conductor that move about freely inside the metal because they are not attached to a particular atom.

Front 53

Free oscillations

Back 53

Oscillations where no damping and no periodic force acting on the system so the amplitude of the oscillations is constant.

Front 54

Forced oscillations

Back 54

Oscillations of a system that is subjected to an external periodic force.

Front 55

Fusion (nuclear)

Back 55

The fusing together of light nuclei to form a heavier nucleus.

Front 56

Fusion (nuclear)

Back 56

The fusing together of light nuclei to form a heavier nucleus.

Front 57

Fusion (thermal)

Back 57

The fusing together of metals by melting them together.

Front 58

Fusion (nuclear)

Back 58

The fusing together of light nuclei to form a heavier nucleus.

Front 59

Fusion (thermal)

Back 59

The fusing together of metals by melting them together.

Front 60

Gamma radiation

Back 60

Electromagnetic radiation emitted by an unstable nucleus when it becomes more stable.

Front 61

Fusion (nuclear)

Back 61

The fusing together of light nuclei to form a heavier nucleus.

Front 62

Fusion (thermal)

Back 62

The fusing together of metals by melting them together.

Front 63

Gamma radiation

Back 63

Electromagnetic radiation emitted by an unstable nucleus when it becomes more stable.

Front 64

Geostationary satellite

Back 64

A satellite that stays above the same point in the Earth’s equator as it orbits the Earth because it’s print is in the same plane as the equator, its period is exactly 24h and it orbits in the same direction as the Earth’s direction of rotation.

Front 65

Gold leaf electroscope

Back 65

A device used to detect electric charge

Front 66

Gold leaf electroscope

Back 66

A device used to detect electric charge

Front 67

Gravitational field strength, g

Back 67

The force per unit mass on a small mass placed in the field.

1) g=F/m, where F is the gravitational force on a small mass m.

2) At distance r from a point mass M, g=GM/r^2

3) At or beyond the surface of a sphere of mass M, g=GM/r^2 where r is the distance to the centre.

4) at the surface of a sphere of mass M and radius R, Gs=GM/R^2

Front 68

Gravitational potential V

Back 68

At a point in a gravitational field is the work done per unit mass to move a small object of mass M, V=-GM/r

Front 69

Gravitational potential energy

Back 69

At a point in a gravitational field is the work done to move a small object from infinity to that point. The change of gravitational potential energy of mass m moved through height h near the Earth’s surface, 🔼Ep=mg🔼h

Front 70

Half life, T(1/2)

Back 70

The time taken for the mass of a radioactive isotope to decrease to half the initial mass for its activity to halve. This is the same as the time taken for the number of nuclei of the isotope to decrease to half the initial number.

Front 71

Hall probe

Back 71

A device used to measure magnetic flux density.

Front 72

Heat capacity

Back 72

The energy needed to change the temperature of an object by 1K

Front 73

Ideal gas

Back 73

A gas under conditions such that it obeys Boyle’s law.

Front 74

Ideal gas equation

Back 74

pV=nRT. Where p is pressure, v is the volume, n is the number of moles of gas, T is the absolute temperature and R is the molar gas constant.

Front 75

Impulse

Back 75

Of a force acting on an object, force x time for which the force acts.

Front 76

Inverse square laws

Back 76

1) Force: Newton’s law of gravitation and coulomb’s law of force between electric charges are inverse square laws because he force between two point objects is inversely proportional to the square of the distance between the two objects.

2) Intensity; the intensity of gamma radiation from a point source varies with the inverse of the square of the distance from the source. The same rule applies to the radiation from any point source that spreads out equally in all directions and is not absorbed.

Front 77

Kepler’s third law

Back 77

For any planet, the cube of its mean radius of orbit r is directly proportional to the square of its time period T.

r^3/T^2 = GM/4(pi)^2

Front 78

Latent heat of fusion

Back 78

The energy needed to change a the state of a liquid without change of temperature.

Front 79

Lenz’s law

Back 79

When a current is induced by EM induction, the direction of the induced current is always such as to oppose the change that causes the current.

Front 80

Magnetic flux

Back 80

= BA for a uniform magnetic field of flux density B that is perpendicular to an area A.

Front 81

Magnetic flux density

Back 81

The magnetic force per unit length per unit current on a current carrying conductor at right angles to the field lines. The unit of magnetic flux density is the Tesla (T). B is sometimes referred to as magnetic field strength.

Front 82

Magnetic flux linkage

Back 82

= NBA where B is the magnetic flux density perpendicular to area A. The unit of magnetic flux me of flux linkage is the Weber (Wb), equal to 1Tm^-1 or 1 V s.

Front 83

Kepler’s third law

Back 83

For any planet, the cube of its mean radius of orbit r is directly proportional to the square of its time period T.

r^3/T^2 = GM/4(pi)^2

Front 84

Natural frequency

Back 84

The frequency of free oscillations of an oscillating system.

Front 85

Newton’s law of gravitation

Back 85

The gravitational force F between two point masses m1 and m2 at distance r is given by F = Gm1m2/r^2

Front 86

Newton’s first law

Back 86

An object continues at rest or in uniform motion unless it is acted on by a result force.

Front 87

Newton’s second law

Back 87

The rate of change of momentum of an object is proportional to the resultant force on it.

Front 88

Newton’s third law

Back 88

When two objects interact, they exert equal and opposite forces on one another.

Front 89

Pair production

Back 89

When a gamma photon changes into a particle and an antiparticle.

Front 90

Potential gradient

Back 90

at a point in a field is the change of potential per unit change of distance along the field line at that point. The potential gradient = - the field strength at any point.

Front 91

Latent heat of fusion

Back 91

The energy needed to change a the state of a liquid without change of temperature.

Front 92

Lenz’s law

Back 92

When a current is induced by EM induction, the direction of the induced current is always such as to oppose the change that causes the current.

Front 93

Magnetic flux

Back 93

= BA for a uniform magnetic field of flux density B that is perpendicular to an area A.

Front 94

Magnetic flux density

Back 94

The magnetic force per unit length per unit current on a current carrying conductor at right angles to the field lines. The unit of magnetic flux density is the Tesla (T). B is sometimes referred to as magnetic field strength.

Front 95

Magnetic flux linkage

Back 95

= NBA where B is the magnetic flux density perpendicular to area A. The unit of magnetic flux me of flux linkage is the Weber (Wb), equal to 1Tm^-1 or 1 V s.

Front 96

Equations for magnetic force

Back 96

1) F = BIlsin(x) - force on a current carrying wire of length l in a uniform magnetic field B at angle x.

2) F = BQvsin(x) - gives the force F on a particle of charge Q moving through a uniform magnetic field B at speed v in a direction at angle x.

3) BQv = mv^2/r - gives the radius of the orbit between the mass of the separated nucleons and the nucleus.

Front 97

Mass defect

Back 97

Mass defect of a nucleus is the difference between the mass of the separated nucleons and the nucleus.

Front 98

Metastable state

Back 98

An excited state of the nuclei of an isotope that lasts long enough after alpha or beta emission for the isotope to be separated from the parent isotope.

Front 99

Moderator

Back 99

Substance in a thermal nuclear reactor that slows down the fission neutrons so they can go on to produce further fission.

Front 100

Kepler’s third law

Back 100

For any planet, the cube of its mean radius of orbit r is directly proportional to the square of its time period T.

r^3/T^2 = GM/4(pi)^2

Front 101

Natural frequency

Back 101

The frequency of free oscillations of an oscillating system.

Front 102

Newton’s law of gravitation

Back 102

The gravitational force F between two point masses m1 and m2 at distance r is given by F = Gm1m2/r^2

Front 103

Newton’s first law

Back 103

An object continues at rest or in uniform motion unless it is acted on by a result force.

Front 104

Newton’s second law

Back 104

The rate of change of momentum of an object is proportional to the resultant force on it.

Front 105

Newton’s third law

Back 105

When two objects interact, they exert equal and opposite forces on one another.

Front 106

Pair production

Back 106

When a gamma photon changes into a particle and an antiparticle.

Front 107

Potential gradient

Back 107

Potential gradient at a point in a field is the change of potential per unit change of distance along the field line at that point. The potential gradient = - the field strength at any point.

Front 108

Pressure law

Back 108

For a fixed mass of an ideal gas at constant volume, its pressure is directly proportional to its absolute temperature.

Front 109

Conservation of momentum

Back 109

When two or more bodies interact, the total momentum is unchanged, provided no external forces act on the bodies.

Front 110

Resonance

Back 110

The amplitude of vibration of an oscillating system subjected to a periodic force is largest when the periodic force has he same frequency as he resonant frequency of the oscillating system. At resonance, the system vibrates such that its velocity is in phase with the periodic force.

Front 111

Latent heat of fusion

Back 111

The energy needed to change a the state of a liquid without change of temperature.

Front 112

Rutherford’s alpha particle scattering experimentsim

Back 112

Demonstrated that every atom contains a positively charged nucleus which is much smaller than the atom and where all the positive charge and most of the mass of the atom is located.

Front 113

Simple harmonic motion

Back 113

Motion of an object if its acceleration is proportional to the displacement of the object from equilibrium and is always directed towards the equilibrium position.

Front 114

Specific heat capacity

Back 114

of a substance is the energy needed to raise the temperature of 1kg of the substance by 1K without change of state. To raise the temperature of mass m of a substance from T1 to T2, the energy needed, Q = mc(T2-T1), where c is the specific heat capacity of the substance.

Front 115

Specific latent heat of fusion

Back 115

of a substance is the energy needed to change the type of unit mass of a solid to a liquid without change of temperature.

Front 116

Specific latent heat of vaporisation

Back 116

for a substance is the energy needed to change the state of unit mass of a solid to a liquid to a vapour without change of temperature. To change the state of mass m of a substance without the change of temperature, the energy needed Q = ml.

Front 117

Strong force

Back 117

Force that holds nucleons together. Range of 2-3fm. Below 0.5fm is repulsive, above is attractive

Front 118

Transformer ratio rulers W

Back 118

The ratio of the secondary voltage to the primary voltage is equal to the ratio of the number of secondary turns to the number of primary turns.

Front 119

Wave particle duality

Back 119

1) Matter particles have a wave-like nature, for example, electrons directed at a thin crystal are diffracted by the crystal, and particle-like behaviour, such as electrons being deflected by a magnetic field.

2) Photons have a particle-like nature, as shown in the photoelectric effect, as well as a wave-like nature as shown in diffraction experiments.

Front 120

Lenz’s law

Back 120

When a current is induced by EM induction, the direction of the induced current is always such as to oppose the change that causes the current.

Front 121

Magnetic flux

Back 121

= BA for a uniform magnetic field of flux density B that is perpendicular to an area A.

Front 122

Magnetic flux density

Back 122

The magnetic force per unit length per unit current on a current carrying conductor at right angles to the field lines. The unit of magnetic flux density is the Tesla (T). B is sometimes referred to as magnetic field strength.

Front 123

Magnetic flux linkage

Back 123

= NBA where B is the magnetic flux density perpendicular to area A. The unit of magnetic flux me of flux linkage is the Weber (Wb), equal to 1Tm^-1 or 1 V s.

Front 124

Equations for magnetic force

Back 124

1) F = BIlsin(x) - force on a current carrying wire of length l in a uniform magnetic field B at angle x.

2) F = BQvsin(x) - gives the force F on a particle of charge Q moving through a uniform magnetic field B at speed v in a direction at angle x.

3) BQv = mv^2/r - gives the radius of the orbit between the mass of the separated nucleons and the nucleus.

Front 125

Mass defect

Back 125

Mass defect of a nucleus is the difference between the mass of the separated nucleons and the nucleus.

Front 126

Metastable state

Back 126

An excited state of the nuclei of an isotope that lasts long enough after alpha or beta emission for the isotope to be separated from the parent isotope.

Front 127

Moderator

Back 127

Substance in a thermal nuclear reactor that slows down the fission neutrons so they can go on to produce further fission.