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Physics: AS definitions + facts
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Gravity
Terms in this set (57)
Specific charge
Charge per unit mass of a particle
Isotope
Nuclei with the same number of protons but differing numbers of neutrons
The four fundamental forces + exchange particle
1. gravity  graviton
2. electromagnetic  photon
3. weak nuclear  gluon
4. strong nuclear  W+/
Hadrons
Baryons (3 quarks or 3 antiquarks) and mesons (quarkantiquark pair)
Conservation laws
1. lepton + baryon number and charge are always conserved
2. energy and momentum are also conserved in particle interactions
3. strangeness is conserved in strong interaction
Threshold frequency
The minimum frequency of electromagnetic radiation required to liberate electrons from the surface of a particular metal
Work function
The minimum energy required to liberate the least bound electron from a metal surface
Stopping potential
The voltage required to transfer sufficient energy to just stop the photoelectric current
Ionisation
When an atom gains sufficient energy such that one or more electrons escape the atom completely, leaving a positively charged ion
Excitation
When an atom gains energy and an electron is promoted to a higher lying binding energy level but remains bound to the atom
Electron volt
The energy transferred to or from an electron passing through a potential difference of 1 volt
Progressive waves
Transfer energy without any net displacement of the medium
Amplitude
Maximum displacement of the medium from its equilibrium position
Frequency
Number of oscillations of the wave per second
Wavelength
Distance between two adjacent points on the wave that are in phase
Longitudinal waves
Medium oscillates parallel to the direction of propagation of the wave
Transverse waves
Medium oscillates perpendicular to the direction of the propagation of the wave
Polarisation
The restriction of a transverse wave to oscillate in one plane only
Stationary waves
Formed by two waves of the same frequency travelling in opposite directions
Nodes
Points of zero displacement

1. Medium between two adjacent nodes vibrates in phase (different amplitudes)
2. Medium either side of a node vibrates in antiphase
Antinodes
Points of maximum amplitude of oscillation to the medium
Coherent waves
Have the same frequency, wavelength, polarisation and wavespeed
Constant phase relationship
Diffraction
The spreading of a wave into the geometrical shadow of an object
Refractive index
The ratio of the speed of light in the material/speed of light in vacuum
The refractive index of air is only just greater than 1
Scalar quantities
Have size only
Vector quantities
Have size and direction
Acceleration
Rate of change of velocity with time
Velocity
The rate of change of displacement with time
Equilibrium
The resultant force acting on a system equals zero
The vector sum of the forces acting is zero
The force vectors form a closed loop
Moment
Force x perpendicular distance from the point of interest to the line of action of the force
Couple
force x perpendicular distance between the lines of action of the forces
The principle of moments
For equilibrium, the sum of the clockwise moments equals the sum of the anticlockwise moments about any point
The centre of mass of a body
The point through which when a force acts no rotational effect is observed
The centre of gravity of an object
The point at which the weight of the object appears to act
Newton's First Law
A body will remain at rest or at constant velocity until acted upon by a resultant force
Newton's Second Law
The rate of change of momentum of an object is equal to the resultant force applied; acceleration is always in the same direction as the force
F = ma
Newton's Third Law
For every force, there exists an equal and opposite force
Momentum
It is conserved providing no external forces act
Impulse
A change in momentum
Elastic collision
Kinetic energy is conserved
Inelastic collision
Kinetic energy is not conserved
Efficiency
useful energy out divided by total energy in
Principle of the conservation of energy
Energy is never lost but only transferred from one form to another
Hooke's Law
Extension is directly proportional to the applied load providing the limit of proportionality is not exceeded
Elastic limit
If an object exceeds its elastic limit, it will not go back to its original shape
Electrical current
Rate of flow of charge
Potential difference
Work done per unit charge
Ohm's Law
Current is directly proportional to applied potential difference providing the temperature remains constant
Superconductivity
The complete disappearance of all electrical resistance of a material once the temperature is reduced below a certain critical temperature
emf (electromotive force)
Work done per unit charge passing completely around a circuit
Also, the terminal pd of a power supply when no current flows
Terminal speed
The maximum speed reached by an object when the drag force on it is equal and opposite to the force causing the motion of the object
Spring constant
The force needed to produce unit extension in a spring
Tensile stress
The tensile force applied per unit cross sectional area in a solid, perpendicular to the cross section
Tensile strain
Extension divided by original length of a solid
Young's modulus
The stress divided by the strain (assuming the limit of proportionality has not been exceeded)
Superposition
The total displacement in a place where two waves meet is the vector sum of the displacements due to each wave individually
Snell's Law
The refractive index of a substance is equal to sin i / sin r where i is the angle of incidence and r is the angle of refraction
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