AQA Physics Definitions Flashcards
Specific charge
the charge in coulombs divided by the mass in kilograms or charge to mass ratio
Isotope
Isotopes are nuclei with the same number of protons but a different number of neutrons
Photo-electric 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 a 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 1V
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 a circuit to another
Resistance
the ratio of potential difference across a component to the current through it
Ohmic conductor or resistor
the ratio of potential difference to current remains constant
Ohms law
the current through a component is proportional to the potential difference across it
Critical temperature
the temperature at or below which the resistivity of a superconductor becomes zero
Kirchoff’s first law
the sum of the currents into the junction is zero
Kirchoff’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 to charges as they pass through the battery or cell (before losing energy to internal resistance)
Internal resistance
the resistance inside a cell, battery, 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
Route mean square
the square root of the mean of all the square values
Time base
the control on an oscilloscope which changes the time it takes for the beam to cross the screen horizontally
Y-gain
the control on an oscilloscope that changes the sensitivity of the vertical voltage scale
Scalar
a physical quantity which has magnitude only
Vector
a physical quantity which has magnitude and direction
Equilibrium
a object is in equilibrium when the resultant force on it is zero and the resultant torque is zero
Couple
two equal an opposite forces acting on a body but not along the same line
Moment/torque
the moment/torque 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 anticlockwise moments. This apples 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, and the point where a single force acting has no turning effect
Displacement
the distance an object has moved in a particular direction
Speed
distance 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
Newton’s first law
an object remains at rest of moves with a constant velocity providing no resultant external force is applied
Newton’s second law
for a constant mass the acceleration of an object is proportional to the resultant force applied to it
Newton’s third law
if object A applies a force F on object B, object B applies a force of -F on object A
Work
work done equals force multiplied by the distance moved in the direction of the force
Power
power equals work done divided by time (rate of transfer of energy)
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 up to the limit of proportionality
Elastic limit
the point beyond which a material or spring stretched will not return to its original length
Tensile stress
tensile force divided by the cross-sectional area of the material
Tensile strain
the extension of a solid divided by its original length when deformed
Plastic behaviour
a material undergoing plastic behaviour will not return to its original shape after deforming forces are removed
Fracture
when a material breaks - materials are compared by their ultimate tensile stress; the tensile stress needed to break the material
Brittleness
a brittle material will not undergo plastic deformation before fracture
Young’s modulus
tensile stress divided by tensile strain (assuming the limit of proportionality has not been exceeded)
Amplitude
the maximum displacement from the equilibrium position of an oscillating object
Frequency
the number of oscillations per second
Wavelength
the distance between two consecutive particles in a wave that are in phase
Wavespeed
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 a 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
TIR occurs when the angle of incidence is greater than the critical angle, the substance at the boundary has a lower refractive index, 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/difference
Fringe spacing
the distance between two consecutive maxima or minima in an interference pattern
Impulse
force multiplied by the time for which the force acts on the body
Principle of conservation of momentum
momentum in a collision is always conserved provided no resultant external forces are acting
Elastic collision
kinetic energy is conserved
Inelastic collision
kinetic energy is not conserved
Explosive collision
kinetic energy is increased due to transfer of energy from the explosion
Centripetal force
the resultant force applied to an object to keep it moving in a circle, directed towards the centre of the circle and at right angles to the direction of motion of the object
Angular speed
the rate of change of angular displacement in radians for an object in circular motion
Simple harmonic motion
the condition for SHM is that 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
Resonance
the frequency of oscillation of the driver equals the natural frequency, and so maximum energy transfer occurs
Phase relationship in resonance
the driver leads the driven by a phase angle of pi/2 radians (90 degrees) or a time of T/4 seconds
Forced vibrations
the oscillator vibrates at the frequency of the driver
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
the force per unit mass placed at that point in the field
Electric potential (at a point)
the work done per unit positive charge in bringing a small test charge from infinity to that point
Electric field strength
the force per unit charge on a small positively charged object placed at a point in the field
Gravitational potential (at a point)
the work done per unit mass in bringing a small test mass from infinity to that point in the field
Inverse square law
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
Circular path
the path taken by a particle when force of constant magnitude is applied at right angles to the motion of the particle
Capacitance
the charge stored per unit p.d. of a capacitor
Time constant
the resistance times the capacitance, measured in seconds
Magnetic flux density
this is a measure of the magnetic field strength, units Tesla
Tesla
1 Weber per metre squared
Magnetic flux
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)
Magnetic flux linkage
the amount of magnetic flux that passes through a coil or complete loop of wire, multiplied by the number of turns of the wire
Fleming’s left hand rule
this predicts the relationship between the direction of force (movement, thuMb), the current (seCond finger) and the field direction (First Finger)
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 magnetic flux linkage
Lenz’s law
the direction of induced current flows opposite to the change that produces it (conservation of energy)
Transformer equation
the ratio of the number of turns in the primary coil and secondary coil equals the ratio of primary and secondary voltages
Rutherford scattering
the deflection of alpha particles using thin gold film
Absorption
when the energy of the ionising radiation is completely used to ionise atoms in a material
Background radiation
the ionising radiation that is present without a radioactive source (from sun, rocks, medical uses, nuclear fallout from weapons)
Random nature of radioactive decay
radioactive decay follows the laws of probability - one cannot predict which nucleus will decay or when
Activity
the total number of particles emitted by a source per second (measured in Becquerel)
Becquerel
the number of counts per second
Half life
the time for half of the undecayed atoms to decay
Decay constant
the constant that relates the rate of decay of a radioactive material to the number of undecayed particles in the sample
Alpha particle
a helium nucleus emitted from a nucleus of an atom at about 1/10th the speed of light
Beta particle
an electron emitted from the nucleus of an atom 9/10th the speed of light
Electron capture
when a nucleus absorbs an electron from an atomic energy level close to the nucleus causing a proton to change to a neutron and an electron neutrino to be emitted, accompanied by the emission of a high energy photon when electrons drop down to fill the gap left by the absorbed electron
Nuclear excited states
when the nucleus is left with excess energy after the emission of a particle - the nucleus has quantised energy levels and a gamma ray of fixed energy is emitted from the nucleus
Radius of closest approach
the closest distance an alpha particle will get to a nucleus when fired directly towards it
Electron diffration
the diffraction of a high energy electron by the nucleus
Nuclear density
the density of nuclear matter
Mass difference
the difference between the mass of all the individual protons and neutrons and the mass when they are combined together in a nucleus
Binding energy
the energy released when the nucleus is formed from individual protons and neutrons
Atomic mass unit (u)
the mass of a particle or nucleus measured in ‘u’ compared to the mass of a carbon 12 atom having a mass of exactly 12 u
Binding energy per nucleon
the binding energy divided by the total number of protons and neutrons in a nucleus
Fission
the splitting of a large nucleus into smaller nuclei
Fusion
the joining together of smaller nuclei to form a larger nucleus
Induced fission
slow moving (thermal) neutrons are absorbed by a large nucleus making it unstable, which then decays releasing energy and producing smaller nuclei
Chain reaction
after induced fission occurs neutrons are released which will go on to produce further induced fission reaction
Critical mass
the mass of nuclear matter is critical when every induced fission causes at least one further fission (this depends upon the mass and shape of the matter)
Moderator
the material in a nuclear reactor that is used to slow the neutrons by collision (material usually water or carbon)
Coolant in a nuclear reactor
a fluid used to stop a machine or device becoming dangerously hot. It is pumped through the core of a nuclear reactor to transfer the thermal energy to the heat exchanger which is used to boil water to drive turbines
Control rods
these are made of a material that absorbs neutrons and changes the amount of neutron flux in the reactor hence changing the rate of fission (usually boron)
Specific heat capacity
the heat energy needed to raise 1kg of the material by 1 kelvin without change of state
Specific latent heat
the energy needed to change the phase of 1kg of a material
Specific latent heat of vaporisation
the energy needed to change the state of 1kg of a material from liquid to gas without change of temperature
Ideal gas
a gas that obeys Boyle’s law (pV=k) or the ideal gas equation (pV=nRT)
Avagadro’s constant
the number of particles in one mole of carbon-12 (in exactly 12g of carbon-12)
Molar mass
the mass of one mole of a substance
Molecular mass
the mass of one particle in a gas
Assumptions in the kinetic theory of gases
- the particles apply a force on the walls of the container by collision
- collisions of particles with the walls and eachother are elastic
- no bonds between particles
- gas particles move with random motion
- there are a large number of particles
- the volume of the particles is negligible compared to the volume of the container
- contact time of collisions with walls is negligible compared with time between collisions with walls
Frequency of a progressive wave
number of complete cycles/wavelengths passing
a point per second
Why is gravitational potential negative
Gravitational potential is defined as zero at infinity, and as the force is attractive, work must be done to reach infinity, hence it is always negative
Black hole
An object with an escape velocity greater than the speed of light
Escape velocity
The velocity at which an objects kinetic energy is equal to minus its gravitational potential energy, or the minimum speed that will allow an object to escape the gravitational field
Modal dispersion
Light rays enter fibre at different angles and so some travel longer distances. Use a single-mode fibre to prevent this
Material dispersion
Different wavelengths of light travel at different speeds in the fibre. Use monochromatic light to prevent this
Yield point
the stress at which a large amount of plastic deformation takes place with a constant load
Internal energy
Internal energy is the sum of the randomly distributed kinetic energies and potential energies of the particles in a body
