Download Glossary Higher Terms

Glossary Higher Terms
a.c.
Alternating current. The current from an a.c. supply constantly changes direction.
absolute refractive index
The absolute refractive index (or more simply, the refractive index), n, of a medium is the
ratio
, where θ1 is in a vacuum, and θ2 is in the medium.
absolute zero
The minimum temperature theoretically attainable, 0 K, equivalent to -273.15 °C.
Absorbed dose
The energy absorbed from a radioactive source per unit mass of tissue.
acceleration
The rate of change of velocity. Acceleration is a vector quantity.
activity
The number of nuclei in a radioactive sample decaying each second.
ampere
1 coulomb of charge passing a point in 1 second
amplifier
An electronic circuit with at least one input and one output, that is designed to process
the waveform obtained from a physical quantity (such as a sound) and produce an output
that is an enlarged (amplified) copy of this waveform.
amplitude
The maximum displacement of the medium from its mean position, measured in metres.
angle of incidence
The angle between the incident ray and the normal.
angle of refraction
The angle between the refracted ray and the normal.
atomic number
The number of protons in an atomic nucleus. It is this number that determines the
element.
breakdown voltage
The limit of operation of a reverse-biased diode.
buoyancy force
The upwards force (sometimes called the upthrust) acting on an object immersed in a
fluid, due to the difference in pressure (and hence force) acting on its upper and lower
surfaces.
capacitance
The ratio of electric charge to potential difference between any two conductors separated
by an insulating material. The capacitance of a system of conductors describes the ability
of the system to store electric charge.
capacitor
Two (or more) conductors separated by an insulator that can be used to store charge.
chain reaction
When a nucleus undergoes fission it releases neutrons that can go on to collide with
other nuclei, causing further fission reactions. If there is a sufficient concentration of
suitable nuclei, the process becomes self-sustaining.
clipped
An output signal is clipped when the peak values that it should have are greater than ±
the supply voltage.
coherent waves
Two or more waves that have the same frequency and wavelength and a constant phase
relationship.
collimator
Part of a spectrometer that is used to produce a parallel beam of light.
components of a vector
Two vectors which act at right angles, the vector sum of which is the original vector.
conductor
A material through which electric charge can flow.
conservation of energy
Energy cannot be created or destroyed, only converted from one form to another.
conservation of momentum
When two or more objects interact, the total momentum is conserved, in the absence of
net external forces.
constructive interference
When two coherent waves that are in phase interfere, there is an increased disturbance
of the medium, because the net disturbance is the sum of the disturbances due to each
individual wave. This process is called constructive interference.
critical angle
The maximum value of the angle between the normal and the ray in glass, θ glass, for
which refraction can occur.
d.c.
Direct current. The current from a d.c. supply always moves in the same direction around
an electric circuit.
density
-3
The density of a substance is its mass per unit volume, measured in kg m .
depletion layer
The area surrounding the p-n junction of a diode where the electrons have combined with
the holes leaving no free charges.
destructive interference
When two coherent waves that are not exactly in phase interfere, there is a reduced
disturbance of the medium. This process is called destructive interference. If the waves
are of equal amplitude and they are exactly out of phase (in anti-phase), the net
disturbance is zero.
differential mode
A mode of operation of an op-amp where the output of the op-amp Vo depends on the
difference between the voltage V1 applied to the inverting input, and the voltage V2
applied to the non-inverting input.
diffraction
An effect that causes waves to bend as they go past the end of an obstacle or through a
small gap in a barrier.
diffraction grating
A transparent slide of glass or plastic that has a very large number of equally-spaced
grooves machined on to its surface. Each groove acts as a source for coherent beams of
light.
digital to analogue converter
A circuit that converts digital signals into an analogue waveform.
dispersion
The process of splitting up light into its constituent colours.
displacement
A specified distance from a fixed point, in a specified direction. Displacement is a vector
quantity.
doping
The process of adding atoms of other materials to alter the resistance of certain
materials. Usually done in the manufacture of semiconductors.
dosimetry
The measurement of the biological effect of ionising radiation.
effective equivalent dose
A quantity used to assess the health risk from ionising radiation, based on the
susceptibility of different tissue to harm.
elastic collision
A collision in which both momentum and kinetic energy are conserved.
electric field
The region around a charged object where the charge exerts a force on other charges.
electromagnetic waves
The spectrum of waves that includes radio, visible light, X-rays etc.
electromotive force
The electromotive force of a source is the electrical potential energy that is given to each
unit of charge that passes through the source.
equivalent dose
A quantity used to compare the biological effects of different types of radiation. It is equal
to the absorbed dose multiplied by the radiation weighting factor of the radiation.
excited state
Any atomic energy level higher than the ground state.
Fission (MUST BE ABLE TO SPELL CORRECTLY!)
The splitting of a large atomic nucleus into smaller fragments, with the resultant release
of excess energy.
The energy produced in a fission reaction is due to the mass after being less than the
mass before the reaction. The mass difference is converted to energy.
forward-biased
A diode connected in a circuit such that the p-type terminal is more positive than the ntype terminal.
frequency
The number of complete events that happen in a given time period. The unit used to
measure frequency is the hertz (Hz) where one hertz is one wave or one cycle per
second.
Fusion (MUST BE ABLE TO SPELL CORRECTLY!)
Occurs when 2 light nuclei combine to form nucleus of large mass number.
ground state
The lowest energy level of an atom.
half-value thickness
The thickness of absorber that reduces the measured count rate of a radioactive source
by half.
high-pass filter
An electrical filter that allows high frequency signals to pass, but blocks low frequency
signals.
impulse
The change of momentum of an object, equal to the product of the force acting on the
object and the time over which the force acts.
induced fission
The deliberate splitting of a large nucleus caused by the collision of the nucleus with a
neutron.
inelastic collision
A collision in which momentum is conserved but kinetic energy is not.
internal resistance
The opposition to current in a source of electrical energy.
intrinsic semiconductors
Semiconductor material with no impurities.
inversion layer
The section of the substrate below the gate that reverses type when a voltage is applied
to the gate.
inverting mode
A mode of operation of an op-amp where the single input voltage V1 is applied to the
inverting input, and a proportion of the output is fed back to the inverting input. The noninverting input is connected to the 0 V line.
ionisation level
The energy level at which an electron can break free from an atom.
irradiance
The power per unit area of radiation incident on a surface.
isotopes
Different forms of the same element. The isotopes of an element contain the same
number of protons but have different numbers of neutrons.
junction voltage
The potential difference between the ends of the depletion layer inside a p-n junction
diode.
kinetic energy
The energy of an object due to its motion.
kinetic theory
A theory that explains the physical properties of a gas by considering the gas to be made
up of a number of molecules moving with random speeds, undergoing elastic collisions
with each other and with the walls of the container.
LASER
Light Amplification by Stimulated Emission of Radiation.
leakage current
The tiny current in a reverse-biased diode.
line absorption spectrum
A spectrum that consists of narrow dark lines across an otherwise continuous spectrum.
line emission spectrum
A spectrum consisting of narrow lines of light, the position of which depend on the
substances producing the light.
load resistor
The resistor, or combination of resistors, that forms the external part of an electrical
circuit.
lost volts
The potential difference that is used to drive a current through the internal resistance of a
source. Lost volts is given by the expression Ir where r is the internal resistance of the
source.
mass defect
The difference between the mass of a nucleus and the total mass of an equal number of
individual nucleons.
mass number
The total number of nucleons in the nucleus of an atom.
momentum
The product of the mass of an object and its velocity. Momentum is a vector quantity,
-1
measured in kg m s .
monochromatic
Radiation consisting of a single frequency.
Monochromatic light
Light of one wavelength (and therefore one colour).
n-type semiconductor
Semiconductor material that has an excess of free electrons.
negative feedback
Negative feedback is when a proportion of the output of an op-amp is fed back to the
inverting input of the op-amp.
newton
One newton is the force that, when applied to an object of mass 1 kg, will cause the
-2
object to accelerate at a rate of 1 m s in the direction of the applied force.
noise
An unwanted electrical signal picked up at the inputs of an op-amp. Most frequently
mains pick-up.
normal
A line drawn at right angles to a surface or the boundary between two different media.
nucleon
The general term for protons and neutrons.
p-type semiconductor
Semiconductor material that has an excess of free holes.
path difference
The difference in path lengths of two sets of waves.
period
The time to make one complete wave. Period is measured in seconds.
phase
A way of describing how far through a cycle a wave is. Usually used when describing
whether two waves are in phase (at the same point in their cycles) or out of phase (at
different points in their cycles).
photocathode
The terminal from which electrons will be emitted due to the photoelectric effect.
photoconductive mode
The mode of operation of a photodiode that allows it to act as a light sensor.
Photodiode
A type of p-n junction diode that responds to light intensity.
photoelectric effect
The emission of electrons from a metal due to the effect of electromagnetic radiation.
photoelectrons
Free electrons produced by the photoelectric effect.
photoemission
The emission of electrons from a material caused by light shining on it.
photon
The particle of electromagnetic radiation.
photovoltaic mode
The mode of operation of a photodiode where it can supply power to a load. This is the
basis of a solar cell.
potential difference
The potential difference between two points is a measure of the work done in moving one
coulomb of charge between the two points.
potential energy
The energy stored in an object due to its position, its shape or its state.
pressure
-2
The force acting per unit area, measured in N m or pascal (Pa).
radiation weighting factor
A multiplying factor based on the ionising effect of different types of radiation.
radioactive decay series
A chain of radioactive decays as a radioactive element changes to eventually become a
stable, non-radioactive element.
recombination energy
The energy released as a photon when an electron combines with a hole in a p-n
junction.
rectifier circuit
A circuit that uses diodes to convert an a.c. signal to a d.c. signal.
refraction (MUST BE ABLE TO SPELL CORRECTLY!)
Refraction occurs when a wave goes from one medium into another. When a wave is
refracted, its speed and wavelength always change; its frequency never changes; its
direction sometimes changes.
resistance
The opposition that a conductor offers to a current through it. Defined as the ratio of
potential difference across the conductor to the current through it.
reverse-biased
A diode connected in a circuit such that the p-type terminal is more negative than the ntype terminal.
saturation
An amplifier is driven into saturation when the input signal is so large that the output is at
its greatest value, ie ± Vs.
scalar
A physical quantity which has magnitude but no direction.
short-circuit current
The maximum current that a source can supply. The current drawn from the supply when
there is zero resistance in the external circuit (when the terminals of the source are joined
together or 'short-circuited').
spectrometer
An instrument that can make precise measurements of the spectra produced by different
light sources.
speed
The speed of a wave is the distance travelled by a wave per unit time, measured in m s
1
.
spontaneous emission of radiation
The random process of an electron jumping to a lower energy level, releasing a photon.
spontaneous fission
The random splitting of a large atomic nucleus due to the internal processes within the
nucleus.
stimulated emission of radiation
The release of a photon by an electron jumping to a lower energy level, caused by the
passing of a photon of energy equal to the energy difference between the two levels.
threshold voltage
The minimum voltage needed to switch on a MOSFET (approximately 2 volts).
total internal reflection
When a ray of light travelling in a more dense substance meets a boundary with a less
dense substance at an angle greater than the critical angle, the ray is not refracted but is
all reflected inside the more dense substance.
vector
A physical quantity which has direction as well as magnitude.
velocity
The rate of change of displacement. Velocity is a vector quantity.
voltage
electrical energy transferred by 1 coulomb of charge
voltage gain
The amount by which an amplifier increases a voltage. Voltage gain is given by the
expression
wavelength
The distance between successive points of equal phase in a wave, measured in metres.
Wheatstone bridge circuit
A resistor network, consisting of a series/parallel combination that can be used to
measure resistance when balanced. In the out-of-balance condition, a small p.d. that is
proportional to the change in resistance is produced.
work function
The minimum energy required to cause photoemission from a particular substance.
Sample Explanation Type Questions
Explain how an LED emits light.
Electrons (negative charge carriers) and hole (positive charge carriers) combine at the junction
layer to release photons.
Explain in terms of waves how maxima and minima are formed.
Both constructive and destructive interference occurs when waves meet. Constructive
interference occurs when waves meet in phase. This forms a maxima. Destructive interference
occurs when waves meet out of phase. This forms a minima.
Explain how amplification is produced in a laser/explain the use of mirrors in a LASER.
Amplification is achieved within a LASER by the reflection of photons between two mirrors. This
triggers more electrons to fall from higher to lower energy levels, emitting more photons. The
number of photons increases = amplification. One mirror totally reflects the light from one end of
the LASER. The other mirror only reflects 99% of the light. The other 1% of light is allowed to
come out of the LASER – emitting a narrow beam of light.
Explain how neutrons produced during a fission reaction can cause further nuclear
reactions.
Neutrons released during the reaction collide with large <uranium> nuclei, splitting it into two
smaller nuclei and releasing more neutrons, which continue the process, thus setting up a chain
reaction.