Vocab Flashcards
(298 cards)
1
Q
absolute scale
A
a temperature scale in which temperature is measured in kelvin (K)
2
Q
absolute zero
A
the temperature at which a system has minimum internal energy; equivalent to -273.15 degree
3
Q
absorption line spectrum
A
a dark line of a unique wavelength seen in a continuous spectrum
4
Q
acceleration
A
the rate of change of an object’s velocity
5
Q
accuracy
A
an accurate value of a measured quantity is one which is close to the true value of the quantity
6
Q
acoustic impedance
A
the product of the density of a substance and the speed of sound in that substance
7
Q
activity
A
the rate of decay or disintegration of nuclei in a radioactive sample
8
Q
ampere
A
the SI unit of electric current (A)
9
Q
amplitude
A
the maximum displacement of a particle from its equilibrium position
10
Q
amplitude modulation
A
a form of modulation in which the signal causes variations in the amplitude of a carrier wave
11
Q
analogue signal
A
a signal that is continuous variable, having a continuum of possible value
12
Q
analogue-to-digital conversion (ADC)
A
conversion of a continuous analogue signal to discrete digital numbers
13
Q
angular displacement
A
the angle through which an object moves in a circle
14
Q
angular frequency
A
the frequency of a sinusoidal oscillation expressed in radians per second. angular frequency = 2pi / period
15
Q
angular velocity
A
the rate of change of the angular position of an object as it moves along a curved path
16
Q
antinode
A
a point on a stationary wave with maximum amplitude
17
Q
atomic mass unit
A
a unit of mass (symbol u) approximately equal to 1.661 x 10^-27 kg. the mass of an atom of carbon-12 is 12.000u exactly
18
Q
attenuation
A
the gradual loss in strength or intensity of a signal
19
Q
average speed
A
the total distance travelled by an object divided by the total time taken
20
Q
Avogadro constant
A
the number of particles in one mole of any substance, 6.02 x 10^23 mol-1, denoted NA
21
Q
band theory
A
the idea that electrons in a solid or liquid can have energies within certain ranges or bands, between which are forbidden values
22
Q
bandwidth (communications)
A
a measure of the width of a range of frequencies being transmitted
23
Q
base units
A
defined units of the SI system from which all other units are derived
24
Q
best fit line
A
a straight line drawn as closely as possible to the points of a graph so that similar numbers of points lie above and below the line
25
binding energy
the minimum external energy required to separate all the neutrons and protons of a nucleus
26
bit
a basic unit of information storage, the amount of information stored by a device that exists in only two distinct states, usually given as the binary digits 0 and 1
27
Boltzmann constant
a fundamental constant given by k = R / NA, which R is the ideal gas constant and NA is the Avogadro constant
28
Boyle's law
the pressure exerted by a fixed mass of gas is inversely proportional to its volume, provided the temperature of the gas remains constant
29
braking radiation
X-rays produced when electrons are decelerated (also called Bremsstrahlung radiation)
30
capacitance
the ratio of charge stored by a capacitor to the potential difference across it
31
carrier wave
a waveform (usually sinusoidal) which is modulated by an input signal to carry information
32
centre of gravity
the point where the entire weight of an object appears to act
33
centripetal force
the resultant force acting on an object moving in a circle; it is always directed towards the centre of the circle
34
characteristic radiation
very intense X-rays produced in an X-ray tube, having specific wavelengths that depend on the target metal
35
charge carrier
any charged particle, such as an electron, responsible for a current
36
Charle's law
the volume occupied by a gas at constant pressure is directly proportional to its thermodynamic (absolute) temperature
37
closed system
a system of interacting objects in which there are no external forces
38
coaxial cable
an electrical cable with an inner conductor surrounded by a tubular insulating layer and an outside conducting layer
39
coherent
two sources are coherent when they emit waves with a constant phase difference
40
collimated beam
a parallel-sided beam of radiation
41
components (of a vector)
the magnitudes of a vector quantity in two perpendicular directions
42
compression
a region in a sound wave where the air pressure is greater than its mean value
43
compressive
describes a force that squeezes an object
44
computerised axial tomography
a technique in which X-rays are used to image the human body in order to produce a computerised 3D image
45
conduction band
a range of electron energies in a solid; electrons in the conduction band are free to move throughout the material
46
conservation of momentum
in a closed system, when bodies interact, the total momentum in any specified direction remains constant
47
constructive interference
when two waves reinforce to give increased amplitude
48
contact force
the force an object exerts on another with which it is in contact
49
contrast
in a high-contrast image, there is a is difference in brightness between bright and dark areas
50
contrast media
materials such as barium that easily absorb X-rays. a contrast medium is used to reveal the outlines or edges of soft tissues in an X-ray image
51
coulomb
the SI unit of electrical charge (abbreviated C). A charge of 1C passes a point when a current of 1A flows for 1s. 1 C = 1 As
52
Coulomb's law
any two point charges exert an electrical force on each other is proportional to the product of their charges and inversely proportional to the square of the distance between them
53
count rate
the number of particles (beta or alpha) or gamma-ray photons detected per unit time by a Geiger-Muller tube. Count rate is always a fraction of the activity of a sample
54
couple
a pair of equal and antiparallel forces having a turning effect but no resultant force
55
damped
describes an oscillatory motion in which the amplitude decreases with time due to energy losses
56
de Broglie wavelength
the wavelength associated with a moving particle, give by the equation:
λ = h/(mv)
57
decay constant
the constant λ for an isotope that appears in the equation A = -λN. It is the probability of an individual nucleus decaying per unit time interval.
58
decibel
a logarithmic unit of measurement that expresses the relative sizes of two powers using the formula 10lg(P1/P2)
59
density
the mass per unit volume of a material: p = m/V unit: kgm-3
60
dependent variable
the variable in an experiment with a value that changes as the independent variable is altered by the experimenter
61
derived units
Units which are combinations of the base units of the SI system
62
destructive interference
when two waves cancel to give reduced amplitude
63
diffraction
the spreading of a wave when it passes through a gap or past the edge of an object
64
digital signal
a signal that has only a few possible values, often only two
65
digital-to-analogue conversion (DAC)
conversion of a series of digital numbers into a continuous analogue signal
66
dispersion
the splitting of light into its different wavelengths
67
displacement
the distance moved by an object in a particular direction (measured from a fixed starting point)
68
Doppler effect
the change in frequency or wavelength of a wave observed when the source of the wave is moving towards or away from the observer (or the observer is moving relative to the source)
69
drag
a force that resists the movement of a body through a fluid
70
drift velocity, mean
the average speed of a collection of charged particles when a current flows.
71
dynamics
the study of motion using quantities such as force and mass
72
e.m.f.
the total work done when unit charge is moved round a complete circuit. Unit: JC-1 or volt (V)
73
efficiency
the ratio of useful output energy to the total input energy for a device, expresses as a percentage:
efficiency = useful output energy/total input energy x 100%
74
Einstein relation
this refers to the equation for the energy of a photon: E = hf or E = hc / λ
75
elastic limit
the value of stress beyond which an object will not return to its original dimensions
76
elastic potential energy
energy stored in a stretched or compressed material
77
electric charge
a property of a body that gives rise to a force on the body when it is within an electric field
78
electric field
a region in which a charged body experiences a force
79
electric field strength
the force per unit positive charge at a point. Unit: Vm-1 or NC-1
80
electric potential
The electric potential at a point is equal to the work done in bringing unit positive charge from infinity to that point
81
electric resistance
the ratio of potential difference to current. Unit: ohm (Ω)
82
electrolyte
an electrically conducing solution. The conduction is due to positive and negative ions in the solution
83
electromagnetic spectrum
the family of waves that travel through a vacuum at a speed of 3.00 x 10^8 ms-1
84
electronvolt
the energy gained by an electron travelling through a p.d. of 1 volt. 1eV = 1.60 x 10^-19J
85
elementary charge
the smallest unit of charge that a particle or an object can have. It has a magnitude of 1.60 x 10^-19C
86
emission line spectrum
a sharp and bright line of a unique wavelength seen in a spectrum
87
energy
a calculated quantity which is conserved during any change; that which is transferred when a force does work
88
energy band
a range of permitted electron energies in a solid
89
energy level
a quantised energy state of an electron in an atom
90
equation of state
for an ideal gas: pV = nRT or pV = NkT (also known as ideal gas equation)
91
equations of motion
```
four interrelated equations that can be used to determine the displacement, initial velocity, final velocity and acceleration of a body moving with constant acceleration:
v = u + at
s = ut + ½at^2
s = (u + v)/2
v^2 = u^2 + 2as
```
92
equilibrium
an object in equilibrium is either at rest or travelling with a constant velocity because the resultant force on it is zero
93
errors
inaccuracies when taking measurements
94
evaporation
the process by which a liquid becomes a gas at a temperature below its boiling point
95
exponential decay
describes the decrease of a quantity when the rate of decrease is proportional to the value of the quantity
96
extension
the change in the length of a material from its originial length
97
farad
the unit of capacitance (abbreviated F). 1F = 1 CV-1
98
Faraday's law of electromagnetic induction
the induced e.m.f. is proportional to the rate of change of magnetic flux linkage
99
field lines
lines drawn to represent the strength and direction of a field of force
100
field of force
a region of space where an object feels a force; the force may be gravitational, electric, magnetic, etc
101
First law of thermodynamics
the increase in internal energy of a body is equal to the thermal energy transferred to it by heating plus the mechanical work done on it
102
Fleming's left-hand (motor) rule
this rule is used to predict the force experienced by a current-carrying conductor placed in an external magnetic field: thumb = motion produced, first finger = magnetic field, second finger = conventional current
103
Fleming's right hand (generator) rule
this rule is used to predict the direction of the induced current or e.m.f. in a conductor moved at right angles to a magnetic field: thumb = motion, first finger = magnetic field, second finger = induced conventional current
104
forbidden gap
a range of energy values which an electron in a solid cannot have
105
force constant
the ratio of force to extension for a spring or a wire. Unit: Nm-1
106
forced oscillation
an oscillation caused by an external driving force; the frequency is determined by the driving force, and is not the natural frequency of the oscillator
107
free-body force diagram
a diagram showing all the forces acting on an object (but not the forces it exerts on other objects
108
free oscillation
an oscillation at is the natural frequency of the oscillator
109
frequency
the number of oscillations per unit time. Unit: hertz (Hz)
110
frequency modulation
a form of modulation in which the signal causes variations in the frequency of a carrier wave
111
fundamental frequency
the lowest-frequency stationary wave for a particular system
112
gain
the voltage gain of an amplifier is the ratio of the output voltage to the input voltage
113
geostationary orbit
the orbit of an artificial satellite which has a period equal to one day and moves from west to east so that the satellite remains above the same point on the Earth's equator, From Earth the satellite appears to be stationary
114
gravitational field
a region where any object with mass experiences a force
115
gravitational field strength
the gravitational field strength at a point is the gravitational force exerted per unit mass on a small object placed at that point. g = F/m
116
gravitational potential
the gravitational potential at a point is the work done per unit mass in bringing a mass from infinity to the point
117
gravitational potential energy
the energy a body has due to its position in a gravitational field
118
ground state
the lowest energy state that can be occupied by an electron in an atom
119
hadron
any particle which is affected by the strong nuclear force, made from two or three quarks or anti-quarks
120
half-life
the mean time taken for half the number of active nuclei in a radioactive sample to decay
121
half-thickness
the mean thickness of an absorbing material required to reduce the intensity of radiation by half
122
Hall effect
the production of a voltage across a conductor when a current flows through the conductor at right angles to a magnetic field
123
Hall voltage
the voltage produced across a conductor when a current flows through the conductor at right angles to a magnetic field; used in a Hall probe to measure B since Vh ∝ B
124
harmonic
a wave of frequency n times the fundamental frequency, where n is an integer
125
Hooke's law
the extension produced in an object is proportional to the force producing it
126
ideal gas
a gas that behaves according to the equation pV = nRT or pV = NkT
127
image intensifier
a device used to change a low-intensity X-ray image into a bright visual image
128
impedance matching
the reduction in intensity of ultrasound reflected at the boundary between two substances, achieved when the two substances have similar acoustic impedances
129
independent variable
the variable in an experiment with a value that is altered by the experimenter
130
inelastic
a collision is inelastic when kinetic energy is not conserved; some is transferred to other forms such as heat. Momentum and total energy are always conserved
131
inertia
a measure of the mass of an object. a massive object has large inertia
132
instantaneous speed
the speed of an object measured over a very short period of time
133
intensity
the power transmitted normally through a surface per unit area: intensity = power / cross-sectional area. Unit: W m^-2
134
interference
the formation of points of cancellation and reinforcement where two coherent waves pass through each other
135
internal energy
the sum of the random distribution of kinetic and potential energies of the atoms or molecules in a system
136
internal resistance
the resistance of an e.m.f. source. The internal resistance of a battery is due to the chemicals within it
137
intrinsic semiconductor
a pure substance whose resistivity is intermediate between that of a conductor and an insulator
138
inverting amplifier
a circuit, involving the use of an amplifier, where the output is 180° out of phase with the input
139
ion
an atom with a net positive or negative charge
140
isotopes
nuclei of the same element with a different number of neutrons but the same number of protons
141
I-V characteristic
a graph of current against voltage for a particular component of an electrical circuit
142
kinematics
the study of motion using quantities such as time, distance, displacement, speed, velocity and acceleration
143
kinetic energy
energy of an object due to its motion
144
kinetic theory of gases
a model based on the microscopic motion of atoms or molecules of a gas
145
Kirchhoff's first law
the sum of the currents entering any point (or junction) in a circuit is equal to the sum of the currents leaving that same point. This law represents the conservation of charge
146
Kirchhoff's second law
the sum of the e.m.f.s round a closed loop in a circuit is equal to the sum of the p.d.s in that same loop
147
Larmor frequency
the frequency of procession of a nucleus in an external field
148
Lenz's law
an induced current or e.m.f. is in a direction so as to produce effects which oppose the change producing it
149
lepton
a sub-atomic particle which is not affected by the strong nuclear force
150
light-dependent resistor (LDR)
a resistor whose resistance decreases as the intensity of light falling on it increases
151
light-emitting diode (LED)
a semiconductor component that emits light when it conducts electricity
152
linear momentum
the product of an object's mass and its velocity, p = mv. Momentum is a vector quantity
153
longitudinal wave
a wave in which the particles of the medium oscillate along the direction in which the wave travels
154
lost volts
the difference between the e.m.f. and the terminal p.d. in a circuit. It is equal to the voltage across the internal resistance
155
magnetic field
a force field in which a magnet, a wire carrying ca current, or a moving charge experiences a force
156
magnetic flux
the product of magnetic flux density normal to a circuit and the cross-sectional area of the circuit. Unit: weber (Wb)
157
magnetic flux density
the strength of a magnetic field. Magnetic flux density B is defined as: B = F / IL, where F is the force experienced by a conductor in the magnetic field, I is the current in the conductor and L is the length of the conductor in the magnetic field. (the conductor is at right angles to the field)
158
magnetic flux linkage
the product of magnetic flux and the number of turns. Unit: weber (Wb)
159
magnetic resonance imaging (MRI)
a medical imaging technique which uses nuclear magnetic resonance
160
mass
a measure of the amount of matter within an object. unit: kilogram (kg)
161
mass defect
the difference between the total mass of the individual, separate nucleons and the mass of the nucleus
162
mass excess
the difference between the mas of a nuclide (in u) and its mass number
163
mean drift velocity
the average speed of a collection of charged particles when a current flows
164
microwave link
a communications system that uses a beam of radio waves in the microwave frequency range to transmit audio, data or video information
165
modulation
the process of using one waveform to alter the frequency, amplitude or phase of another waveform
166
mole
the amount of matter which contains 6.02 x 10^23 particles
167
moment
the moment of a force about a point is the magnitude of the force, multiplied by the perpendicular distance of the point from the line of the force. Unit: N m
168
monochromatic
describes light of a single frequency
169
natural frequency
the unforced frequency of oscillation of a freely oscillating object
170
negative feedback
the output of a system is used to oppose changes to the input of the system, with the result that the changes are reduced
171
neutrino
a lepton, released during beta-decay
172
neutron number
the number of neutrons in the nucleus of an atom
173
newton
the force that will give a 1 kg mass an acceleration of 1 m s ^-2 in the direction of the force. 1N = 1 kg m s^-2
174
Newton's first law of motion
an object will remain at rest or keep travelling at constant velocity unless it is acted on by a resultant force
175
Newton's law of gravitation
any two point masses attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of their separation
176
Newton's second law of motion
the resultant force acting on an object is equal to the rate of change of its momentum. the resultant force and the change in momentum are in the same direction
177
Newton's third law of motion
when two bodies interact the forces they exert on each other are equal and opposite
178
node
a point on a stationary wave with zero amplitude
179
noise
an unwanted random addition to a transmitted signal
180
non-inverting amplifier
a circuit, involving the use of an amplifier, in which the output is in phase with the input
181
nuclear fission
the splitting of a nucleus into two large fragments and a small number of neutrons
182
nuclear fusion
a nuclear reaction in which two light nuclei join together to form a heavier but more stable nucleus
183
nuclear magnetic resonance
a process in which radio waves are absorbed or emitted by nuclei spinning in a magnetic field
184
nuclear model of the atom
a model of the atom in which negative charges (electrons) are distributed outside a tiny nucleus of positive charge
185
nucleon number
the number of neutrons and protons in the nucleus of an tom (also called mas number
186
nucleon
a particle found in an atomic nucleus, i.e. a neutron or a proton
187
nucleus
the tiny central region of the atom that contains most f the mass of the atom and all of its positive charge
188
nuclide
a specific combination of protons and neutrons in a nucleus
189
number density
the number of particles, such as free electrons, per unit volume in a material
190
Ohm's law
the current in a metallic conductor is directly proportional to the potential difference across its ends, provided its temperature remains constant
191
operational amplifier (op-amp)
a high-gain electronic d.c. voltage amplifier with differential inputs and, usually, a single output
192
optic fibre
a glass or plastic fibre that carries light along its length
193
oscillation
a repetitive back-and-forth or up-and-down motion
194
parallel
describes components connected side-by-side in a circuit
195
path difference
the difference in the distances travelled by two waves from coherent sources at a particular point
196
perfectly elastic
a collision is perfectly elastic when kinetic energy is conserved. momentum and total energy are always conserved
197
period
the time taken by an object (e.g. a planet) to complete one cycle (e.g. an orbit). the period is also the time taken for one complete oscillation of vibrating object. unit:second
198
phase
refers to the point that an oscillating mass has reached in a complete cycle
199
phase difference
the difference in the phases of two oscillating particles, expressed in degrees or radians
200
photoelectric effet
an interaction between a photon and an electron in an atom, in which the electron is removed from the atom
201
photon
a particle of electromagnetic radiation
202
piezo-electric crystal
a material that produces an e.m.f. when it is stressed, causing its shape to change. also, when a voltage is applied across it in one direction, it changes its dimensions slightly
203
piezo-electric effect
the production of an e.m.f. between the faces of a crystal when the crystal is compressed
204
Planck constant
the constant which links the energy of a photon and its frequency, in the equation: E = hf
205
plum-pudding model
a model of the atom in which negative charges are distributed throughout a sphere of positive charge
206
positron
an anti-electron
207
potential difference (p.d.)
the energy lost per unit charge by charges passing through a component. Unit: J C^-1 or volt (V)
208
potential divider
a circuit in which two or more components are connected in series to a supply. the output voltage from the circuit is taken across one of the components
209
potentiometer
a circuit which allows the measurement of an e.m.f. by comparison with a known e.m.f.
210
power
the rate at which energy is transferred or the rate at which work is done. Unit: watt (W)
211
precession
the movement of the axis of a spinning object (proton) around another axis
212
precision
the smallest change in value that can be measured by an instrument or an operator. a precise measurement is one made several times, giving the same, or very similar, values
213
pressure
the force acting normally per unit area of a surface: p = F / A. Unit: N m^-2 or pascal (Pa)
214
principle of conservation of energy
the idea that, within a closed system, the total amount of energy in all its forms is unchanged during any change
215
principle of superposition
when two or more waves meet at a point, the resultant displacement is the sum of the displacements of the individual waves
216
progressive wave
a wave that carries energy from one place to another
217
projectile
any object thrown in the Earth's gravitational field
218
proton number
the number of protons in the nucleus of an atom (also called atomic number)
219
quarks
the fundamental particles of which hadrons are made
220
radian
one radian is the angle subtended at the centre of a circle by an arc with equal length to the radius of the circle
221
range
the horizontal distance covered by an object
222
rarefaction
a region in a sound wave where the air pressure is less than its mean value
223
rectification
the process of converting alternating current (a.c.) into direct current (d.c.)
224
red shift
the change in frequency or wavelength of a spectral line observed when the source of light is moving away from the observer; see Doppler effect
225
reflection
the bouncing back of a wave from a surface
226
refraction
the change in direction of a wave as it crosses an interface between two materials where its speed changes
227
regeneration
restoration of a signal to its original form, usually by removing or reducing noise and increasing signal strength
228
relative speed
the magnitude of the difference in velocities between two objects
229
relaxation time
the time taken for a nucleus or other excited system to fall back to a lower energy state
230
relay
an electrically operated switch, caused to open and close by current in a coil
231
repeater
an electronic device that receives a signal and retransmits it
232
resistivity
a property of a material, a measure of its electrical resistance, defined by: ρ = RA / L. Unit: Ω m
233
resistor
an electrical component whose resistance in a circuit remains constant, is independent of current or potential difference
234
resonance
the forced motion of an oscillator characterised by maximum amplitude when the forcing frequency matches the oscillator's natural frequency. a system absorbs maximum energy from a source when the source frequency is equal to the natural frequency of the system
235
rest mass
the mass of an isolated stationary particle
236
resultant force
the single force that has the same effect as all of the forces acting on an object
237
right-hand grip rule
a rule for finding the direction of the magnetic field inside a solenoid, if the right hand grips the solenoid with the fingers following the direction of the conventional current around the solenoid, then the thumb points in the direction of the magnetic field
238
sampling
taking the value of a continuous signal at regular intervals
239
scalar quantity
a scalar quantity has magnitude but no direction
240
semiconductor diode
an electrical component made from a semiconductor material (e.g.silicon) that only conducts in one direction, a diode in 'reverse bias' has an infinite resistance
241
sensor
a device that produces an output (usually a voltage) in response to an input
242
series
a term used when components are connected end-to-end in a circuit
243
sharpness
the degree of resolution in an image, which determines the smallest item that can be identified
244
simple harmonic motion
motion of an oscillator in which its acceleration is directly proportional to its displacement from its equilibrium position and is directed towards that position
245
solenoid
a long current-carrying oil used to generate a uniform magnetic field within its core
246
specific heat capacity
the energy required per unit mass of a substance to raise its temperature by 1 K (or 1 degree). unit: Jkg^-1 K^-1
247
specific latent heat of fusion
the energy required per unit mass of a substance to change it from solid to liquid without a change in temperature, unit: J kg^-1
248
specific latent heat of vaporisation
the energy required per unit mass of a substance to change it from liquid to gas without a change in temperature, unit: Jkg^-1
249
Speed
The rate of change of the distance moved by an object
Speed = distance/time
Unit ms^-1
250
Spin
A fundamental property of subatomic particles which is conserved during atomic and nuclear reactions
251
Stationary waves
A wave pattern produced when two progressive waves of the same frequency travelling in opposite directions combine. It is characterised by nodes and antinodes. Also known as a standing wave
252
Strain
The extension per unit length produced by tensile or compressive force
Strain = extension / original length
253
Strain energy
The potential energy stored in an object when it is deformed elastically
254
Strain gauge
A device that contains a fine wire sealed in plastic. Its electrical resistance changes when the object to which it is attached changes shape
255
Stress
The force acting per unit cross-sectional area
| Stress = force / cross-sectional area
256
strong nuclear force
a fundamental force which acts between hadrons
257
systematic error
an error in readings which is repeated throughout an experiment, producing a constant absolute error or a constant percentage error
258
tensile
associated with tension or pulling, e.g. a tensile force
259
terminal p.d.
the potential difference across an external resistor connected to an e.m.f. source
260
terminal velocity
the maximum velocity of an object travelling through a fluid, The resultant force on the object is zero
261
tesla
the SI unit of magnetic flux density (abbreviated T). 1 T = 1 N A^-1 m^-1
262
thermal energy
energy transferred from one object to another because of a temperature difference; another term for heat energy
263
thermal equilibrium
a condition when two or more objects in contact have the same temperature so that there is no net flow of energy between them
264
thermistor
a device whose electrical resistance changes when its temperature changes
265
thermocouple
a device consisting of wires of two different metals across which an e.m.f. is produced when the two junctions of the wires are at different temperatures
266
thermodynamic scale
a temperature scale in which temperature is measured in kelvin (K)
267
threshold frequency
the minimum frequency of electromagnetic radiation that will eject electrons from the surface of a metal
268
threshold voltage
the minimum forward bias voltage across a light-emitting diode (LED) when it starts to conduct and emit light
269
time constant
the time taken for the current, stored charge or p.d. to fall to 1/e (about 37%) of its original value when a capacitor discharges through a resistor. It is also equal to the product of capacitance and resistance
270
torque of a couple
the product of one of the forces of a couple and the perpendicular distance between them, Unit: N m
271
tracers
radioactive substances used to investigate the function of organs of the body
272
transducer
a general term used for any device that changes one form of energy into another
273
transition
when an electron makes a 'jump' between two energy levels
274
transverse wave
a wave in which the particles of the medium oscillate at right angles to the direction in which the wave travels
275
triangle of forces
a closed triangle drawn for an object in equilibrium. the sides of the triangle represent the forces in both magnitude and direction
276
turns-ratio equation
an equation relating the ratio of voltages to the ratio of numbers of turns on the two coils of a transformer:
Vs/Vp = Ns/Np
277
unified atomic mass unit
a convenient unit used for the mass of atomic and nuclear particles (1 u is equal to the mass of a C-12 carbon atom)
1 u = 1.66 x 10^-27 kg
278
uniform acceleration
acceleration that remains constant
279
uniform motion
motion of an object travelling with a constant velocity
280
upthrust
the upward force that a liquid exerts on a body floating or immersed in a liquid
281
valence band
a range of electron energies in a solid; electrons in the valence band are bound to individual atoms
282
vector addition
using a drawing, often to scale, to find the resultant of two or more vectors
283
vector quantity
a quantity which has both magnitude and direction
284
vector triangle
a triangle drawn to determine the resultant of two vectors
285
velocity
the rate of change of the displacement of an object:
velocity = change in displacement / time
unit: m s^-1
286
virtual earth approximation
an approximation in which the two inputs of an op-amp are nearly at the same potential
287
viscous forces
forces that act on a body moving through a fluid that are caused by the resistance of the fluid
288
voxel
a small cube in a three-dimensional image
289
wave
a periodic disturbance travelling through space, characterised by a vibrating medium
290
wavelength
the distance between two adjacent peaks or troughs in a wave
291
weak nuclear force
a fundamental force, involved in radioactive beta-decay
292
weight
the force on an object caused by a gravitational field acting on its mass:
weight = mass x acceleration of free fall
Unit: newton (N)
293
wire-pair
a type of electrical wiring in which the two conductors needed to carry a signal are placed close together
294
work done
the product of the force and the distance moved in the direction of the force
295
work function
the minimum energy required by a single electron to escape a metal surface
296
X-ray tube
a device that produces X-rays when accelerated electrons hit a target metal
297
Young modulus
the ratio of stress to strain for a given material, resulting from tensile forces, provided Hooke's law is obeyed:
Young modulus = stress / strain
Unit: pascal (Pa; or MPa, GPa)
298
zero error
a systematic error in an instrument that gives a non-zero reading when the true value of a quantity is zero
