c block definitions Flashcards
(92 cards)
1
Q
quantity
A
In S.I. a quantity is represented by a number a unit, (e.g. m = 3.0 kg).
2
Q
scalar
A
A scalar is a quantity that has magnitude only.
3
Q
vector
A
A vector is a quantity that has magnitude and direction.
4
Q
Resolving a vector
A
This means finding vectors (the so-called components) in these directions, which add together vectorially to make the original vector, and so, together, are equivalent to this vector.
5
Q
density
A
density mass Unit: kg m3 or g cm-3 volume
in which mass and volume apply to any sample of the material.
6
Q
moment
A
The moment (or torque) of a force about a point is defined as the force x the perpendicular distance from the point to the line of action of the force,
i.e. moment = F x d
Unit: N m [N.B. the unit is not J]
7
Q
principle of moments
A
For a system to be in equilibrium, sum of the anticlockwise moments about a point = sum of the clockwise moments about the same point.
8
Q
centre of gravity
A
The centre of gravity is the single point within a body at which the entire weight of the body may be considered to act.
9
Q
displacement
A
The displacement of a point B from a point A is the shortest distance from A to B, together with the direction. Unit: m
10
Q
mean speed
A
Mean speed= total distance travelled/
totaltimetaken, dx/dt
11
Q
instantaneous speed
A
Instantaneous speed = rate of change of distance
Unit: ms-1
12
Q
Mean velocity
A
Mean velocity = total displacement/total time taken
13
Q
instantaneous velocity
A
The velocity of a body is the rate of change of displacement.
14
Q
mean acceleration
A
Mean acceleration = change in velocity/time taken, dv/dt
15
Q
instantaneous acceleration
A
The instantaneous acceleration of a body is its rate of change of velocity. Unit: m s-2
16
Q
terminal velocity
A
The terminal velocity is the constant, maximum velocity of an object when the resistive forces on it are equal and opposite to the ‘accelerating’ force (e.g. pull of gravity).
17
Q
force
A
A force on a body is a push or a pull acting on the body from some external body. Unit: N
18
Q
N3
A
If a body A exerts a force on a body B, then B exerts an equal and opposite force on A.
19
Q
resultant force
A
The mass of a body x its acceleration is equal to the vector sum of the forces acting on the body. This vector sum is called the resultant force.
20
Q
momentum
A
The momentum of an object is its mass multiplied by its velocity. (p = mv). It is a vector.
UNIT: kg m s-1 or Ns
21
Q
N2 in terms of momentum
A
The rate of change of momentum of an object is proportional to the resultant force acting on it, and takes place in the direction of that force.
22
Q
principle of conservation of momentum
A
The vector sum of the momenta of bodies in a system stays constant even if forces act between the bodies, provided there is no external resultant force.
23
Q
elastic collision
A
A collision in which there is no change in total kinetic energy.
24
Q
inelastic collision
A
A collision in which kinetic energy is lost.
25
work done
Work done by a force is the product of the magnitude of the force and the distance moved in the direction of the force.( W.D. = Fxcos θ )
Unit: J
26
principle of conservation of energy
Energy cannot be created or destroyed, only transferred from one form to another. Energy is a scalar.
27
gravitational potential energy
This is energy possessed by an object by virtue of its position. gpe = mgh Unit: J
28
KE
This is energy possessed by an object by virtue of its motion. Ek = 1⁄2mv2 Unit: J
29
EPE
This is the energy possessed by an object when it has been deformed due to forces acting on it.
Eelastic = 1⁄2 Fx or 1⁄2 kx2 Unit: J
30
energy
The energy of a body or system is the amount of work it can do. Unit: J
31
power
This is the work done per second, or energy transferred per second.
32
hookes law
The tension in a spring or wire is proportional to its extension from its natural length, provided the extension is not too great.
33
spring constant
The spring constant is the force per unit extension. Unit: Nm-1
34
stress
Stress is the force per unit cross-sectional area when equal opposing forces act on a body.
Unit Pa or N m-2
35
strain
Strain is defined as the extension per unit length due to an applied stress. Unit: none
36
Youngs modulus
E=tensile stress/tensile strain, Unless otherwise indicated this is defined for the Hooke’s law region. Unit: Pa or N m-2
37
crystal
Solid in which atoms are arranged in a regular array. There is a long range order within crystal structures.
38
crystalline solid
Solid consisting of a crystal, or of many crystals, usually arranged randomly. The latter is strictly a polycrystalline solid. Metals are polycrystalline.
39
amorphous solid
A truly amorphous solid would have atoms arranged quite randomly. Examples are rare. In practice we include solids such as glass or brick in which there is no long range order in the way atoms are arranged, though there may be ordered clusters of atoms.
40
polymeric solid
A solid which is made up of chain-like molecules.
41
ductile material
A material which can be drawn out into a wire, can plastically deform and can be reshaped without breaking. This implies that plastic strain occurs under enough stress.
42
elastic strain
This is strain that disappears when the stress is removed, that is the specimen returns to its original size and shape.
43
plastic strain
This is strain that decreases only slightly when the stress is removed. In a metal it arises from the movement of dislocations within the crystal structure.
44
elastic limit
This is the point at which deformation ceases to be elastic. For a specimen it is usually measured by the maximum force, and for a material, by the maximum stress, before the strain ceases to be elastic.
45
dislocations in crystals
Certain faults in crystals which (if there are not too many) reduce the stress needed for planes of atoms to slide. The easiest dislocation to picture is an edge dislocation: the edge of an intrusive, incomplete plane of atoms.
46
grain boundaries
The boundaries between crystals (grains) in a polycrystalline material.
47
ductile fracture
The characteristic fracture process in a ductile material. The fracture of a rod or wire is preceded by local thinning which increases the stress.
48
brittle material
Material with no region of plastic flow, which, under tension, fails by brittle fracture.
49
brittle fracture
This is the fracture under tension of brittle materials by means of crack propagation.
50
elastic hysterises
When a material such as rubber is put under stress and the stress is then relaxed, the stress-strain graphs for increasing and decreasing stress do not coincide, but form a loop. This is hysteresis.
51
black body
A black body is a body (or surface) which absorbs all the electromagnetic radiation that falls upon it. No body is a better emitter of radiation at any wavelength than a black body at the same temperature.
52
Wien's displacement law
The wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body. max wavelength =W/T
53
absolute temperature
The temperature, T in kelvin (K) is related to the temperature, θ, in celsius (°C) by:
T / K= θ / °C + 273.15
At 0 K (-273.15°C) the energy of particles in a body is the lowest it can possibly be.
54
stefans law
The total electromagnetic radiation energy emitted per unit time by a black body is given by power = A σT4 in which A is the body’s surface area and σ is a constant called the Stefan constant. [σ = 5.67 10-8 W m-2 K-4]
55
luminosity of a star
The luminosity of a star is the total energy it emits per unit time in the form of electromagnetic radiation. UNIT: W
[Thus we could have written luminosity instead of power in Stefan’s law (above).]
56
intensity
The intensity of radiation at a distance R from a source
is given by, I = P / 4pir^2
57
leptons
Leptons are electrons and electron-neutrinos [and analogous pairs of particles of the so-called second and third generations].
58
hadrons
Hadrons are particles consisting of quarks or antiquarks bound together. Only hadrons (and quarks or antiquarks themselves) can ‘feel’ the strong force.
59
baryon
A baryon is a hadron consisting of 3 quarks or 3 antiquarks. The best known baryons are the nucleons, i.e. protons and neutrons.
60
meson
A meson is a hadron consisting of a quark-antiquark pair.
61
current
This is the rate of flow of electric charge. I = dQ/dt
62
efficiency of a system
%Efficiency = 100 x useful work (or energy)out / work (or energy) put in
63
potential difference
The pd between two points is the energy converted from electrical potential energy to some other form per coulomb of charge flowing from one point to the other. Unit: V [= J C-1]
64
ohms law
The current in a metal wire at constant temperature is proportional to the pd across it.
65
electrical resistance
The resistance of a conductor is the pd (V) placed across it divided by the resulting current (I) through it. R = V/I
66
resistivity
The resistance, R, of a metal wire of length L and cross- sectional area A is given by r = resistivity x L/A, in which ρ the resistivity, is a constant (at constant temperature) for the material of the wire. Unit: Ω m
67
superconducting transition temperature
The temperature at which a material, when cooled, loses all its electrical resistance, and becomes super-conducting. Some materials (e.g. copper) never become superconducting however low the temperature becomes.
68
the law of conservation of charge
Electric charge cannot be created or destroyed, (though positive and negative charges can neutralise each other). Charge cannot pile up at a point in a circuit.
69
EMF
The emf of a source is the energy converted from some other form (e.g. chemical) to electrical potential energy per coulomb of charge flowing through the source. Unit: V
70
progressive wave
A pattern of disturbances travelling through a medium and carrying energy with it, involving the particles of the medium oscillating about their equilibrium positions.
71
transverse waves
A transverse wave is one where the particle oscillations are at right angles to the direction of travel (or propagation) of the wave.
72
longitudinal waves
A longitudinal wave is one where the particle oscillations are in line with (parallel to) the direction of travel (or propagation) of the wave.
73
polarised wave
A polarised wave is a transverse wave in which particle oscillations occur in only one of the directions at right angles to the direction of wave propagation.
74
in phase
Waves arriving at a point are said to be in phase if they have the same frequency and are at the same point in their cycles at the same time.
[Wave sources are in phase if the waves have the same frequency and are at the same point in their cycles at the same time, as they leave the sources.]
75
wavelength of a progressive wave
The wavelength of a progressive wave is the minimum distance (measured along the direction of propagation) between two points on the wave oscillating in phase.
76
frequency of a wave
The frequency of a wave is the number of cycles of a wave that pass a given point in one second, [or equivalently the number of cycles of oscillation per second performed by any particle in the medium through which the wave is passing.]
77
speed of a wave
The speed of a wave is the distance that the wave profile moves per unit time.
78
diffraction
Diffraction is the spreading out of waves when they meet obstacles, such as the edges of a slit. Some of the wave’s energy travels into the geometrical shadows of the obstacles.
79
principle of superposition
The principle of superposition states that if waves from two sources [or travelling by different routes from the same source] occupy the same region then the total displacement at any one point is the vector sum of their individual displacements at that point.
80
phase difference
Phase difference is the difference in position of 2 points within a cycle of oscillation. It is given as a fraction of the cycle or as an angle, where one whole cycle is 2π or 360], together with a statement of which point is ahead in the cycle.
81
coherence
Waves or wave sources, which have a constant phase difference between them (and therefore must have the same frequency) are said to be coherent.
82
stationary wave
A stationary wave is a pattern of disturbances in a medium, in which energy is not propagated. The amplitude of particle oscillations is zero at equally-spaced nodes, rising to maxima at antinodes, midway between the nodes.
83
snells law
At the boundary between any two given materials, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. For light, Snell's law may be written:
n1 sin theta1 = n2 sin theta2
in which theta1 and theta2 are angles to the normal for light passing between medium 1 and medium 2; n1 and n2 are called the refractive indices of medium 1 and medium 2 respectively. The refractive index of a vacuum is fixed by convention as exactly 1. For air, n = 1.000
84
critical angle
When light approaches the boundary between two media from the 'slower' medium, the critical angle is the largest angle of incidence for which refraction can occur. The refracted wave is then travelling at 90° to the normal.
85
photoelectric effect
When light or ultraviolet radiation of short enough wavelength falls on a surface, electrons are emitted from the surface.
86
work function
The work function of a surface is the minimum energy needed to remove an electron from the surface. Unit: J or eV
87
electron volt
This is the energy transferred when an electron moves between two points with a potential difference of 1 V between them. 1 eV = 1.60 x 10-19 J
So for an electron being accelerated it is the kinetic energy acquired when accelerated through a pd of 1 V.
88
ionisation
The removal of one or more electrons from an atom.
89
ionisation energy
The ionization energy of an atom is the minimum energy needed to remove an electron from the atom in its ground state. Unit: J
90
stimulated emission
This is the emission of a photon from an excited atom, triggered by a passing photon of energy equal to the energy gap between the excited state and a state of lower energy in the atom. The emitted photon has the same frequency, phase, direction of travel and polarisation direction as the passing photon.
91
population inversion
A population inversion is a situation in which a higher energy state in an atomic system is more heavily populated than a lower energy state (i.e. a less excited state or the ground state) of the same system.
92
pumping
Pumping is feeding energy into the amplifying medium of a laser to produce a population inversion.
