Physics Flashcards
(131 cards)
1
Q
Analogue reading uncertainty =
A
± 1/2 smallest unit measurable
2
Q
Digital reading uncertainty =
A
± smallest unit measurable
3
Q
Random uncertainty =
A
max. value - min. value / no. measurements
4
Q
Remember directions for vectors eg. gravity on earth =
A
-9.8ms²
5
Q
Newton’s 1st Law
A
An object will remain at rest or travelling at a constant speed in the same direction unless acted on by an unbalanced force.
6
Q
Newton’s 2nd Law
A
When a force acts on an object it will accelerate. F = ma
7
Q
Newton’s 3rd Law
A
For every action there is an equal but opposite reaction.
8
Q
Forces in a lift can be calculated by adding or subtracting what from the weight of the lift?
A
Fun = ma
9
Q
Forces on a slope:
The force of weight acting parallel to the slope =
A
Wsinθ
10
Q
Forces on a slope:
The force of weight acting perpendicular in to the slope =
A
Wcosθ
11
Q
Momentum (p) =
A
mv (kgms-1)
12
Q
Law of Conservation of momentum
A
Total momentum before collision is equal to the total momentum after IN THE ABSCENCE OF EXTERNAL FORCES.
m1u1 + m2u2 = m1v1 +m2v2 {or (m1 + m2) v if the objects stick together upon collision}
13
Q
Momentum is a vector and so requires
A
direction.
14
Q
What is impulse?
A
The force x the time it acts over (Ft) measured in Ns.
The time should be as great as possible to reduce damage.
15
Q
What is terminal velocity?
A
Velocity at which air resistance = weight of falling object.
16
Q
For projectiles at an angle, never
A
directly use the velocity at angle. Only the horizontal and vertical components.
17
Q
For projectiles at an angle at the highest point of the path
A
the path is symmetrical about either side.
The vertical velocity is 0ms-1.
18
Q
Newton’s Thought Experiment was?
A
That if an object had a high enough sideways velocity it would fall vertically at the same rate as the Earth fell away from it (satellites).
19
Q
Unlike magnetic or electrostatic force, gravity always
A
attracts.
20
Q
The 2 principles of special relativity are:
A
- speed of light in a vacuum is the same for all observers
- when 2 observers are moving at constant speeds relative to each other they will observe the same laws of physics
21
Q
Description of time dilation:
A
Time is different for observers in different reference frames because the path they observe for a moving object is different.
22
Q
Description of length contraction:
A
Length contraction is the shortening of an object travelling at 0.1c or faster. It is observed by the stationary observer.
23
Q
Description of the Doppler effect:
A
When a source producing waves is moving the wave fronts are closer together and so a higher frequency is observed in front of the moving source than behind. Doppler effect is related to sound waves.
24
Q
Description of Redshift and Blueshift:
A
the Doppler effect
25
Redshift (z) of galaxies =
v (of galaxy) / c
26
What 2 formulas combine to calculate the age of the universe?
```
v = Ho x d
t = d/v
```
27
Evidence for the Big Bang theory / expanding universe:
Olber's Paradox:
The sky should be bright with stars but since the universe is expanding the light from some stars has not reached us yet.
28
What are the four fundamental forces:
- Gravitational force
- Electromagnetic force
- Strong nuclear force
- Weak nuclear force
29
Electric fields are weaker the
further away from the charge something is.
30
Field lines between charged plates must be curved
at the edges of the plates.
31
Work done to move a charge from the attractive side to the repellant side =
QV
32
Magnetic fields are stronger
at the poles.
33
Left hand rule shows the direction of
a magnetic field around moving charges (current).
34
Right hand motor rule shows the movement of
a NEGATIVE charge in a magnetic field.
35
Left hand motor rule shows the movement of
a POSITIVE charge in a magnetic field.
36
Atomic number is
the number of protons in the nucleus.
37
Mass number is
the number of protons + neutrons in the nucleus.
38
The atomic number (no. protons) is equal to
no. electrons, if the atom is not ionized.
39
Nuclear Fission is
when a big nucleus splits into 2 or more smaller nuclei, emitting some neutrons. Induced fission is when a neutrons is fired at the nucleus, spontaneous is when an unstable nucleus breaks apart.
40
Nuclear Fusion is
when 2 or more small nuclei combine to make a big nucleus.
41
Energy released from a nuclear reaction =
mass defect x speed of light squared
42
E0 in terms of the photoelectric effect is the
work function.
43
F0 in terms of the photoelectric effect is the
threshold frequency.
44
A crest and a trough are said to be
exactly out of phase.
45
Two crests are said to be
in phase.
46
What is the diffraction grating equation?
dsinθ = nλ
d = 1 / no. lines per metre
47
For a maximum the path difference =
nλ
48
For a minimum the path difference =
(n + 1/2) λ
49
If the angle of incidence = critical angle then
the ray is refracted at 90° to the normal.
50
If the angle of incidence > critical angle then
the ray is totally internally refracted (and θr = θi).
51
What is the name given to E0 in an emission spectra?
Ground state
52
In an emission spectra the 0J level is also known as
the ionisation level.
53
In an emission spectra levels E1 and onwards are called the
excited states.
54
If the energy level increases then the spectra is an
absorption spectra.
55
If the energy level decreases then the spectra is an
emission spectra.
56
emf stands for
electromotive force.
57
emf =
lost volts + p.d. across terminals
58
Each lepton has a 'ghost like' particle called a neutrino associated with it, how were these discovered?
During beta decay processes as the total momentum wasn't conserved.
59
Around a moving charge/electron there is
a magnetic field as well as an electric field.
60
Photoelectric effect is much more likely given a radiation with a higher frequency eg.
UV
61
The higher the irradiance,
the more photons.
62
Power or Irradiance =
no. photons x energy per photon (N x hf)
63
Vo stands for what?
Stopping potential, negative voltage required to stop electrons from jumping a gap in a circuit (photoelectric effect).
64
Frequency =
1 / T or no. waves / time
65
What is the root mean square (RMS)?
The RMS of an alternating current or voltage is equal to the value of if it were direct current or voltage.
66
V r.m.s =
| I r.m.s =
1 / √2 x peak voltage
| 1 / √2 x peak current
67
What is emf?
Electromotive force is the electrical energy supplied to each coulomb of charge which passes through the source. (Basically the same a voltage)
68
In terms of emf, what is:
r?
R?
```
Internal resistance
Load resistance (resistance of all components added up)
```
69
When the terminals of a supply are joined (short circuited) what is the emf?
E = Ir since R = 0Ω
70
Graph questions:
| Values can be obtained from a graph by
rearranging the relevant formula to match y = mx + c.
71
Circuit Rules - Series
Is =
Vs =
RT =
I1 = I2 = I3 ....
V1 + V2 + V3 ....
R1 + R2 + R3 ....
72
Circuit Rules - Parallel
Is =
Vs =
1 / RT =
I1 + I2 + I3 ....
V1 = V2 = V3 ....
1 / R1 + 1 / R2 + 1 / R3 ....
73
Total power in a circuit =
P1 + P2 + P3 ....
74
What is a semiconductor?
An insulator eg. silicon that is doped with an impurity eg. indium to make it conduct better.
75
What property of conductors make them conduct better than insulators?
More electrons.
76
What is doping?
When an impurity with either more or less electrons than the insulator is added to it.
77
N - type semiconductor:
An insulator lattice, eg. silicon with 4 electrons, is doped with an atom with MORE electrons, eg. phosphorus with 5 electrons. This leaves 1 free electron to move around making the n - type conduct better.
78
P - type semiconductor:
An insulator lattice, eg. silicon with 4 electrons, is doped with an atom with LESS electrons, eg. indium with 4 electrons. This leaves a free hole which 'moves' as the electrons move making the p - type conduct better.
79
Band Structure:
An atom has electrons of different energy levels orbiting it. There can be many energy level but no electrons can exist in the band gaps between levels.
80
Filled Band:
Electrons that are in filled orbital bands are in the filled band.
81
Valence Band:
Electrons in outer orbital shells will be in the valence band (valence electrons).
82
Fermi Level:
The energy level that most electrons will be at for a temperature of 0 Kelvin.
83
Conduction Band:
A partially filled band where electrons are able to move.
84
How doping affects band structure:
| N - type
Extra electrons, Fermi level closer to conduction band making it easier for electrons to move up to conduction band.
85
How doping affects band structure:
| P - type
Fewer electrons, Fermi level closer to valence band making it easier for holes to be created in valence band.
86
What is another name for a p-n junction?
Diode.
87
What causes the depletion layer in a p-n junction?
Diffusion - when electrons from the n - type move across into the p - type and holes move to the n - type.
88
What is the depletion layer in a p-n junction?
A region where there are no free charge carriers which results in a potential barrier (usually 0.7 V for silicon).
89
What is the p - type side of the diode also called?
Anode.
90
What is the n - type side of the diode also called?
Cathode.
91
Why is a n - type given its name?
Because electrons are negatively charge, NOT because the n - type is negatively charged.
92
Why is a p - type given its name?
Because the holes are positively charge, NOT because the p - type is positively charged.
93
What is capacitance?
The ability of a component to store charge. A charged capacitor stores electrical energy.
94
What is a capacitor made of?
2 conducting layers with an insulator in between.
95
Capacitance (C) =
Charge (Q) / Voltage (V)
96
What is capacitance measured in?
Farads (F), usually micro or nano farads.
97
1 farad =
1 coulomb per volt
98
When a current is switched on electrons will flow
onto one plate and away from the other.
99
When charging a capacitor the negatively charged plate will
repel the electrons approaching it. Energy from the Vs must be used to overcome this repulsion.
100
Energy stored in a capacitor =
1/2 QV
(work done moving a charge in an electric field = QV but since current and voltage constantly change in a capacitor the 1/2 is needed as an averaging factor)
101
As a capacitor charges:
- current decreases from I max
| - voltage increases until Vc = Vs
102
As a capacitor discharges:
- current increases until I max
| - voltage decreases until Vc = 0V
103
Factors affecting rate of charge and discharge of a capacitor:
- resistance of resistor in circuit with capacitor
| - capacitance of capacitor
104
What is the 'time constant'?
Resistance x capacitance (unit is seconds)
It is related to the time taken for a capacitor to charge r discharge.
105
Large capacitance and large resistance increase
the charge and discharge time of a capacitor.
106
When the p - n junction is unbiased what happens to the Fermi level?
It is flat across the junction (straight line).
107
When the diode is unbiased, work done to move an electron across the junction =
e x Vi
Vi - potential barrier
e - charge of electron
108
What is forward bias?
N - type connected to negative terminal and vice versa. Electrons are given energy by the supply so that they can move across the junction.
109
What is reverse bias?
N - type connected to positive terminal and vice versa. The voltage applied causes the depletion layer to increase so virtually no conduction takes place.
110
Which side of a cell symbol is the negative terminal?
The short side.
111
What is the name given to current that flows when a diode is in the reverse bias?
Leakage current.
112
What effect does the forward bias have in terms of band structure?
Lowers the bands on the p - type side so it is easier for electrons to flow across the junction as less energy is required.
113
When the diode is in forward bias, work done to move an electron across the junction =
e(Vi - Va)
Va - supply voltage
Vi - potential barrier
e - charge of electron
114
What does a diode in forward bias create?
An LED.
115
What does a diode in reverse bias create?
A photodiode.
116
How does an LED emit light?
Electrons drop from conduction band in n - type to valence band in p - type when they cross the junction. When they drop they recombine with a hole. Due to conservation of energy the lost energy from this recombination is emitted as a photon.
117
Energy (released by recombination in an LED) =
hf
f - frequency of light emitted
118
What are the 2 modes a photodiode can be in?
- Photovoltaic
| - Photoconductive
119
What effect does the reverse bias have in terms of band structure?
Raises the bands on the p - type side so it is harder for electrons to flow across the junction as more energy is required. No conduction takes place as the voltage supply is trying to make electrons gain energy.
120
When the diode is in forward bias, work done to move an electron across the junction =
e(Vi + Va)
Va - supply voltage
Vi - potential barrier
e - charge of electron
121
In photovoltaic mode, the voltage generated by the diode is proportional to
the intensity of the light.
122
How does a diode in photovoltaic mode generate a voltage?
Photons incident on the diode have their energy absorbed which creates electron hole pairs (frees electrons). A voltage is generated by the separation of these pairs.
123
What is photoconductive mode?
When a photodiode is connected to a supply voltage in reverse bias so it acts like an LDR.
124
In photoconductive mode, the greater the intensity of light means
the more free electrons and so the less resistance since conduction is better.
125
Irradiance (I) =
Power (P) / Area (A)
Area in m squared
Power in Watts
126
Definition of irradiance:
The power per unit of area.
127
What is irradiance measured in?
Wm squared
Watts per metre squared
128
Irradiance (I) =
k / d squared
```
k = a constant
d = distance
```
129
The irradiance equations only apply to point sources, give an example of a source of light which is not a point source?
A laser, as it produces a parallel beam.
130
Stars and gas in most galaxies are moving quicker than their luminosity predicts so what is adding to gravity that we cannot see?
Dark matter.
131
What is dark energy?
Something that overcomes the force of gravity.
