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AS physics > Electrons, waves and photons > Flashcards

Electrons, waves and photons Flashcards

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1
Q

Charge carriers

A

in liquids(electrolytes), generally ions
in metals, free delocalised electrons
if one end of a wire is positive and one end is negative, electrons will flow

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2
Q

factors affecting current

A

temperature
cross sectional area
speed of electrons

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3
Q

conventional current

A

positive to negative

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4
Q

number density

A

number of free charge carriers per unit volume.
for conductors n ≈ 10^28
for semi conductors n≈10^17
for insulators, n is much lower

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5
Q

derrivation of equation for current

A

I =Q/t
no. electrons = nV(density x volume)
charge = neV (no. electrons x e)
I = (neV)/t
V/t = (Axl)/t = Av where v is mean drift velocity
I =Anev

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6
Q

How does an electron gun work

A

Thermionic emission - metal filament is heated by electrical current; some electrons gain enough KE to escape surface of metal

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7
Q

What does an electron gun do and what is it used for

A

Fires narrow beam of electrons
Can be used to ionise particles
used in electron microscopes, mass spectrometers or oscilloscopes

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8
Q

why does higher temperature increase resistance

A

if temperature increases, positive ions have more internal energy and vibrate with greater frequency about their mean positions
frequency of collisions with charge carriers increases, so resistance increases as more work is done by charge carriers

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9
Q

factors affecting resistance

A

temperature
material
length
cross sectional area

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10
Q

relationship between length and resistance

A

R is proportional to L

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11
Q

relationship between cross sectional area and resistance

A

R is proportional 1/A

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12
Q

Resistivity

A

How hard it is for charge to flow through a material
ρ(rho), measured in ohm meters

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13
Q

Resistivity of conductors and insulators

A

Good conductors : ρ is of the order of10^-8
Insulators: ρ is of the order of 10^16
Semiconductors are in between

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14
Q

Negative temperature coefficient

A

As temperature increases, resistance decreases
In some semiconductors, as temp increases increases, number density increases

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15
Q

Uses of thermistors

A

Used in thermometers, thermostats and inside electrical devices

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16
Q

How do LDRs work

A

Made from semiconductors in which number density changes depending on light intensity
As light intensity increases, number density increases so resistance decreases

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17
Q

Derivation of power equation

A

P =W/t
W=QV
P=QV/t
I=Q/t
P=IV

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18
Q

What is one unit of energy

A

1 kWh (kilowatt-hour)

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19
Q

Laws in series circuits

A

Current is the same in every position
e.m.f is shared between components- components with greater resistance take greater share

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20
Q

Laws in parallel circuit

A

Current is split between branches - branch with higher resistance have smaller current
Each loop has equal p.d and this must be equal to the emf

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21
Q

Derivation of resistance equation (series)

A

Vt = V1+V2…
It = I1=I2
V=IR
IRt =IR1+IR2…
Rt = R1+R2…

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22
Q

Derivation of resistance equation (parallel)

A

I=I1+I2…
Vt = V1=V2…
R=V/I
It/V = I1/V = I2/V…
1/Rt = 1/R1 = 1/R2 …

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23
Q

lost volts

A

difference between emf and terminal pd(measured at the terminals of the power source)

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24
Q

Calculating lost volts

A

r= internal resistance
lost volts = Ir

draw a graph of I against V
emf = Ir +V
y-intercept = emf
gradient = -r

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25
Q

should r be high or low

A

some devices must have a very low r in order to have a high current through them
other must have high r as a safety feature

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26
Q

How do potential dividers work

A

Pd is shared across components depending on ratio of resistance
Pd across each resistor must add up to emf

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27
Q

Sensing circuits

A

Connecting a thermistor or LDR in a potential divider circuit creates a circuit where pd is dependent on temp/light

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28
Q

Potentiometer

A

Variable resistor with three terminals and a sliding contact
Adjusting this contact varies the pd
Can be made very compact
V-out can be changed across range of 0v-emf
Can be either linear or logarithmic

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29
Q

Types of waves from an earthquake

A

primary (P-waves) - longitudinal
secondary(S-waves) - transverse

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30
Q

Progressive wave

A

An oscillation that travels through matter/ a vacuum
transfer energy but not matter
particles vibrate but don’t move along the wave
a displaced particle (not in equilibrium position) experiences restoring force and it pulled back

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31
Q

wave profile

A

graph showing displacement against distance

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32
Q

phase difference

A

The difference between the displacements of particles along a wave
One complete = 360 degrees or 2pi radians

33
Q

Reflection

A

When a wave changes direction at a boundary between two different media, remaining in the original medium

34
Q

Law of reflection

A

angle of incidence = angle of reflection

35
Q

Refraction

A

When a wave changes direction as it changes speed when it passes from one medium to another
If wave slows down, it will bend toward the normal
If wave speeds up it will bend away from the normal
Refraction effects wavelength, frequency stays the same

36
Q

When do waves change speed

A

Sound waves speed up in denser mediums
Light waves slow down in denser mediums
Water waves slow down when they enter shallower water

37
Q

Diffraction

A

When waves pass through a gap or travel through an obstacle, they spread out
Speed, wavelength and frequency stay the same
Diffraction effects are most significant when the size of the gap or obstacle is about the same as the wavelength of the wave

38
Q

Polarisation

A

Particles oscillate along one direction only so wave is confined to a single plane
The wave is plane polarised

39
Q

Partial polarisation

A

When transverse waves reflect off a surface they become partially polarised - there are only more waves in one plane, but waves are not completely polarised

40
Q

Relationship between intensity and distance

A

I is proportional to 1/r^2

41
Q

Relationship between intensity and amplitude

A

Intensity is proportional to amplitude^2

42
Q

What are EM waves

A

EM (electromagnetic) waves can be though of as electric and magnetic fields oscillating at right angles to each other
A wave is a period disturbance in a material or space; an EM wave is a disturbance in an electric and magnetic field

43
Q

Wavelengths of EM waves

A

> 10^6 Radiowaves 10^-1 Microwaves 10^-3 Infrared 7x10^-7 Visible 4x10^-7 Ultraviolet 10^-8 X-rays {overlap from 10^-10 to 10^-13} gamma <10^-16

44
Q

How are X rays and gamma defined in the area of overlapping wavelength

A

X rays are emitted by fast-moving electrons, whereas gamma rays come from the unstable atomic nuclei

45
Q

Are EM waves plane polarised

A

Most naturally occurring EM waves are unpolarised
Microwaves produced artificially tend to be plane polarised

46
Q

Refractive index

A

Ratio of speed of light through a vacuum to speed of light through the material
n of air = 1

47
Q

Refraction law

A

n sinθ = k
n1sinθ1 = n2sinθ2

48
Q

Total internal reflection

A

TIR occurs at the boundary between two different media
When light strikes the boundary at a large angle to the normal, all the light is reflected back into original medium - no light energy is refracted
This only happens when moving from a higher refractive index into a lower one

49
Q

Critical angle formula derivation

A

n1sinθ1 = n2sinθ2
nsinC = n(air)sin90
nsinC = 1
sinC =1/n

50
Q

How to calculate C

A

Use a semi-circular block and a ray box - find the angle where it is no longer refracted at all

51
Q

Optical fibres

A

Designed to totally internally reflected pulses of visible light travelling through them
Used for transmitting data and imaging during keyhole surgery
A simple optical fibre has a fine glass core surrounded by a glass cladding with a lower refractive index

52
Q

Young double slit experiment

A

Pass light through a colour filter to make it monochromatic
Pass light through a double slit - it arrives in phase
Light diffracts through the slits so each slit acts as a source of coherent waves, forming an interference patterns that can be seen as alternating bright and dark regions

53
Q

How to calculate wavelength from double slit experiment

A

λ = ax/D

54
Q

Stationary waves

A

When two progressive waves with the same frequency travelling in opposite directions are superposed, it forms a stationary/standing wave
In certain points(nodes) they are in antiphase
At other points (antinodes) they are in phase

55
Q

λ in a stationary wave

A

The separation between two adjacent nodes or antinodes = half the wavelength of the original progressive wave

56
Q

Energy in a stationary wave

A

Stationary waves have no net transfer of energy

57
Q

What points on a stationary wave are in phase/antiphase

A

In between adjacent nodes, all particles in a stationary wave are in phase
On different sides of a node the particles are in antiphase

58
Q

Fundamental mode of vibration

A

the wavelength of the progressive wave is double the length of the string

59
Q

Stationary waves on a string

A

Nodes at each end.
First harmonic: λ=2L, f = f0
Second harmonic: λ=L, f=2f0
Third harmonic: λ=2/3L, f=3f0

60
Q

f0

A

fundemental frequency is the minimum frequency of a stationary wave for a string

61
Q

Stationary waves in a tube closed at one end

A

There must be an antinode at the open end and a node at the closed end
Harmonics are all odd multiples of f0
First harmonic: λ=4L, f = f0
Second harmonic: λ=4/3L, f=3f0
Third harmonic: λ=4/5L, f=5f0

62
Q

Investigation wavelength in a tube closed at one end

A

Use a tuning fork or speaker
Hold it above a tube submerged in water. Change the length above water until sound it louder
Find multiple loud points - the distance between these is 1/2 wavelength

63
Q

Stationary waves in an open tube

A

Antinodes at each end.
All integer number of nf0 are possible
First harmonic: λ=2L, f = f0
Second harmonic: λ=L, f=2f0
Third harmonic: λ=2/3L, f=3f0

64
Q

Photons

A

small particles that make up light

65
Q

Equation for energy of a photon

A

E = hf where h=Planck constant

66
Q

eV

A

electron volts
the energy transferred to or from an electron when it moves through a p.d of 1V
= 1.60 x10^-19 J

67
Q

Experiment to find Planck constant

A

Find threshold voltage for LEDs (voltage where they only just turn on)
At this voltage, E= energy of emitted photon = QV
eV=hf
plot a graph of V against 1/lambda ; gradient = hc/e

68
Q

Photoelectric effect

A

When electrons above a certain frequency are shone on metals, electrons are emitted

69
Q

Gold leaf electroscope

A

When the gold leaf and stem both have a negative charge, they will repel
If zinc is placed on top of the negatively charged plate:
- visible light has no effect
- UV light causes gold leaf to fall back down

70
Q

Observations from gold leaf electroscope

A

Photoelectrons are only emitted above the threshold frequency for each model
Above this frequency emission of photoelectrons is instantaneous
Increasing intensity doesn’t effect KE of waves - it only leads to more electrons being emitted. Only way to increase energy is increasing frequency

71
Q

Work function

A

Minimum energy required to free an electron from the surface of the metal

72
Q

Einstein’s photoelectric equation

A

energy of a single photon = minimum energy to free an electron + max energy of emitted electron

hf = work function + KEmax

73
Q

Why is KE max not KE of all electrons

A

If electrons are not on the surface, the energy taken to release them is more than the work function of the metal

74
Q

Finding work function of a metal

A

At exactly threshold frequency, KEmax =0 so hf = phi
In a graph KEmax = hf - phi
work function = negative y-intercept
gradient = h

75
Q

Wave particle duality

A

A model used to describe how all matter has both wave and particle properties

76
Q

Electron diffraction

A

Under certain conditions, electrons can be made to diffract- if an electron gun fires electrons at a thin piece of polycrystalline graphite, electrons pass between carbon atoms and diffraction takes place
A diffraction pattern of rings is seen on the end of the tube

77
Q

De Broglie equation

A

wavelength = h/momentum

78
Q

Relationship between wavelength and Ek

A

λ∝1/√Ek

AS physics (7 decks)

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