Electrons, waves and photons Flashcards
Charge carriers
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
factors affecting current
temperature
cross sectional area
speed of electrons
conventional current
positive to negative
number density
Definition and values
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
derrivation of equation for current
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
How does an electron gun work
Thermionic emission - metal filament is heated by electrical current; some electrons gain enough KE to escape surface of metal
What does an electron gun do and what is it used for
Fires narrow beam of electrons
Can be used to ionise particles
used in electron microscopes, mass spectrometers or oscilloscopes
why does higher temperature increase resistance
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
factors affecting resistance
temperature
material
length
cross sectional area
relationship between length and resistance
R is proportional to L
relationship between cross sectional area and resistance
R is proportional 1/A
Resistivity
How hard it is for charge to flow through a material
ρ(rho), measured in ohm meters
Resistivity of conductors and insulators
Good conductors : ρ is of the order of10^-8
Insulators: ρ is of the order of 10^16
Semiconductors are in between
Negative temperature coefficient
As temperature increases, resistance decreases
In some semiconductors, as temp increases increases, number density increases
Uses of thermistors
Used in thermometers, thermostats and inside electrical devices
How do LDRs work
Made from semiconductors in which number density changes depending on light intensity
As light intensity increases, number density increases so resistance decreases
Derivation of power equation
P =W/t
W=QV
P=QV/t
I=Q/t
P=IV
What is one unit of energy
1 kWh (kilowatt-hour)
Laws in series circuits
Current is the same in every position
e.m.f is shared between components- components with greater resistance take greater share
Laws in parallel circuit
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
Derivation of resistance equation (series)
Vt = V1+V2…
It = I1=I2
V=IR
IRt =IR1+IR2…
Rt = R1+R2…
Derivation of resistance equation (parallel)
I=I1+I2…
Vt = V1=V2…
R=V/I
It/V = I1/V + I2/V…
1/Rt = 1/R1 +1/R2 …
lost volts
difference between emf and terminal pd(measured at the terminals of the power source)
Calculating lost volts
r= internal resistance
lost volts = Ir
draw a graph of V against I
emf = Ir +V
y-intercept = emf
gradient = -r
should r be high or low
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
How do potential dividers work
Pd is shared across components depending on ratio of resistance
Pd across each resistor must add up to emf
Sensing circuits
Connecting a thermistor or LDR in a potential divider circuit creates a circuit where pd is dependent on temp/light
Potentiometer
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
Types of waves from an earthquake
primary (P-waves) - longitudinal
secondary(S-waves) - transverse
Progressive wave
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
wave profile
graph showing displacement against distance
phase difference
The difference between the displacements of particles along a wave
One complete = 360 degrees or 2pi radians
Reflection
When a wave changes direction at a boundary between two different media, remaining in the original medium
Law of reflection
angle of incidence = angle of reflection
Refraction
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
When do waves change speed
Sound waves speed up in denser mediums
Light waves slow down in denser mediums
Water waves slow down when they enter shallower water
Diffraction
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
Polarisation
Particles oscillate along one direction only so wave is confined to a single plane
The wave is plane polarised
Partial polarisation
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
Relationship between intensity and distance
I is proportional to 1/r^2
Relationship between intensity and amplitude
Intensity is proportional to amplitude^2
What are EM waves
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
Wavelengths of EM waves
> 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
How are X rays and gamma defined in the area of overlapping wavelength
X rays are emitted by fast-moving electrons, whereas gamma rays come from the unstable atomic nuclei
Are EM waves plane polarised
Most naturally occurring EM waves are unpolarised
Microwaves produced artificially tend to be plane polarised
Refractive index
Ratio of speed of light through a vacuum to speed of light through the material
n of air = 1
Refraction law
n sinθ = k
n1sinθ1 = n2sinθ2
Total internal reflection
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
Critical angle formula derivation
n1sinθ1 = n2sinθ2
nsinC = n(air)sin90
nsinC = 1
sinC =1/n
How to calculate C
Use a semi-circular block and a ray box - find the angle where it is no longer refracted at all
Optical fibres
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
Young double slit experiment
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
How to calculate wavelength from double slit experiment
λ = ax/D
Stationary waves
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
λ in a stationary wave
The separation between two adjacent nodes or antinodes = half the wavelength of the original progressive wave
Energy in a stationary wave
Stationary waves have no net transfer of energy
What points on a stationary wave are in phase/antiphase
In between adjacent nodes, all particles in a stationary wave are in phase
On different sides of a node the particles are in antiphase
Fundamental mode of vibration
the wavelength of the progressive wave is double the length of the string
Stationary waves on a string
Nodes at each end.
First harmonic: λ=2L, f = f0
Second harmonic: λ=L, f=2f0
Third harmonic: λ=2/3L, f=3f0
f0
fundemental frequency is the minimum frequency of a stationary wave for a string
Stationary waves in a tube closed at one end
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
Investigation wavelength in a tube closed at one end
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
Stationary waves in an open tube
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
Photons
small particles that make up light
Equation for energy of a photon
E = hf where h=Planck constant
eV
electron volts
the energy transferred to or from an electron when it moves through a p.d of 1V
= 1.60 x10^-19 J
Experiment to find Planck constant
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
Photoelectric effect
When photonsabove a certain frequency are shone on metals, electrons are emitted
Gold leaf electroscope
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
Observations from gold leaf electroscope
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
Work function
Minimum energy required to free an electron from the surface of the metal
Einstein’s photoelectric equation
energy of a single photon = minimum energy to free an electron + max energy of emitted electron
hf = work function + KEmax
Why is KE max not KE of all electrons
If electrons are not on the surface, the energy taken to release them is more than the work function of the metal
Finding work function of a metal
At exactly threshold frequency, KEmax =0 so hf = phi
In a graph KEmax = hf - phi
work function = negative y-intercept
gradient = h
Wave particle duality
A model used to describe how all matter has both wave and particle properties
Electron diffraction
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
De Broglie equation
wavelength = h/momentum
Relationship between wavelength and Ek
λ∝1/√Ek
