ALL OF TURNING POINTS

Question 1

Describe the process by which the discharge tube begins to glow

Answer 1

1. The high potential difference across the discharge tube will ionise the gas atoms, forming positive ions and electrons.
2. The positive gas ions are accelerated towards the cathode and when they collide with it they release even more electrons.
3. The electrons are accelerated along the tube (because the gas is at low pressure, the electrons are accelerated to high speeds) and collide with gas atoms causing them to become excited. The atoms will quickly de-excite and release photons of light.

The glow is brightest at the cathode because here the gas ions and electrons can recombine and emit photons of light.

Question 2

What is thermionic emission

Answer 2

Where a metal is heated until the free electrons on its surface gain enough energy and are emitted.

Question 3

What do electron guns do and how

Answer 3

Electron guns use a potential difference in order to accelerate electrons, which are released from the cathode by heating it (thermionic emission).

Question 4

Describe the method using ONLY a magnetic field to measure the specific charge of an electron

Answer 4

1.Electrons are accelerated using an electron gun and enter the fine beam tube perpendicular to the direction of the field.

3. 2. The magnetic force on the electrons acts perpendicular to their motion, and therefore the electrons move in a circular path because the magnetic force acts as a centripetal force.
4. As the electrons move through the fine beam tube, they collide with gas atoms causing them to become excited, the gas atoms then de-excite releasing photons of light meaning the path of the electrons is visible, so the radius of their circular path can be measured.

Question 5

Describe the method using a magnetic field AND an electric field to measure the specific charge of an electron

Answer 5

1. Electrons are accelerated using an electron gun and enter the apparatus perpendicular to the direction of both fields. The electrons will be deflected upwards by the electric field, while being deflected downwards by the magnetic field (due to Fleming’s left hand rule).
2. The strengths of these fields are adjusted until the electron beam passes through the crossed fields undeflected, therefore the electric and magnetic forces are equal and opposite.

Question 6

Describe Milikans experiment to determine the charge of an electron

Answer 6

An atomizer is used to spray tiny droplets of oil, which are negatively charged due to friction.

These droplets fall until they reach two parallel plates which form a uniform electric field, as the
droplets are charged they will experience an electric force.

The strength of the field can be adjusted by changing the potential difference between the plates, until the observed oil droplet becomes stationary.

Oil drops mass must be measured first.

The potential difference across the plates is removed so the droplet no longer experiences an electric force upwards, and so will begin to fall.

The droplet will experience a resistive force upwards (known as a viscous drag force) and its weight downwards, the viscous drag force (F) can be calculated using Stokes’ law

The terminal velocity of the droplet can be measured by using a microscope with a calibrated graticule and measuring the distance travelled by the droplet in a certain amount of time.

When the droplet is moving at terminal velocity, the viscous force and weight are equal

Using the formula, the radius of the oil droplet can be found meaning that its mass can be measured, and so the charge of the droplet can be calculated.

Question 7

What did Milikan observe in his oil drop experiment and why was it significant

Answer 7

Charge of all the oil droplets he observed was an integer multiple of 1.60x10^-19 this is significant because it shows that charge is quantised, meaning it exists in discrete packets of 1.60x10^-19C, which is the smallest possible magnitude of charge

Question 8

How did Newtons corpuscular theory of light explain reflection

Answer 8

The corpuscles collide with the surface and a repulsive force pushes them back, causing their component of velocity perpendicular to the surface to change direction, while their component of velocity parallel to the surface stays the same.

Question 9

How did Newtons corpuscular theory of light explain refraction

Answer 9

As the corpuscles approach a denser medium, short-range forces ofattraction cause their component of velocity perpendicular to the surface to increase, while the parallel component of velocity stays the same, therefore the light will bend towards the normal. According to Newton’s explanation light travels faster in denser mediums.

Question 10

What was Huygens’ wave theory of light

Answer 10

Light was a wave and that every point on a wavefront is a point source to secondary wavelets, which spread out to form the next wavefront

Question 11

How did Huygens wave theory of light explain reflection

Answer 11

As the whole wavefront will not reach the surface at once (unless it is travelling perpendicular to the surface), wavelets spread away from the surface once they reach it and rejoin with others to reform the reflected wavefront.

Question 12

How did Huygens wave theory of light explain refraction

Answer 12

It was assumed that light travels slower in a more dense medium, therefore as it entered a more optically dense medium it would slow down and therefore bend towards the normal.

Question 13

Why was Newtons theory of light accepted over Huygens’

Answer 13

Newton had a very high reputation at the time, also diffraction had not yet been observed and the speed of light hadn’t been measured.

Question 14

What would be observed in Youngs double slit experiment if Newtons corpuscular theory of light was true

Answer 14

An interference pattern wouldn’t be formed during the above experiment, instead there would only be two bright fringes corresponding to the two slits in the apparatus since corpuscles travel in straight lines

Question 15

What experiment proved Huygens theory to be correct and why

Answer 15

Youngs double slit experiment as it demonstrated demonstrated diffraction and interference of light,
which are both wave properties

Question 16

What are electromagnetic waves

Answer 16

Electromagnetic waves are formed of an alternating magnetic and electric fields travelling in phase and at right angles to each other. The direction of wave travel is perpendicular to the oscillations of the electric and magnetic fields.

Question 17

What does the permeability of free space, μ0, relate to

Answer 17

the magnetic flux density produced by a wire to the current in the wire in free space.

OR

Relates to the magnetic field strength in free space

Question 18

What does the permittivity of free space ,ε0, relate to

Answer 18

Electric field strength in free space

Question 19

Equipment used by Hertz to discover radiowaves

Answer 19

Dipole reciever
Loop of wire with a gap

Question 20

What does a dipole receiver do

Answer 20

This detects the waves’ electric field. This is made by placing a second set of charged plates parallel to those forming the high voltage sparks.

Question 21

What does a loop of wire with a gap do

Answer 21

This will detect the waves’ alternating magnetic field as the field will enter the loop causing a change in magnetic flux, inducing a potential difference which will cause a spark to cross the gap in the wire.

Question 22

How did Hertz discover radiowaves waves and confirm they were EM waves

Answer 22

By placing a metal sheet in front of the apparatus, the radio waves are reflected back onto themselves causing stationary waves to be formed. By using one of the detectors above, you can find the distance between adjacent nodes in order to find the wavelength, and using the frequency of the waves, calculate their speed.

The speed calculated by Hertz was found to be the same as Maxwell’s predicted value of the speed of electromagnetic waves, which helped confirm that radio waves were EM waves.

Question 23

What would happen in the toothed wheel experiment to determine speed of light if the speed of rotation of the wheel was doubled after the light can no longer be seen

Answer 23

If this speed is doubled, the light would be visible again because the light will now return through the gap next to the one it initially passes through.

Question 24

How did Fizeau measure the speed of light

Answer 24

A pulsed beam of light is passed through a gap in a toothed wheel rotating at a slow speed.

The beam of light reflects on a mirror a large distance behind the wheel causing it to return
back through the same gap between teeth in the wheel.

The speed of rotation of the wheel is increased until the light beam can no longer be seen because it is blocked by a tooth in the wheel next to the gap it could previously pass through.

Question 25

What is a black body

Answer 25

A black body absorbs and emits all possible wavelengths of radiation

Question 26

What did wave theory predict

Answer 26

Wave theory predicted that as the wavelength of radiation decreases, the intensity of the radiation would increase, leading to a prediction of infinite amount of ultraviolet radiation being emitted

Question 27

Why did the wave theory lead to the ultraviolet catastrophe

Answer 27

Wave theory predicted an impossible amount of UV radiation and it could not be used to explain experimental measurements.

Question 28

How can the UV catastrophe be resolved

Answer 28

Using Planck's interpretation of EM waves

Question 29

What was Planck's interpretation of EM waves

Answer 29

EM waves travel in discrete packets called quanta, which have an energy directly proportional to their frequency (E=hf).

Question 30

Give 4 reasons why photoelectric effect could not be explained by wave theory

Answer 30

1.Wave theory suggests that any frequency of light should be able to cause photoelectric emission as the energy absorbed by each electron will gradually increase with each incoming wave, and so can’t explain the existence of a threshold frequency. 2. The photoelectric effect is immediate, which contradicts wave theory which suggests time is needed for the energy supplied to the electrons to reach the work function 3. Increasing the intensity of the light does not increase the speed of photoelectric emission as would be suggested by wave theory, but instead it increases the number of photoelectrons released per second. 4. Photoelectrons are released with a range of kinetic energies.

Question 31

If a potential is applied across a metal surface which makes it positive is increased, explain what happens to the number of photoelectrons released

Answer 31

The kinetic energy of the photoelectrons will decrease as they must do work against the electrostatic force of attraction towards the surface. As this potential is increased, the number of photoelectrons released will decrease, because the number of photoelectrons with a high enough kinetic energy to be emitted decreases.

Question 32

What is stopping voltage

Answer 32

The potential difference you would need to apply across the metal to stop the photoelectrons with the maximum kinetic energy

Question 33

What is the De Broglie hypothesis

Answer 33

All particles have a wave-like nature and a particle nature, and that the wavelength of any particle can be found using the following equation: λ = h/mv

Question 34

What was experimental evidence for the De Broglie hypothesis

Answer 34

Electron diffraction

Question 35

Describe and explain the change in the diffraction pattern when the accelerating voltage is increased

Answer 35

The speed of electrons increases and so their de Broglie wavelength decreases, so the electrons are diffracted more and the fringe spacing will decrease (the rings will move closer)

Question 36

Why do electron microscopes have a much higher resolving power than light microscopes

Answer 36

Wavelength of an electron beam is much smaller than that of light,

Question 37

Describe the relationship between wavelength and resolving power of microscopes

Answer 37

As the wavelength of the electrons decreases the resolving power of the microscope increases.

Question 38

Describe how a TEM microscope works

Answer 38

In a TEM electrons are accelerated by an electron gun, and pass through a set of magnetic lenses all of which have different purposes, passing through an extremely thin sample so that the electrons do not slow down, and their wavelength doesn’t change. All of the lenses will leave electrons at the centre of the lens undeflected, but will deflect electrons at the edge of the lens towards the axis (centre) of the microscope.

Question 39

Name all of the lenses in a TEM microscope

Answer 39

Condensor lens Objective lens Projector lens

Question 40

What is the function of the condenser lens

Answer 40

This is the first lens the electrons beams pass through and this lens deflects the electrons so that they form a wide parallel beam, which is directed at the sample

Question 41

What is the function of the objective lens

Answer 41

This lens will form an image of the sample, which is directly above it.

Question 42

What is the function of the projector lens

Answer 42

This will magnify the image made by the objective lens and project it onto the fluorescent screen.

Question 43

What factor affect the resolving power of a TEM

Answer 43

Wavelength Sample thickness Electrons travelling at a range of speeds

Question 44

How does sample thickness affect the resolving power of a TEM

Answer 44

As electrons pass through the sample they will slow down, causing their wavelength to increase and so the resolving power is decreased THICKER SAMPLE = LOWER RESOLVING POWER

Question 45

How does electron travelling at a range of speeds affect the resolving power and image quality of a TEM

Answer 45

As the electron gun emits electrons through thermionic emission some electrons may lose kinetic energy while leaving the metal (due to collisions). This leads to electrons travelling at different speeds, having different wavelengths and therefore being diffracted by different amounts which causes blurring of the image (known as aberration).

Question 46

What must the De Broglie wavelength of electrons have to be to resolve details around the size of an atom

Answer 46

Around the diameter of an atom which is 1x10^-10m or 0.1nm

Question 47

Why does quantum tunnelling occur

Answer 47

Quantum tunnelling occurs due to the wave nature of electrons, meaning that if the barrier they are trying to cross is small enough electrons can move across it just like light waves would be able to. The smaller the barrier/gap, the more likely it is that tunnelling will occur.

Question 48

What are the 2 ways an STM can operate

Answer 48

Constant height mode Constant current mode

Question 49

Describe constant height mode in an STM

Answer 49

The probe is kept at a constant height as it moves across the surface, and the tunnelling current is recorded and used to image the surface of the object.

Question 50

Describe constant current mode in an STM

Answer 50

The current is constantly monitored and fed back to the microscope allowing it to adjust the probe’s height so the current is kept constant. The movement of the probe can then be used to image the surface

Question 51

What is absolute motion

Answer 51

The idea that everything moves relative to the ether, which they believed was a substance which permeated the entire universe.

Question 52

Why was it believed rotating the apparatus by 90 degrees would cause a change in interference pattern

Answer 52

It was believed the light moving parallel to the Earth’s motion will take longer to travel, therefore rotating the apparatus by 90° would cause a shift in the interference pattern. However, the interference pattern experienced no shift, showing that the time taken for light to travel was unaffected by rotation of the apparatus

Question 53

What were the conclusions made from the results of the Michelson-Morley experiment

Answer 53

The ether doesn’t exist or the Earth drags the ether along with it as it moves The speed of light is invariant in free space, meaning that the speed of light is independent of the motion of the source or the observer.

Question 54

What is an inertial frame of reference

Answer 54

Inertial frames of reference are those which move at a constant velocity relative to each other. No acceleration.

Question 55

What are Einstein's two postulates

Answer 55

1. The speed of light in free space is invariant - The speed of light is independent of the motion of the source or the observer 2. The laws of physics have the same form in all inertial frames

Question 56

What is the experimental evidence for time dilation

Answer 56

Muon decay

Question 57

Describe how to measure muon decay

Answer 57

In order to measure muon decay you must place one detector at a high altitude and one much further down to measure the change in muon count rate. You will also have to measure the distance between the detectors (d) and the speed that they muons are travelling at (v).

Question 58

Why can an object not reach the speed of light

Answer 58

As an object’s speed approaches the speed of light, its mass approaches infinity and so its energy also approaches infinity, which is impossible as you cannot have an infinite amount of energy.