Turning Points

Question 1

How was the electron discovered?

Answer 1

Cathode rays: gases at low pressure in discharge tubes conduct electricity and emit light of a certain colour. Charged particles move through the gas when it conducts electricity.

Question 2

Who discovered the electron? What did he prove?

Answer 2

J.J. Thompson proved that cathode rays have energy momentum and mass; have negative charge and have the same properties regardless of gas used.

Question 3

Describe the process of emission of light from a discharge tube.

Answer 3

If a high voltage is applied to the tube some gas atoms are ionised. Positive ions are attracted to the cathode surface causing free electrons to be emitted.
Glowing gas near the cathode are because of the photons emitted when the positive ions and electrons recombine.
Some electrons move towards the anode and the glowing gas there is due to the deexcitation of the gas atoms.

Question 4

What is thermionic emission?

Answer 4

When a metal is heated some free electrons gain enough energy to leave the metal surface. The electrons can be accelerated through an electric field.

Question 5

How is an electron beam deflected?

Answer 5

By an electric or magnetic field. The force on each electron in the electric field is constant. The field is uniform so the electron curves in a parabolic path.
By using a uniform magnetic field. Electrons experience force because the magnetic field is perpendicular to the direction of motion. No work is done by the magnetic field so speed is constant and electron moves in circular path.
Deflection can be cancelled out by applying magnetic and electric fields.

Question 6

How is the specific charge of an electron worked out using a fine beam tube?

Answer 6

It makes the beam visible and collisions between some electrons in beam and a small amount of gas. If the magnetic field is strong enough the beam will be a complete circle.

Question 7

What was the significance of thompsons discovery?

Answer 7

He showed that the electron specific charge was 1860 times larger than hydrogen was.

Question 8

How was the charge of an electron discovered?

Answer 8

Milikan’s oil drop experiment. He controlled the motion of the droplet using the electric field between charges plates. He made the charged droplet stationary by adjusting the pd between the plates until the electric force was equal and opposite to the weight of the droplet.

Question 9

What was the significance of Milikan’s experiment?

Answer 9

He measured the mass of each droplet by measuring the terminal speed. He found the charge was always a multiple of 1.6x10^-19.
Stokes law was developed.

Question 10

Describe Newton’s corpuscular theory of light.

Answer 10

Light was made up of tiny particles (corpuscles).
Reflection- corpuscles bounce off mirror without loss of speed and the force pushed the particles away from the surface.
Refraction- when light passes through a transparent substance from air the corpuscles are attracted to the substance so they move faster.

Question 11

Explain Hygen’s wave theory of light.

Answer 11

Assumed light waves travel slower in a transparent substance than air.
Light should diffract around tiny objects and 2 coherent light sources should interfere with each other.

Question 12

Why did scientists prefer Newtons theory?

Answer 12

Newton had a stronger scientific reputation.
The wave theory was considered in terms of longitudinal waves so couldn’t explain polarisation of light.
It wasn’t possible to measure the speed of light.

Question 13

What is the significance of youngs double slit experiment?

Answer 13

Shows interference- a wave property and challengers corpuscular theory.
Produced an interference pattern of equally spaces light and dark fringes.
Corpuscular theory couldn’t explain fringes but wave theory could (newton only predicted 2 bright fringed).
Later realised light was a transverse wave explaining polarisation.

Question 14

How were electromagnetic waves discovered?

Answer 14

Maxwells theory of electromagnetic waves showed waves are transverse and electric waves are in phase with and perpendicular to magnetic waves.
An alternating current in the wire creates alternating electric and magnetic fields that radiate from the wire.
Showed the speed of electromagnetic waves in free space.

Question 15

What did Maxwells theory show?

Answer 15

Light consists of electromagnetic waves and predicted electromagnetic waves outside the known spectrum.

Question 16

How did Hertz discover radio waves?

Answer 16

Radio waves produced by the spark gap transmitter spread out from the spark gap and pass through the detector loop. The waves passing through the detector loop cause voltage to be induced in the detector loop which makes sparks jump across the detector gap.
Radio waves do not pass through they reflect.
Insulators do not stop radio waves.

Question 17

How do you measure the wave length and speed of radio waves?

Answer 17

Hertz produced stationary radio waves by using a flat metal sheet to reflect the waves back towards the transmitter.
He measured the wavelength and calculated frequency of the waves using charge and capacitance.
Using radio waves of know frequency he calculated the speed.
Concluded radio waves travel at the same speed as light and that they are electromagnetic waves.

Question 18

How was the transverse nature of radio waves discovered?

Answer 18

Hertz discovered that when a reflector and dipole detector were parallel to the transmitter a Steph signal was observed.
The detector signal decreased and became zero at 90•.
Conclude radio waves were polarised and signal was zero because dipole was perpendicular to the plane of polarisation.

Question 19

What did Fizeu discover and how?

Answer 19

He determined the speed of light.
Passed a beam of light between 2 cog teeth to a reflector. The cog was rotated at exactly the right speed so that the reflected beam passed through the next gap in the cog teeth. He used the frequency of rotation and the number of gaps to calculate the time taken for the light to travel to the reflector and back.

Question 20

What is the ultraviolet catastrophe?

Answer 20

All glowing objects emit electromagnetic radiation due to their temperature. The radiotions consists of a continuous range of wavelengths. A black body is a body that absorbs and emits all wavelengths of electromagnetic radiation.
Each black body curve has an intensity peak at a particular wavelength that depends on the temperature which couldn’t be explained by the wave theory. Wave theory suggests the intensity becomes infinite at smaller wavelengths- ultraviolet catastrophe.
Planck used his quantum theory to explain black body curves.

Question 21

Describe the photoelectric effect.

Answer 21

Metals emit electrons when supplied with sufficient energy illuminating the metal with light. Threshold frequency- minimum frequency of light needed to emit electrons. Photoelectrons have a range of kinetic energies from zero up to a maximum value. The number of photoelectrons emitted is proportional to intensity.
The wave theory couldn’t explain this- light of any frequency should cause emission. It couldn’t explain threshold frequency, instant emission and max kinetic energy.

Question 22

What was Einsteins photon theory?

Answer 22

Electromagnetic radiation consists of wave packets of energy he referred to as photons. The work function is the minimum amount of energy needed to emit an electron. In order for an electron to escape it must absorb a single photon and use energy equal to the work function. Stopping potential is energy needed to stop fastest moving electrons.

Question 23

What is the significance of the photon theory?

Answer 23

Shows the wave packet nature.
Particle like- photoelectric effect
Wave like- diffraction, interference

Question 24

What did de Broglie do?

Answer 24

He said all particles have a wave like nature and remained a hypothesis until a beam was diffracted when passed through a thin metal foil.
Photos of the diffraction patterns were similar to using X-rays and confirmed his hypothesis.

Question 25

Describe what an electron microscope does.

Answer 25

Makes use of particle nature electrons- uses electric and magnetic fields. Uses wave nature to obtain detailed images.
To form an image of an atom the electrons must have a certain de Broglie wavelength.

Question 26

What is a TEM?

Answer 26

Transmission electron microscope. A beam of electrons is directed at a thin sample in an evacuated tube. As the electrons pass through the sample some of them are scattered by the structures in the sample. Electromagnetic coils focus on the scattered electrons onto a fluorescent screen at the end of the tube to form a magnified image of the sample structure.

Question 27

How does an electron microscope work?

Answer 27

An electron gun produces electrons by thermionic emission from a heated filament. The electrons are accelerated through a hole in a metal anode at constant pd relative to filament and emerge through the hole at same speed.
Magnetic condenser lens produces magnetic field that forces electrons into parallel beams directed at a sample.
Objective lens deflects the scattered electrons to form an inverted image.
Magnifier lens focuses the electrons to form a magnified final image.

Question 28

What are the limitations of TEM?

Answer 28

Sample thickness- electrons passing through the sample suffer loss of speed increasing the de Broglie wavelength reducing the resolving power.
Lens aberrations- magnetic field in the lens gap may focus electrons to a different point on the screen creating a blurred image.

Question 29

What is an STM?

Answer 29

Scanning tunnelling microscope.
Fine tip initial probe scans small areas of surface at a height of 1nm. The gap means electrons can tunnel through it. The probe is at a constant negative pd and insured the electrons only tunnel across the gap in one direction.
The tunnelling current increases is the gap decreases.
In constant height mode the tunnelling current is recorded. In constant current mode the gap is kept constant and recorded with the electric transducer controlling the height.

Question 30

What is absolute motion?

Answer 30

Electromagnetic waves were believed to be vibrations in an invisible substance they called ether. The earth must be moving through the ether. Light was thought to move at a fixed speed relative to the ether.

Question 31

What was the Michelson Morley experiment?

Answer 31

Used a interferometer to test other hypothesis. The light beam is split into 2 beams at the back surface of the glass. One beam reflects at mirror 1 the other at mirror 2. When the reflected beams meet again they form an interference pattern. The compensator block makes sure both beams travel through the same thickness of glass so they overlap. They were unable to detect the predicted fringe shift so the ether doesn’t exist. The speed of light has the same value for all observers.

Question 32

What is relative motion?

Answer 32

Observe an object against your frame of reference.
Einsteins theory- physical laws have the same form in all inertial frames (frames that move at a constant velocity relative to each other), the speed of light in free space is invariant.

Question 33

Describe time dilation.

Answer 33

A moving clock runs more slowly than a stationary clock. The time between events is stretched out according to the moving observer.

Question 34

Describe length contraction.

Answer 34

A rod moving in the same direction as its length appears shorter than when stationary.

Question 35

Explain relativistic mass.

Answer 35

Mass of an object increases with speed.

Question 36

What is the cosmic speed limit?

Answer 36

No material object can ever reach the speed of light as mass would become infinite and can never travel at the speed of light.

Question 37

What is Bertozzis experiment?

Answer 37

The first direct evidence for special relativity.
He used linear particle accelerators to accelerate pulses of electrons from rest through a measured pd. Time taken for the electrons to reach a target electrode at a fixed distance was measured and speed calculated. He checked the kinetic energy of the electrons by moving the metal targets temperature rise. He plotted a graph which matched Einsteins prediction.