Turning Points In Physics

Question 1

What is an anode?

Answer 1

A positively charged electrode.

Question 2

What is the Bertozzi Experiment?

Answer 2

An experiment that investigated the velocity of electrons emitted with different kinetic energies. It showed that the speed of light was never exceeded and that as the kinetic energy continually increases, the velocity of the electrons continually approaches the speed of light.

Question 3

What is a black body?

Answer 3

A body that emits all wavelengths of radiation, dependent on its temperature.

Question 4

What are cathode rays?

Answer 4

Beams of electrons emitted from a cathode through thermionic emission.

Question 5

What is a cathode?

Answer 5

A negatively charged electrode.

Question 6

What are condenser lenses?

Answer 6

The lens in a TEM that is responsible for deflecting the electrons into a wide beam, that is then incident of the sample.

Question 7

What are constant-current STM?

Answer 7

A form of scanning tunneling microscope in which the current of the probe remains constant, and the probe’s height changes with changes in the sample’s surface. The current that tunnels across varies depending on the height, allowing an image to be formed.

Question 8

What are dipole detectors?

Answer 8

A pair of aligned metal rods placed at the centre of a concave reflector.

Question 9

What are discharge tubes?

Answer 9

A sealed glass tube held at low pressure, with an anode at one end and a cathode at the other. When the electrodes are connected to a supply, the gas in the tube will glow and conduct electricity.

Question 10

What is a drag force?

Answer 10

A force acting against the oil drop’s motion in Millikan’s Oil Drop experiment. All objects travelling through a fluid will experience drag.

Question 11

What is an electron gun?

Answer 11

A pair electrodes that produce a thin and fast moving beam of electrons. Electrons are emitted from the cathode by thermionic emission and are then accelerated through a gap by the anode.

Question 12

What is ether?

Answer 12

A substance that was believed to fill all space and be responsible for light propagation.

Question 13

What is ether drift?

Answer 13

A theory that the Earth’s ether carried light along with it. This implied that light should travel faster when travelling in the direction of the ether, however this was later proved to be false.

Question 14

What is Fizeau’s Determination of the Speed of Light?

Answer 14

An early method of determining the speed of light. It involved timing the length of time it took for a beam of light to travel to and back from a mirror, by passing it through a rotating cog wheel

Question 15

What was Hertz’s discovery of radio waves?

Answer 15

Hertz discovered that radio waves are produced when sparks jump across a gap of air. They can be detected with a dipole detector or a wire loop with a gap in.

Question 16

What is Huygens’ wave theory?

Answer 16

A theory stating that all wavefronts consist of a series of points from which secondary wavelets are emitted.

Question 17

What is the inertial frame of reference?

Answer 17

A frame of reference which is not accelerating (frames which travel at a constant velocity relative to each other).

Question 18

What is invariance of the speed of light?

Answer 18

The speed of light is identical for all observers, whatever the speed of the source is and whatever the observer’s state of motion is.

Question 19

What is length contraction?

Answer 19

For an observer moving parallel to a rod, the r.

Question 20

What is Maxwell’s electromagnetic theory?

Answer 20

Maxwell predicted that a type of wave, known as an electromagnetic wave, consisted of oscillating, perpendicular electric and magnetic fields. He then went onto produce an equation for their speed, which combined with experiments, proved his theory.

Question 21

What is the Michelson-Morley interferometer?

Answer 21

An experimental set-up involving a monochromatic light source, a semi-silvered glass block, a plane glass block, two mirrors and a viewing telescope. It proved that the speed of light is invariant.

Question 22

What is negative glow?

Answer 22

A glow found near the cathode in a cathode ray tube. It is caused by the recombination of positive ions with the electrons produced by ionisation. This recombination results in photons being emitted.

Question 23

What is Newton’s corpuscular theory?

Answer 23

A theory that light existed as discrete packets, referred to as corpuscles, that travel in straight lines and have momentum.

Question 24

What is an objective lens?

Answer 24

The second lens in a TEM microscope. It is responsible for magnifying and focussing the image of the sample.

Question 25

What are paddle wheels?

Answer 25

​A wheel found in a discharge tube that demonstrates the presence of cathode rays.

Question 26

What is a photon?

Answer 26

A packet of energy. Light consists of photons.

Question 27

What are projector lens’?

Answer 27

The final lens in a TEM that is responsible for focussing the image onto a fluorescent screen.

Question 28

What is the quantisation of electric charge?

Answer 28

Electric charge is quantised. This means it can only be found in packets of energy with a charge that is a multiple of the electron charge.

Question 29

What is quantum tunnelling?

Answer 29

For a sufficiently thin barrier, the amplitude of a matter wave passing through it will not become zero - this means the matter wave can ‘tunnel’ through the barrier.

Question 30

What is relativistic energy?

Answer 30

When an object approaches relativistic speeds, the mass will increase, resulting in an increase of energy known as relativistic energy.

Question 31

What is a scanning tunnelling microscope?

Answer 31

A microscope that uses a probe controlled by piezoelectric transducers to scan the surface of a sample. It uses electron tunnelling and has a resolution of around 0.001nm.

Question 32

What is specific charge?

Answer 32

The ration of charge to mass.

Question 33

What is Stokes’ law?

Answer 33

A law that gives the drag force that acts on a spherical object, such as an oil droplet.

Question 34

What is stopping potential?

Answer 34

A value of potential difference at which photoelectric emission is stopped. This is because the electrons no longer have sufficient kinetic energy to cross the potential difference.

Question 35

What is thermionic emission?

Answer 35

The emission of electrons from a metal plate due to heating it.

Question 36

What is threshold frequency?

Answer 36

The minimum frequency of photons required for photoelectrons to be emitted from the surface of a metal plate through the photoelectric effect. It is equal to the metal’s work function divided by Planck’s constant.

Question 37

What is time dilation?

Answer 37

A consequence of the invariance of the speed of light, that means that time will appear to run slowly when you are moving.

Question 38

What is the transmission electron microscope?

Answer 38

A microscope that passes a beam of electrons through lenses and across a sample, to form an image on a fluorescent screen. The resolution is around 0.1nm.

Question 39

What is the ultraviolet catastrophe?

Answer 39

The discrepancy between practical measurements and classical theoretical predictions of the energy intensities at different wavelengths, emitted from a black body.

Question 40

What is the work function?

Answer 40

The minimum energy required to remove an electron from a metal’s surface.