Section 13: Option D - Turning Points in Physics

Question 1

What is meant by thermionic emission?

Answer 1

A filament is heated up using an electric current, this causes the delocalised electrons to gaine energy. Eventually they gain enough energy for the electron to leave the metak surface as a beam of electrons

Question 2

How are cathode rays made in a discharge tube?

Answer 2

• Electrons are released by thermionic emission
• The electrons are repelled by the cathode and accelerated towards an anode

Question 3

Why is light emitted from a discharge tube?

Answer 3

• The strong potential difference between the anode and cathode cause atoms in the discharge tube to ionise
• When this occurs the ionised atoms (which are now positive ions) are attracted to the cathode
• They accelerate towards and then collide into the cathode
• When this happens electrons leave the cathode and go on to excite other atoms
• When these atoms de excite they release photons of light

Question 4

What is the speed, v, of each electron leaving the anode in a cathode ray?

Answer 4

• The work done on each electron by the potential difference V between the anode and the cathode is eV (e- electron charge)
• The kinetic energy of each electron, with speed v, passing through the hole is 1/2mv^2
• THe work done on each electron increases KE at the cathode, then the speed, v, of each electron leaving the anode id given by 1/2mv^2=eV
• So v= sqrt(2eV/m)

Question 5

How do you work out the specific charge of an electron?

Answer 5

The charge of an electron/ the mass of an electron

Question 6

State 3 methods used to wrok out the specific charge of an electron

Answer 6

1. Using a magnetic field
2. Using m=mv/Be
3. Using e/m = v/Br

Question 7

Why must electron tubes be evacuated when working out specific charge?

Answer 7

So the electrons do not collide with air particles and lose energy in the collisions

Question 8

If gas is pumped into an electron tube, why must the pressure be low?

Answer 8

• A low pressure means less molecules which is beneficial because too many gas molecules could disrupt the path of the electrons
• This is because the more air particles, the more interaction between them and the electrons
• This could mean the electrons won’t be able to travel the whole length of the tube

Question 9

Who was Thomson?

Answer 9

• A physicist who conducted experiments to investigate cathode rays and the particles that are produced
• This included him carrying out experiments determining the specific charge of an electron

Question 10

Thomson found out that the specific charge of an electron was how many times larger than the specific ion? And why was this significant?

Answer 10

• 1800 times
• It was significant becasue before finding out the specific charge of an electron, hydorgen ions had the largest known specific charge

Question 11

Why were Thomson’s experiments important?

Answer 11

• Showed that electrons were negatively charged
• Showed that the specific charge of a particle is a characteristic of that type of particle as all electrons have the same specific charge
• The specific charge was very high which showed that an electron had to have little mass

Question 12

What was the aim of Millikan’s experiments?

Answer 12

To determine the charge of the electrons

Question 13

In Millikan’s experiment, what forces are acting on the droplet when it is stationary?

Answer 13

Gravity and an electric force which is equal and opposite to the gravitational force

Question 14

In Millikan’s experiments, explain the journey of a falling droplet when there is no electric field?

Answer 14

• The droplet will begin falling and accelerating as it does the drag acting on it will increase
• Eventually the weight will equal drag force and the droplet will fall at terminal velocity

Question 15

What is Stoke’s law?

Answer 15

F = 6πηrv
Which is used to work out the force on a droplet due to drag

Question 16

How is Stoke’s law used to work out the radius of the droplet?

Answer 16

• As the droplet is moving at terminal velocity the forces acting on it will balance
• Therefore 6πηrv = mg
• We assume the droplet is a sphere which would suggest its volume is 4πηr^3/3
• Mass = density/volume so 6πηrv = p x g / (4πηr^3/3)
• Which if you rearrange you get r^2 = 9ηv/2pg

Question 17

How did Millikan use the value of the radius to determine the charge of an electron?

Answer 17

• He used the radius to determine the mass of the droplets
• That way he could calculate the weight of a droplet
• He then knew the od required for an object to remain stationary
• Therefore the charge could be worked out using QV/d=mg

Question 18

How did Millikan cause the droplet to move down from stationary in the presence of an electric field?

Answer 18

• Millikan decreased the pd which would have reduced the electric field
• Therefore unbalancing the forces and making the resultant force on the object act downwards

Question 19

Why were Millikan’s experiments significant?

Answer 19

• It helped him to conclude the charge of an electron, by assuming that the charge on each droplet would be a multiple of the charge of an electron
• He founs the common factor between the different charges, and there was no charge less than approx 1.6x10^-19
• Millikan showed that the charge on all material is quantised

Question 20

What is wave particle duality?

Answer 20

The idea that particles can also behave like a wave

Question 21

Outline Newton’s corpuscular theory?

Answer 21

• Light is made up of small particles
• He proposed that white light is made up of different clours which he observed when he split light put in a prism

Question 22

Outline Huygen’s wave theory on light?

Answer 22

• States that all of the points on a wave front are sources for secondary wavelets
• Light was a longitudinal wave but as longitudinal waves must travel through a medium and space was deemed to be ‘empty’ Huygen also suggested that space was filled with aether, a substance we cannot see that has mass

Question 23

Explain reflection in terms of Newton’s corpuscular theory

Answer 23

• Newton suggested that the particles are like balls, and when they bounce off the surface they ‘reflect’ due to a conservation of momentum between the surface and the particles
• As the speed only changes in the direction perpendicular to the surface then it must be reflected at the same angle

Question 24

Describe the difference between Newton’s theory and Huygen’s theory on refraction

Answer 24

• Newton suggested that light diffracts due to the material exerting an attractive force on the light particles causing htem to increase verticle speed as it entered the material. The change in speed is what leads to the change in angle when entering/leaving the medium
• Huygen’s theory suggests that the wavefronts hit the bounday at an angle. This causes the light to take an inifinite amount of time to slow down at the boundary

Question 25

Which theory of light was preferred? Why?

Answer 25

• Newton’s was preferred
• This is because Newton had a better reputation at the time than Huygen and was seen to be more important

Question 26

How did Young’s double slit theory support Huygen’s wave theory?

Answer 26

• The double slit theory displayed a pattern of fringes due to interference between different wavefronts
• Young’s double slit theory occurs due to diffraction and only waves can diffract
• Therefore suggesting that light is like a wave like Huygen suggested

Question 27

How are fringes formed in Young’s double slit?

Answer 27

• Monochromatic light passes through 2 slits
• The 2 slits act as coherent light sources
• This causes bright fringes where the light is in phase and constructive interference occurs
• Dark fringes where destructive interference occurs as the light is out of phase

Question 28

Why can Newton’s theory NOT explain Young’s interference pattern?

Answer 28

If light acted like particles like Newton suggested then there would be 2 light fringes where the slits are, not a repeating pattern of light and dark fringes

Question 29

Why was Newton’s theory eventually rejected?

Answer 29

Newton’s theory of light was rejected many years after Young’s discovery of interference, in which the speed of light in water was measured and found to be less than the speed in air

Question 30

What were Maxwell’s predictions about electromagnetic waves?

Answer 30

• He predicted that an electromagnetic wave has an electric and magnetic field that oscillate perpendicular to each other
• He also derived an equation for the speed of an electromagnetic wave
• He showed that light and electromagnetic waves travelled at the same speed in a vacuum therefore showing that light is an EM wave as well as ultraviolet, infrared and others

Question 31

What does ε0 represent? And what does it relate to?

Answer 31

Represents the permittivity of free space and is related to the strength of the electric field as a result of a charged object in free space

Question 32

What is µ0? What does µ0 relate to in the equation?

Answer 32

It stands for the permeability of free space, which is related to the magnetic flux density because of a current carrying wire in free space

Question 33

How are stationary waves produced when radio waves are reflected?

Answer 33

The reflection produces 2 coherent progressive waves travelling in opposite directions
The 2 waves interfere and cause constructive interference at antinodes and destructive interference at nodes

Question 34

How did Hertz discover radio waves?

Answer 34

Hertz created electromagnetic waves by producing them when a high voltage spark jumped from one metal plate to another
The spark produced radio waves which he detected using a dipole detector
As the radio waves produced a voltage in the detector which causes a spark in the detector