Turning Points

Question 1

What is thermionic emission?

Answer 1

A filament is heated up using an electric current, this causes the delocalised electrons to gain energy. Eventually they gain enough energy for the electron to leave the metal 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 causes atoms in the discharge tube to ionise. When this occurs the ionised atoms, 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 of each electron leaving the anode in a cathode ray?

Answer 4

The work done on each electron by the potential different between the anode and the cathode is eV.

The kinetic energy of each electron passing through the whole is 1/2mv^2

The work done on each electron increases KE at the cathode, then the speed of each electron leaving is given by 1/2mv^2=eV.

Question 5

What is specific charge of an electron?

Answer 5

The charge of an electron/the mass of an electron.

Question 6

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

Answer 6

• using a magnetic field.
• using m=mv/Be.
  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 distrust the path of the electrons. This is because the more air particles, the more interaction between them and the electrons. Which 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 conduced experiments to investigate cathode rays, and the particles that are produced. This included him carrying out experiments deterring the specific charge of an electron.

Question 10

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

Answer 10

1800 times.

It was significant because before finding out the specific charge of an electron, hydrogen ions had the largest known specific charge.

Question 11

Why were Thomason’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 vertically 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 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 magnetic 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=6(pi)nrv - 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, 6(pi)nrv=mg
• assume the droplet is a sphere.
• mass=density/volume.
• r^2=9nv/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 pd 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 forces on the object act downwards.

Question 19

Why were Millikan’s experiments significant?

Answer 19

It helped him 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 found the common factor between the different charges and found there were no charges less than approx 1.6x10^-19. Millikan showed that the charge on all material is quantised.

Question 20

What is Newton’s corpuscular theory?

Answer 20

That light is made up of small particles. He proposed that white light is made up of different colours which he observed when he split light in a prism.

Question 21

What is Huygen’s wave theory of light?

Answer 21

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 and that has no mass.

Question 22

What is the aim of Michelson-Morley experiment?

Answer 22

To prove that there was aether by showing that the speed of light changed with different rotations in respect to the aether.

Question 23

What was the expectations of the Michelson-Morley experiment?

Answer 23

It was expected that when rotating the blocks and the mirrors by 90 degrees, the interference pattern seen should change as the speed of light should be changing in respect to the aether.

Question 24

What was the significance of the failure of the Michelson-Morley experiment?

Answer 24

The interference pattern didn’t change with the rotation therefore concluding that the speed of light is constant and there is no aether.

Question 25

What are the two postulates of Einstein’s theory of special relativity?

Answer 25

• Physical laws have the same form in all inertial frames.
• The speed of light in free space is invariant.

Question 26

What is time dilation?

Answer 26

Time runs slower for an observer when observing a moving object than it does for the object itself. This effect increases greatly as you approach the speed of light.

Question 27

Why is time dilation a consequence of special relativity?

Answer 27

Since the speed of light is the same in all frames, the relative speed of an object moving at the speed of light would vary to that of another observer, observing both objects and their relative speeds.

Question 28

What is proper time?

Answer 28

The time observed by the object, or an observer that is stationary with respect to the moving object. Known as t0.

Question 29

How does muon decay provide evidence for time dilation?

Answer 29

The intensity of muons with distance decreased less than expected. This is because the half life to human observers is much larger than the actual half life of the muon due to time dilation.

Question 30

How does special relativity effect lengths?

Answer 30

Length contraction is another conclusion to be drawn, an observer parallel to a moving object will observe a shorter length than an observer at rest relative to the rod.