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