9.3 Electron gun

Question 1

what does an electron gun do?

Answer 1

it produces a narrow beam of electrons

Question 2

why is an electron gun used?

Answer 2

to ionise particles by adding or removing electrons -> as it produces electrons with specific kinetic energies

Question 3

where are electrons guns used?

Answer 3

analogue TVs, electron microscopes etc

Question 4

what is thermionic emission?

Answer 4

when some electrons in the hot cathode filament gain enough kinetic energy to escape the metal surface

Question 5

what do the free electrons (released in thermionic emission) do afterwards? why?

Answer 5

they accelerate towards the positive anode because of the high PD voltage between the cathode and anode

Question 6

what creates the beam in an electron gun?

Answer 6

they pass through a hole in the anode, which creates a beam with a specific kinetic energy

Question 7

what is work done on a single electron (symbol)?

Answer 7

eV

e = charge of electron
V = voltage

Question 8

what is the equation for the kinetic energy of the anode?

Answer 8

eV = 1/2mv^2
NOTE: varying the PD changes electron velocity (v)

Question 9

CALCULATION:
Calculate the velocity of an electron from an electron gun with an accelerating PD of 4.0kV.

Answer 9

STEP 1: select the equation eV = 1/2mv^2

STEP 2: rearrange to make v subject

STEP 3: substitute values including e (1.6 x 10^-19) and m (9.11 x 10^-31)

STEP 4: complete calculation

ANSWER: 3.7 x 10^7 ms^-1

Question 10

what is a linear particle accelerator (LINAC)? (i.e what does it do)

Answer 10

a machine that uses a series of cylindrical electrodes (“drift tubes”) to accelerate subatomic particles

Question 11

how does a linear particle accelerator (LINAC) work?

Answer 11

the polarity of the tubes is alternated between +ve and -ve at exactly the right moment
-> so when the particle leaves one tube its accelerated towards the next

Question 12

how does a linear particle accelerator (LINAC) continue working?

Answer 12

the tubes get longer to accommodate for increasing speed

Question 13

CALCULATION:
A linear particle accelerator uses 100 drift tubes, each with a maximum alternating EMF of 50V. Calculate the maximum speed of the electrons from the LINAC.

Answer 13

STEP 1: find total V
100 x 50V = 500V

STEP 2: use eV = 1/2mv^2

ANSWER: 4.2 x 10^7 ms^-1

Question 14

why is it necessary for the length of the drift tubes inside a LINAC to increase as they get further along the accelerator?

Answer 14

• electrons must spend the same amount of time inside each electrode (drift tube) -> to ensure they leave the electrode at the same time as the PD changes so they can accelerate towards the next one
• the electrons move faster as they move along the LINAC -> therefore, electrodes must increase in length

Question 15

QUESTION:
Describe how an electron gun produces a high-speed beam of electrons
(4 marks)

Answer 15

• electrons are emitted from hot wire/filament at the rear of the electron gun
• there is a large PD between the filament and anode
• electrons are accelerated towards an anode
• they pass through the hole in the anode

Question 16

QUESTION:
Describe what is happening to the atoms in the screen as they are hit by electrons.
(4 marks)

Answer 16

• electrons in the beam hit atoms in the screen and electrons in the atom are promoted to higher energy levels
• as they fall back to lower levels, energy is emitted in the form of light
• the energy levels involved in the emission of light are only a few eV apart so there is plenty of energy in each incident electron to cause transition between all of the energy states in the atom
• as a result, a number of different energy photons are emitted