Discovery of the electron

Question 1

What is the set up diagram for the determining specific charge of an electron using an electron gun?

Answer 1

https://www.vedantu.com/question-answer/the-diagram-below-shows-a-cathode-ray-tube-in-class-12-physics-cbse-5fbc03fc0c0f15778e7a5bfa
specific-charge-e-m-of-an-electron

Question 2

What is the purpose of the cathode ?
(Determining specific charge)

Answer 2

used to reduce the electrons to a narrow beam

Question 3

What happens to the beam when the magnetic field is switched on?
(Determining specific charge)

Answer 3

beam deflects downwards/upwards
has a curved path

Question 4

What happens to the beam when both the electric and magnetic fields are switched off?
(Determining specific charge)

Answer 4

the beam follows a straight line

Question 5

What happens to the beam when the magnetic field is already switched on, but then also the electric field is switched on too?
(Determining specific charge)

Answer 5

originally has a curved path
but then as the electric field is increased, the beam returns to its straight path

Question 6

When the electron beam is straight, what must be true?
(Determining specific charge)

Answer 6

Electric force = Magnetic force
Eq = Bqv
E=Bv
(v=velocity)

Question 7

What are the formulas required for the derivation of the specific charge using magnetic/electric fields?

Answer 7

v=EB
1/2mv² = Vₐ * q
electron is accelerated to a kinetic energy by voltage Vₐ
which leads to :
q/m = E² / 2Vₐ* B²

Question 8

What are the 3 voltages in the experiment to determine the specific charge of an electron?
(using an electron gun)

Answer 8

V= potential difference between the 2 deflecting plates
Vₐ = accelerating voltage
heater voltage (heats the filament for thermionic revision)

Question 9

What is the comparison between the specific charge of the electron and the hydrogen ion?

Answer 9

electron specific charge is much higher

Question 10

What could Thomson conclude from his experiment to determine the specific charge of an electron?

Answer 10

-specific charge of a cathode ray is independent of the gas used
-specific charge of the cathode rays is much larger than that of a hydrogen ion
-so electrons are particles of large charge compared to its mass

Question 11

The experiment to determine the specific charge of an electron using a fine beam tube uses what field?

Answer 11

magnetic field

Question 12

What formulas can be used to derive the specific charge of an electron using a fine beam tube?

Answer 12

F=Bqv
F=mv²/r
1/2mv² = Vq
leads to :
q/m = 2V / B²r²

Question 13

What is the process of thermionic emission?

Answer 13

-a metal is heated
-energies of its electrons increase
-some of them acquire sufficient energy
-escape from the metal surface

Question 14

What is the diagram for the cathode ray discharge tube?

Answer 14

https://www.easyelimu.com/high-school-notes/physics/item/1112-cathode-rays

Question 15

At low pressure, the gas in the tube begins to glow, why?
(Cathode rays)

Answer 15

-due to excitations and de-excitations of gas molecules
-& emissions of photons

Question 16

What does the colour of the glow in a cathode discharge tube depend on?
(Cathode rays)

Answer 16

-nature of the gas in the tube
-pressure in the tube
-potential difference applied across the tube

Question 17

When the pressure is decreased even further, then what happens and why?
(Cathode rays)

Answer 17

-glow disappears
-walls of the tubes start to fluoresce

Question 18

When the pressure is decreased even further, then what happens and why?
(Cathode rays)

Answer 18

-glow disappears
-walls of the tubes start to fluoresce
-due to cathode rays emitted from the cathode

Question 19

What does the colour of fluorescence depend on in a cathode ray tube?

Answer 19

impurities present in the glass
(not the gas)

Question 20

What terminal is the cathode connected to?

Answer 20

negative terminal

Question 21

What terminal is the anode connected to?

Answer 21

positive terminal

Question 22

What are cathode rays known as today?

Answer 22

electron beams

Question 23

What are the properties of cathode rays?

Answer 23

1. cathode rays travel in straight lines
2. cathode rays can be deflected by both electric and magnetic plates
3. cathode rays can cause fluorescence
4. cathode rays are emitted at 90° to the surface of the cathode
5. cathode rays can be focused using a concave cathode
6. cathode rays produce heating effects
7. cathode rays can pass through thin metal foils without puncturing them
8. cathode rays blacken photographic plates
9. cathode rays travelling in a high voltage discharge tube causes X-rays to be emitted from a suitable anode

Question 24

How was it proved that cathode rays produce heating effects?

Answer 24

a small piece of platinum placed in the path of the rays
becomes hot

Question 25

How was it proved that cathode rays travel in straight lines?

Answer 25

-when a metal cross placed in the path of the rays
-shadow is made onto the fluorescence screen

Question 26

How are the oil droplets for Millikan’s experiment made?

Answer 26

-atomiser used to produce tiny oil droplets
-friction effects make the droplets negatively charged

Question 27

If an oil drop is stationary when a p.d is applied across the two plates, what are the 2 forces acting on the droplet?

Answer 27

the droplet can stay stationary because the force due to the electric field and its weight are balanced

Question 28

What happens after the potential difference is applied across the 2 plates?
(Millikan’s oil drop experiment)

Answer 28

-the oil drop starts to move downwards
-soon reaches terminal velocity

Question 29

What are the 2 forces on the oil droplet when its reached terminal velocity?
(Millikan’s oil drop experiment)

Answer 29

-viscosity force
-weight

Question 30

What was the significance of Millikan’s oil droplet experiment?

Answer 30

-measured the charge on hundreds of oil droplets
-charges were always integral multiples of 1.6 x10⁻¹⁹
-concluded that charge can never exist in fractions of this amount
-quantisation of charge