Thermionic emission

Question 1

Increase anode potential causes

Answer 1

the velocity and kinetic energy of the electrons to increase because they are attracted towards the anode with a greater acceleration or force (or gain more kinetic energy in crossing a greater pd)

Question 2

Increase filament current causes

Answer 2

an increase in the number of electrons per second in the beam because the filament becomes hotter and will emit more electrons per second

Question 3

Explain the need for both voltage supplies in thermionic emission and what voltages are required

Answer 3

voltage supply to supply current/heat to filament <50V. Other voltage supply makes anode +ve with respect to filament to attract electrons. Increasing this voltage makes electron travel faster as more kinetic energy as force of attraction to the anode increases: > 250V

Question 4

Explain why thermionic emission occurs:

Answer 4

Filament heated by electric current delocalised/conduction electrons give metal its conduction properties so when heated, they gain kinetic energy to overcome attractive electrostatic forces and can leave surface

Question 5

Thermionic emission vs discharge tube which is better to produce electron beams

Answer 5

thermionic emission is much simpler to produce electron beam as doesn’t need a gas present

Question 6

What is typically used for the metal in thermionic emission and how is it heated

Answer 6

a wire filament or cathode is heated electrically by passing a current through it

Question 7

Explain 0.5mv^2 = eV

Answer 7

work done on particle with charge e when accelerated through pd of V volts is QV joules which is converted into the particle’s kinetic energy

Question 8

1 electron volt is

Answer 8

the kinetic energy carried by an electron after accelerating through a pd of 1V

Question 9

Why is the anode potential made positive

Answer 9

to attract electrons

Question 10

Anode voltage - equation and use

Answer 10

0.5mv^2 = eV –> anode voltage (to make it positive with respect to filament)

Question 11

Cathode voltage - use

Answer 11

Cathode voltage used to produce electrons via thermionic emission

Question 12

E = V/d , V is the

Answer 12

potential difference between the plates for the electric field strength

Question 13

temperature of filament depends on

Answer 13

current

Question 14

kinetic energy of electrons depends on

Answer 14

current

Question 15

electrons must do work to… work done on electron =

Answer 15

leave metal and overcome thermionic work function = eV

Question 16

Electron gun diagram

Answer 16

Question 17

Top plate is negative relative to lower plate state the direction of:

a) electric field between plates
b) force on electron in field
c) acceleration of electron in field

Answer 17

a) field is direction positive charge goes e.g. positive to negative therefore bottom to top therefore upwards
b) electrons negative so move opposite to field so downwards
c) downwards in direction of force

Question 18

An electron beam is directed horizontally into a uniform electric field which acts vertically downwards. Explain the direction of the electron beam and why this happens

Answer 18

Electrons move upwards with increasing speed

Electrons experience vertical electrostatic force (as negatively charged so opposite to direction of field) which doesn’t change as the electrons move across the field therefore constant vertidcal acceleration

Constant horizontal velocity so direction of motion becomes closer and closer to vertical line as velocity increases.

Question 19

An electron beam is directed horizontally into a uniform magnetic filed that is perpendicular to the initial direction of the beam. Explain why th ebeam curves round in a vertical line at constant speed

Answer 19

Magnetic field is perpendicular to direction of force

so no work done as W=FdcosX and there’s zero distance travelled in the direction of force.

Force causes direction of motion to change - centripetal force acting towards centre of curvature.

No work done so no acceleration therefore constant speed

Question 20

A beam of electrons moving at a speed v passes through perpendicular electric and magnetic fields without being deflected. The flux density is B calculate the strenght of the electric field.

Answer 20

QV/d = BQV

V/d = E

E=Bv (V/m or N/C)