DISCOVERY OF THE ELECTRON

Question 1

Which of ‘cathode’ and ‘anode’ is positive and which is negative?

Answer 1

Cathode is negative and anode is positive.

Question 2

What happens when the high voltage is applied to the gas discharge tube?

Answer 2

Some gas atoms in the tube are ionised.

Question 3

What is created as a result of ionisation and where are they attracted to?

Answer 3

Positive ions which are attracted to the cathode.

Question 4

What is emitted as a result of the attraction of positive ions and where do they go?

Answer 4

Free electrons from the cathode surface which are then accelerated towards the anode.

Question 5

What causes the glowing gas near the cathode?

Answer 5

The electrons from the cathode ionise gas atoms. When some of these positive ions recombine with electrons, photons are emitted.

Question 6

What causes the ‘cathode rays’?

Answer 6

Electrons which don’t recombine accelerate towards the anode and cause excitation by collision of gas atoms.

Question 7

What did Thomson determine about cathode rays?

Answer 7

The cathode rays were negatively charged particles, all with the same specific charge (electrons) for any gas.

Question 8

Do positive ions have the same specific charge for any gas?

Answer 8

No, different gases will have ions with different masses.

Question 9

Describe how a cathode ray is produced in a discharge tube.

Answer 9

Electric field ionises gas atoms. Positive ions in tube are accelerated to cathode and strike electrons in surface. Electrons emitted from cathode accelerate towards the anode.

Question 10

Explain why the gas in a discharge tube emits light.

Answer 10

Positive ions and electrons collide and recombine, emitting photons. Electrons excite gas atoms by collision and photons are emitted when de-excitation occurs.

Question 11

Why must the gas be at low pressure in a discharge tube?

Answer 11

Particles in the gas must be widely spaced; otherwise ions/electrons would be stopped by gas atoms.

Question 12

What is thermionic emission?

Answer 12

When a metal is heated, some of the free electrons gain sufficient kinetic energy to leave the metal surface.

Question 13

How is thermionic emission achieved in practice?

Answer 13

Current is passed through a wire filament.

Question 14

Why is thermionic emission negligible when the filament current is too low?

Answer 14

Low current results in small heating effect, leading to insufficient kinetic energy for electrons to overcome the work function.

Question 15

Describe the process that releases the electrons emitted at the filament.

Answer 15

Filament is heated due to current through it. Free electrons gain sufficient kinetic energy to leave the metal surface.

Question 16

Is the filament from which thermionic emission takes place the cathode or the anode?

Answer 16

Cathode.

Question 17

How are the electrons produced by thermionic emission accelerated?

Answer 17

They are attracted to the anode due to a high voltage between the anode and cathode.

Question 18

Why are electron tubes evacuated?

Answer 18

To prevent collisions with gas molecules that would cause electrons to lose kinetic energy.

Question 19

How can the speed of electrons accelerated by a high voltage be calculated?

Answer 19

eV = ½mv².

Question 20

Why do the electrons not lose kinetic energy after leaving the anode?

Answer 20

The tube is evacuated.

Question 21

State 2 assumptions needed to equate eV and ½mv².

Answer 21

• Each electron starts from the cathode with negligible kinetic energy compared to the work done on it by the voltage
• The speed of the electrons is much less than the speed of light so relativistic effects can be ignored.

Question 22

What happens when the current through the filament is increased?

Answer 22

More electrons per second as the filament becomes hotter.

Question 23

What happens when the anode potential is increased?

Answer 23

Faster electrons as they are accelerated by a greater force due to the greater electric field strength.

Question 24

What are the names of the 2 voltage supplies in a vacuum tube?

Answer 24

• Filament voltage
• Anode voltage

Question 25

What is the role of filament voltage?

Answer 25

To cause current through filament which heats the filament so electrons are released through thermionic emission.

Question 26

What is the role of anode voltage?

Answer 26

To make the anode positive relative to the filament so that electrons are accelerated towards the anode.

Question 27

What is a typical voltage for filament voltage?

Answer 27

0 – 50 V.

Question 28

What is a typical voltage for anode voltage?

Answer 28

> 250 V.

Question 29

What did Thomson’s experiments determine?

Answer 29

The specific charge of the electron.

Question 30

An electron beam can be deflected by what 2 fields?

Answer 30

* Electric field * Magnetic field

Question 31

What is the path of an electron beam in an electric field?

Answer 31

Parabolic.

Question 32

What is the path of an electron beam in a magnetic field?

Answer 32

Circular.

Question 33

What is the equation for the force on each electron in terms of d and VP?

Answer 33

F = eVP/d.

Question 34

What is the equation for the acceleration of each electron towards the positive plate?

Answer 34

a = eVP/md.

Question 35

What is the equation for the time spent by each electron in the electric field?

Answer 35

t = L/v.

Question 36

What is the equation for the vertical deflection, y, of the electron?

Answer 36

y = ½at².

Question 37

What happens to the speed of electrons if the anode voltage is increased?

Answer 37

Speed of the electrons increases.

Question 38

What happens to the time spent in the electric field if the anode voltage is increased?

Answer 38

Time they spend in the electric field decreases.

Question 39

What happens to the vertical deflection if the anode voltage is increased?

Answer 39

Vertical deflection decreases.

Question 40

What is the equation for the radius of curvature, r, of the beam of electrons in a magnetic field?

Answer 40

r = mv/Be.

Question 41

How can the radius of curvature of an electron beam be decreased?

Answer 41

* Decrease the anode pd * Increase the current in the coils

Question 42

What direction does the magnetic field have to be to keep the electron beam horizontal?

Answer 42

West to East.

Question 43

What is the equation for the speed of an electron being kept on a horizontal path by balanced electric and magnetic fields?

Answer 43

v = VP/Bd.

Question 44

Define specific charge and state its units.

Answer 44

Charge per unit mass. Ckg⁻¹.

Question 45

When the magnetic field is turned off, what path will the electrons follow?

Answer 45

Parabolic.

Question 46

What happens when only the electric field is acting on the beam of electrons?

Answer 46

Electrons deflect towards the positive plate.

Question 47

What is the equation for the specific charge of the electron in terms of measurable quantities?

Answer 47

e/m = 2yVP/L²B²d.

Question 48

Why does the beam of electrons curve at an increasing angle to its initial direction in a vertical electric field?

Answer 48

Force due to electric field acts vertically on electrons. Vertical component of velocity increases; horizontal component doesn’t change.

Question 49

What is the equation for the radius of curvature when only the magnetic field is acting on the beam of electrons?

Answer 49

r = mv/Be.

Question 50

How can the specific charge of the electron be calculated using the radius of curvature?

Answer 50

e/m = v/Br.

Question 51

What happens to the speed of electrons in a magnetic field?

Answer 51

The speed remains constant.

Question 52

How is the electron beam visible in a fine beam tube?

Answer 52

A small amount of gas is left in the tube, leading to excitation and de-excitation, emitting visible photons.

Question 53

What is the equation for the specific charge of the electron derived from kinetic energy and radius of circular path?

Answer 53

e/m = 2VA/B²r².

Question 54

What does the gradient correspond to when plotting r against 1/B?

Answer 54

Gradient = √(2mVA/e).

Question 55

How can the gradient be used to calculate e/m?

Answer 55

e/m = 2VA/gradient².

Question 56

How is flux density varied?

Answer 56

By varying the current in the coils.

Question 57

What does the gradient correspond to when plotting r against 1/B?

Answer 57

Gradient = root(2mVA/e)

Question 58

How can the gradient be used to calculate e/m?

Answer 58

e/m = 2VA/gradient2

Question 59

Why is the path followed by an electron beam in a magnetic field circular?

Answer 59

The force is always perpendicular to the velocity, acting as a centripetal force.

Question 60

What happens to the speed of electrons in a fine beam tube due to collisions with gas atoms?

Answer 60

The speed decreases.

Question 61

What occurs when electrons collide with gas atoms in a fine beam tube?

Answer 61

Electrons excite gas atom electrons to higher energy levels, leading to photon emission.

Question 62

Why is the electron beam not a perfect circle in a fine beam tube?

Answer 62

Electrons collide with gas atoms, decreasing velocity and causing an inward spiral.

Question 63

What was the specific charge of cathode ray particles found by Thomson?

Answer 63

1.76 x 10^11 Ckg^-1

Question 64

What was significant about Thomson's measurement of specific charge?

Answer 64

It was 1860 times greater than that of the hydrogen ion.

Question 65

What were two conclusions drawn from Thomson's experiments into cathode rays?

Answer 65

* Cathode rays were negatively charged particles. * The specific charge is 1860 times greater than that of the hydrogen ion.

Question 66

How does the trace of an electron beam show that all electrons have the same speed?

Answer 66

The beam does not spread out.

Question 67

What did Millikan's oil-drop experiment measure?

Answer 67

The charge of the electron.

Question 68

What is the equation for the charge of an oil droplet in Millikan's experiment?

Answer 68

Q = mgd/VP

Question 69

If a positively charged oil droplet is used in Millikan's experiment, which plate must be at the top?

Answer 69

Negative plate.

Question 70

What did Millikan find when measuring the charge of many oil droplets?

Answer 70

The charge was always a whole number multiplied by 1.6 x 10^-19 C.

Question 71

What is the significance of Millikan's results?

Answer 71

Electric charge is quantized in whole number multiples of 1.6 x 10^-19 C.

Question 72

Why does the oil drop reach terminal velocity?

Answer 72

The weight of the oil drop equals the drag force.

Question 73

What is Stokes’ law for drag force on a spherical object falling through air?

Answer 73

FD = 6πηrv

Question 74

What is the equation for the balanced forces when the oil drop is at terminal velocity?

Answer 74

mg = 6πηrv

Question 75

What is the equation for the volume of a sphere?

Answer 75

Vsphere = 4/3 πr^3

Question 76

How do you calculate the speed of an electron accelerated by a high voltage?

Answer 76

eV = ½mv^2

Question 77

What are the equations to calculate vertical displacement of electrons passing through two plates?

Answer 77

* F = eE = eVP/d * a = F/m * t = L/v * y = ½at^2

Question 78

What equations are used to determine the radius of the circular path of an electron beam in a magnetic field?

Answer 78

Bev = mv^2/r

Question 79

What equations allow you to work out the velocity of electrons in balanced electric and magnetic fields?

Answer 79

* Bev = eVp/d * Bev = eE

Question 80

What equations are used to get the specific charge of electrons when the magnetic field is switched off?

Answer 80

* Bev = eVp/d * a = F/m = eVp/md

Question 81

What equations are used to get the specific charge of electrons when the electric field is switched off?

Answer 81

* Bev = eVp/d * Bev = mv^2/r

Question 82

What equations are used for calculating the charge of an oil drop in Millikan's experiment?

Answer 82

QVP/d = mg

Question 83

What equations are used for calculating the mass of an oil drop in Millikan's experiment?

Answer 83

* mg = 6πηrv * Vsphere = 4/3 πr^3