9. Turning Points

Question 1

Why does a discharge tube apply a high voltage?

Answer 1

To ionise the gas atoms in the tube

Question 2

What happens to gas atoms after they have been ionised in a discharge tube?

Answer 2

They are postive ions and are attracted to the cathode

Question 3

What effect does positive ions being attracted onto the cathode surface have?

Answer 3

Free electrons from the cathode surface are emitted

Question 4

Where do the electrons come from in a discharge tube?

Answer 4

They are both emitted by the cathode and produced in the production of positive ions

Question 5

Why were electrons called cathode rays?

Answer 5

Because they were observed to originate at the cathode

Question 6

How do electrons cause a glow in the discharge tube?

Answer 6

By collisions with gas atoms, causing them to be excited, emitting visible light on de-excitation

Question 7

At what point is visible light emitted in cathode ray tubes?

Answer 7

When a gas atom de-excites, after an electron has collided with it and excited it

Question 8

When are photons emitted in cathode ray tubes?

Answer 8

When positive ions and electrons recombine

Question 9

Which way do electrons travel in cathode ray tubes?

Answer 9

Towards the anode

Question 10

What type of pressures are the gases in discharge tubes?

Answer 10

Low

Question 11

What does the colour of light emitted by a discharge tube depend on?

Answer 11

The type of gas inside the tube

Question 12

What is created when a high voltage is applied between the anode and cathode in a discharge tube?

Answer 12

A strong electric field

Question 13

Why is the gas in a cathode ray tube kept at low pressure?

Answer 13

So there is space between gas atoms to allow positive ions to be accelerated towards the cathode

Question 14

What is thermionic emission?

Answer 14

When a metal filament is heated by passing a current through it, and some of the free electrons gain enough Ek to leave the metal surface

Question 15

During thermionic emission, what is the anode and cathode?

Answer 15

Cathode - the wire filament

Anode - Metal plate with a small hole in it

Question 16

In thermionic emission, how are electrons accelerated through the hole in the anode?

Answer 16

A high pd between the cathode and anode

Question 17

In thermionic emission, what is work done on the electrons, eV, equal to?

Answer 17

Kinetic energy gained (1/2mv²)

Question 18

What does thermionic emission essentially produce?

Answer 18

An electron beam

Question 19

How is the metal heated in thermionic emission?

Answer 19

By passing an electric current through it

Question 20

Does the glass tube used in thermionic emission have gas molecules in it?

Answer 20

No - it is evacuated

Question 21

In thermionic emission, what is the equation that gives the speed of each electron in the beam from the anode?

Answer 21

eV = 1/2mv²

Question 22

Why can the speed of electrons in thermionic emission be calculated by eV = 1/2mv²?

Answer 22

Their Ek doesn’t change after they pass the anode, as the electric field is no longer acting

Question 23

Which is a more effective way of freeing electrons from a metal to create electron beams; thermionic emission or cathode ray tubes?

Answer 23

Thermionic emission

Question 24

What is the work function of a metal?

Answer 24

The minimum energy needed to remove one of the free electrons to just beyond the surface of a metal

Question 25

What are the assumptions when using eV = 1/2mv²?

Answer 25

• each electron starts from the cathode with negligible Ek compared to the work done on it by the accelerating pd
• the speed of the electrons in the beam is much less than the speed of light in free space, so the non-relativistic formula for Ek applies

Question 26

How is the specific charge on an electron found in a deflection tube using a magnetic field?

Answer 26

• Bev = mv²/r so r=mv/Be
• combine this with 1/2mv²=eV (rearrange r for v)
• e/m = 2V/B²r²

Question 27

Why are electron tubes usually evacuated?

Answer 27

So that the electrons are not slowed down by collisions

Question 28

How is the specific charge on an electron found in a deflection tube using a magnetic field to deflect the beam and an electric field to balance the deflection?

Answer 28

• radius of curvature of electron beam in magnetic field is measured
• electron field applied to beam path is straight and forces due to fields are balanced
• eE = BeV and E = V/d so V = E/B
• r = mv/Be
• so e/m = v/E

Question 29

How is the specific charge on an electron found in a deflection tube using an electric field to deflect the beam and a magnetic field to balance the deflection?

Answer 29

• horizontal speed of electron beam found from eE = BeV and E = V/d so V = E/B
• vertical deflection y of beam at end of plates is measured
• s = ut + 1/2at²
• hence e/m = ad/V where d is separation of plates

Question 30

Before the specific charge of the electron was measured, what was known to have the largest specific charge of any charged particle?

Answer 30

Hydrogen at 9.6 x 10⁷ Ckg-1

Question 31

Why could Thomson not conclude that the electron has a much smaller mass than a the hydrogen atom from e/m?

Answer 31

As neither the mass nor the charge of the electron was known at that time

Question 32

How does the specific charge of an electron compare to that of a hydrogen atom?

Answer 32

The e/m of an electron is 1860 times larger

Question 33

How did Millikan measure the charge on the electron?

Answer 33

• used oil spray and oppositely charged parallel metal plates
• plates horizontal so field vertical
• could make charged droplet stay stationary by adjusting pd between plates until electric force was equal and opposite to weight of droplet

Question 34

In Millikan’s oil droplet experiment, what equations are used?

Answer 34

QV/d = mg

so charge on droplet, Q = mgd/V

Question 35

In Millikan’s oil droplet experiment, how do the oil droplets become charged?

Answer 35

Frictional effects between the sides of the nozzle and the oil (or using X-rays)

Question 36

In Millikan’s oil droplet experiment, why do some droplets move up and some move down?

Answer 36

• some have +ve/-ve charges

* some are heavy/light compared to their charge - move in different directions

Question 37

In the equation Q = mgd/V, which quantity is hard to measure? How can this be solved?

Answer 37

• mass - use density
• so m =4/3πr³
• of which radius is hard to measure
• so use stokes law F (mg) = 6πηrv to find r

Question 38

What significance did the results from Millikan’s oil droplet experiment have?

Answer 38

• could measure charge of each droplet
• found that the charge was always a whole number multiplied by 1.6 x 10⁻¹⁹
• so charge is quantised in whole number multiples