12) Turning Points in Physics

Question 1

Is a cathode or anode positive

Answer 1

Anode is -ve
Cathode is +ve

Question 2

Where is the brightest glow in a discharge tube

Answer 2

Cathode

Question 3

What did Thomson discover about cathode rays (4)

Answer 3

Have a mass
-ve charge
Same properties regardless of gas sed
Large charge to mass ratio

Question 4

What process causes a discharge tube to glow

Answer 4

1) High potential difference across tube pulls electrons off gas atoms, forming ion and electron pairs
2) Positive gas ions are accelerated towards the cathode and when they collide with it they release more electrons
3) Electrons are accelerated along the tube (as the gas is at low pressure) and collide with gas atoms causing them to become excited
4) The atoms quickly e-excite and release photons of light

Question 5

Why is the glow brightest at the cathode

Answer 5

The gas ions and electrons recombine to emit photons of light

Question 6

What is thermionic emission

Answer 6

A metal is heated until the free electron on its surface gain enough energy and are emitted

Question 7

How do electron guns work

Answer 7

1) A potential difference is used in order to accelerate electrons which are released from the cathode by heating it (thermionic emission)
2) The electrons are accelerated towards the anode, which has a small gap
3) The electrons which pass through the gap form a narrow electron beam which travels at a constant velocity beyond the anode

Question 8

What is the work done on an electron accelerated through a potential difference V

Answer 8

• Change in work done = eV

Question 9

Once the electrons reaches the anode, what will its kinetic energy be equal to

Answer 9

Work done on the electron by the electric field
1/2 * m * v^2 = eV