C9 (Energy, Power, and Resistance)

Question 1

What is an electron gun

Answer 1

An electrical device used to produce a narrow beam of electrons, which can be used to ionise partciles by adding or removing electrons from atoms.

Question 2

How does an electron gun work

Answer 2

All electron guns need a source of electrons (usually by heating a small filament by electric current). Eelctrons gain KE, some gain enough to escape from the surface of the metal (thermionic emmision).

If heated filament is placed in a vacuum and a high p.d applied between filament and an anode (the filament acts as a cathode) and the freed electrons accelerate towards the anode (gaining KE).
If anode has a small hole in it, the elcetrons in line with this hole can pass through it, creating a beam of electrons with a specific KE.

Question 3

Electron gun equation

Answer 3

eV= 1/2(mv^2)
Work done on electron = gain in KE at anode
This assumes the electrons have negligible KE at the cathode

Question 4

In an electron gun changing the acceleration p.d changes what?

Answer 4

The KE of the electrons within the beam. The greater the p.d the more energy is transferred to the electrons and so the faster they move.

Question 5

What are the two factors that determine how much it costs to run an electrical device.

Answer 5

The power of the device

How long the device is used for

Question 6

Equation for power

Answer 6

Power equals energy transferred divided by time taken

P=W/t

Question 7

Equation for energy transferred

Answer 7

W=Pt

Units: kWh