9.3 Electron Gun | 9.4 Resistance

How does an electron gun work?

• Small metal filament is heated by an electric current

• The electrons in this piece of wire gain kinetic energy.

• Some gain enough energy to escape from the surface of the metal (thermionic emission- emission of electrons through action of heat)

• If the the heated filament is placed in a vacuum and a high p.d applied between the filament and an anode, the filament acts as a cathode and the freed electrons accelerate towards the anode, gaining kinetic energy.

• If the anode has a small hole in it then electrons in line with this hole can pass through it, creating a beam of electrons with specific kinetic energy.

• As the electrons accelerate towards the anode they gain kinetic energy.

What is the equation for

What is equation for energy transfers?

From the defintion of p.d, the work done on a single electron travelling from the cathode to the anode is equal to eV, where e the elementary charge, the charge on each electron, V is the accelerating p.d.

eV=1/2 mv²

This assumes the electrons have negligble kinetic energy at the cathode.

How does changing the p.d affect electrons in the beam?

It changes the kinetic energy of the electrons. The greater the p.d the more energy is transferred to the electrons so they move faster.

What is resistance?

All components have resistance. Each component resists the flow of charge carriers through it. It takes energy to push electrons through a component, and the higher the resistance of that component the more energy it takes. 

How do you determine resistance?

The resistance if a component in a circuit can be determined by

R = V/I

Resistance of component R = p.d across component/ current in component.

What is the unit for resistance?

An ohm Ω. 1Ω = 1VA^-1

What is Ohm’s law?

For a metallic conductor kept at a constant temperature, the current the wire is directly proportional to the p.d across its ends.

When the p.d across the wire (kept at constant temperature) doubled, the current in the wire also doubled.

What’s the relationship between temperature and resistance?

As a wire gets hotter, its resistance increases.

• The positive ions have more internal energy and vibrate with greater amplitude about their mean position.

• The frequency of the collisions betwen the charge carriers (free electrons in the metal) and the positive ions increases, and so the charge carriers do more work, transfer more energy as they travel throughout the wire.