Energy, Power and Resistance

Question 1

Potential difference (p.d.)

Answer 1

Potential difference, V, is used to measure the work done by charge carriers, which lose energy as they pass through the components in a circuit.

Question 2

Define pd

Answer 2

It is defined as the energy transferred from electrical energy to other forms, per unit charge.
It is measured in volts (V), where a potential difference of 1 volt is defined as 1 joule of energy transferred per coulomb.

Question 3

Electromotive force (e.m.f.)

Answer 3

Electromotive force, ε, is used to measure the work done to charge carriers, when they gain energy as they pass through a cell or power supply.

Question 4

The electron gun

Answer 4

An electron gun is a device used to produce a thin beam of electrons, which are accelerated to high speeds. A small metal filament, which acts as a cathode, is heated by passing a potential difference through it. Some of the electrons in the metal gain enough kinetic energy to escape the metal, in a process known as thermionic emission. The circuit is in a vacuum tube, with a high p.d., V, between the filament and the anode, so the freed electrons are accelerated towards the anode. If the anode has a small hole in it, a beam of electrons can pass through at a specific kinetic energy.

Question 5

Define emf

Answer 5

It is defined as the energy transferred from chemical energy to electrical energy per unit charge.

Question 6

What is resistivity and what its purpose

Answer 6

The resistivity, ρ, of a material is a physical property of the material. It is the same for any shape of a given material at a set temperature, and it acts as a constant to link the resistance of the material with its area and length.

Question 7

Resistivity is given using the formula

Answer 7

𝜌 = 𝑅A/L

Question 8

Resistivity is measured in

Answer 8

Ohm-meters (Ωm).

Question 9

The resistivity of a material varies

Answer 9

with temperature for the same material.

Question 10

Describe what happens to metals when the temperature is increased.

Answer 10

For metals, when the temperature is increased, the fixed metal ions will vibrate at a greater frequency and amplitude. This increases
the number of collisions of electrons with the ions, increasing the resistance, thus increasing their resistivity.

Question 11

Describe what happens to semiconductors when the temperature is increased.

Answer 11

For semiconductors, the number density of charge carriers increases with increasing temperature, so the resistance of the material decreases, thus decreasing their resistivity.