TOPIC 3 - KEY POINTS

Question 1

What is electric current ?

Answer 1

The rate of flow of charge

Question 2

What is the equation that works out current ?

Answer 2

current = charge/time or current = voltage/resistance

Question 3

What is electric current measured with ?

Answer 3

An ammeter

Question 4

What is Kirchoff’s first law ?

Answer 4

That the total current entering a junction is equal to total current leaving a junction

Question 5

What happens to the current in series?

Answer 5

The current stays the same along the whole circuit as the rate of entrance must be equal to rate of exit, as otherwise electrons would be lost.

Question 6

What happens to the current in parallel?

Answer 6

The current splits because it has more than one way to go.

Question 7

What is drift velocity ?

Answer 7

The average speed of a particle when a material attains an electrical field.

Question 8

What needs to be present for current to flow?

Answer 8

Suitable charge carriers in a material e.g.electrons

Question 9

What happens when electrons are charge carriers?

Answer 9

They drift from negative to positive terminal, opposite to conventional current.

Question 10

Equation to work out drift velocity?

Answer 10

I = nAvq

Question 11

What happens to particle velocity if the cross-sectional area gets smaller?

Answer 11

Electrons move quicker since there is less of them

Question 12

What is thermal velocity

Answer 12

Before electrons attain an electrical field to travel they always move in random motion

Question 13

The ability to conduct depend upon…

Answer 13

material or the amount of charge carriers available

Question 14

How can we increase the ability to conduct ?

Answer 14

By either increasing temperature or adding impurity atoms

Question 15

Name some semiconductors..

Answer 15

Germanium Silicion

Question 16

What is a conduction band ?

Answer 16

Range of energy where electrons are delocalised and can move freely

Question 17

What is a valence band?

Answer 17

Range of energy where electrons are kept close to an atom

Question 18

How big is the band gap in insulators.

Answer 18

Massive, even increasing temperature doesn’t give enough energy to the electrons before the material is damaged.

Question 19

Gap in semiconductors

Answer 19

Fairly close, can be bridged by thermal excitations

Question 20

Gap in conductors?

Answer 20

There isn’t one since bands overlap.

Question 21

What is superconductivity?

Answer 21

Where the resistance drops to 0

Question 22

Where can superconductivity be used ?

Answer 22

A particle accelerator to maximise current

Question 23

What happens to metals and semi conductors when temperature is increased?

Answer 23

Metals have a decrease in electron density due to thermal expansion.

Semi-conductors become better conductors since electrons absorb thermal energy and so there is more conduction electrons.

Question 24

What is p.d.

Answer 24

Work done per unit of charge to transfer electrical energy into components.

Question 25

What pushes p.d.

Answer 25

Chemical reaction in the cell which creates an emf, since electrons experience a force.

Question 26

How to work out p.d.

Answer 26

Workdone/ charge or current x resistance