Module 4

Question 1

Define electric current

Answer 1

The rate of flow of charge

Question 2

What is the coulomb?

Answer 2

The unit for charge

Question 3

What is the charge on an electron, in terms of elementary charge?

Answer 3

-e

Question 4

What is the charge on a proton in terms of elementary charge?

Answer 4

+e

Question 5

What is current?

Answer 5

The movement of electrons in metals and movement of ions in electrolytes

Question 6

What is convential current, and how is this different from electron flow?

Answer 6

Convential current is the flow of charge from positive to negative. Electron flow, which is what actually happens, is electrons moving from negative to positive, so has the opposite direction

Question 7

Define Kirchhoff’s first law

Answer 7

For any point in an electrical circuit, the sum of the currents into that point is equal to the sum of currents out of that point

Question 8

Define number density of a material

Answer 8

Number of free electrons per cubic metre of material

Question 9

Describe how materials can be classified as conductors, semiconductors and insulators

Answer 9

Their number density is used, where a high number density suggests that it is a conductor, and a very low number density suggests an insulator

Question 10

Define mean drift velocity

Answer 10

The average velocity of electrons as they move through a wire

Question 11

Define potential difference

Answer 11

The measure of the transfer of energy by charge carriers.

Question 12

Define the volt

Answer 12

The potential difference across a component when 1 J of energy is transferred per unit charge passing through the component

Question 13

Define electromotive force

Answer 13

The energy transfered from chemical to electrical energy per unit charge

Question 14

Give the difference between e.m.f. and p.d.

Answer 14

• Potential difference is used to describe when work is done by the charge carries.
• Electromotive force is used to describe when work is done on the charge carriers

Question 15

Define resistance

Answer 15

A property of a component calculated by dividing the potential difference across it by the current in it.

Question 16

Define the ohm

Answer 16

The resistacne of a component when a p.d. of 1 V is produced per ampere of current

Question 17

Define ohm’s law

Answer 17

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

Question 18

Describe the I-V characteristics of a resistor

Answer 18

The graph of I against V gives a straight line throught the origin. The steeper the gradient, the lower the resistance

Question 19

Describe the I-V characteristics of a filament lamp

Answer 19

• The p.d. across the filament lamp is not directly proportional to the current through the resistor
• The resistance of the filament lamp is not constant
• The graph of I against V gives a ‘s’ shape through the origin

Question 20

Define ohmic conductor

Answer 20

A conductor which obeys Ohm’s law, e.g. a resistor

Question 21

Define non-ohmic component

Answer 21

A component which does not obey Ohm’s law, e.g. a filament lamp

Question 22

Describe the I-V characteristics of a diode/LED

Answer 22

• When the voltage is negative, there is no current
• As the voltage is increased, the resistance gradually starts to drop when the threshold p.d. of the specific diode is reached.
• Above this voltage, the resistance drops sharply for every small increase in the p.d., so the diode has very little resistance