2.1 Current, potential difference and resistance

Question 1

What is electric current?

Answer 1

The flow of electrical charge

Question 2

State the equation linking charge, current and time. Give the units for the quantities involved.

Answer 2

Q = It

Charge (Coulombs), Current (Amperes), Time (Seconds)

Question 3

What can be said about the value of current at any point in a single closed loop?

Answer 3

Current is the same at all points in a closed loop

Question 4

What two factors does the current of a circuit depend on?

Answer 4

1. Potential Difference (V)
2. Resistance (R)

Question 5

What equation should be used to calculate potential difference if current and resistance are known? State the units for all 3 quantities.

Answer 5

V = IR

Potential Difference (Volts), Current (Amperes), Resistance (Ohms)

Question 6

What is an ‘Ohmic Conductor’? State the condition required.

Answer 6

A conductor for which current and potential difference are directly proportional.

Resistance remains constant as current changes. Temperature must be constant.

Question 7

List four components for which resistance is not constant as current changes

Answer 7

1. Lamps
2. Diodes
3. Thermistors
4. Light Dependent Resistors (LDR)

Question 8

What happens to the resistance of a filament lamp as the temperature increases?

Answer 8

Resistance increases - ions in the metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow.

Question 9

What is different about current flow through a diode?

Answer 9

The resistance only flows In one direction. Resistance is very high in the other direction, preventing current flow.

Question 10

State what happens to the resistance of a thermistor as temperature increases

Answer 10

The thermistor’s resistance decreases

Question 11

Give two examples of when a thermistor may be used

Answer 11

In a thermostat, to turn a heater on below a certain temperature.

In a freezer, to turn on a cooler when the temperature becomes too high.

Question 12

State what happens to the resistance of a LDR as light intensity increases

Answer 12

The LDR’s resistance increases

Question 13

Give an application of a LDR

Answer 13

Street lights often use LDRs. When the light levels become too low, the light gains sufficient current to turn on.