4.2.1 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 in a circuit depend on?

Answer 4

• Potential Difference (V)
• 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 (V), Current (A), Resistance (Ω)

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 Dependant Resistors (LDRs)

Question 8

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

Answer 8

● Resistance increases
● Ions in 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 current 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

1. In a thermostat to turn a heater on below a certain temperature
2. 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 decreases.

Answer 12

The LDR’s resistance increases.

Question 13

Give an application for a LDR.

Answer 13

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

Question 14

Potential difference in parallel circuit:

Answer 14

Question 15

Current in parallel circuit:

Answer 15