P4. Electric Circuits

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 = I t

Charge (coulombs) = Current (amperes) x 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

1. Potential difference
2. Resistance

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 = I R

Potential difference (V) = Current (A) × 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 remain 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. Doodles
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 increase

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 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

What are the two ways that a component can be connected in a circuit?

Answer 14

1. Series (same loop)
2. Parallel (adjacent loop)

Question 15

How does the potential difference across two components vary when connected in series and parallel?

Answer 15

Series: Total Potential difference between each component
Parallel: Potential difference across each component is the same

Question 16

If two resistors are connected in parallel, what can be said about their combined total resistance?

Answer 16

Their total resistance is less than the smallest of the two individual resistances

Question 17

If two resistors are connected in series, what can be said about their total resistance?

Answer 17

Their total combined resistance is equal to the sum of the two individual resistances

Question 18

Describe the current in a series circuit

Answer 18

In a series circuit, the current is the same at all positions since the charge only has one path to flow through

Question 19

Describe the current in a parallel circuit?

Answer 19

In a parallel circuit, the current is shared between the different branches. When the charge reaches a junction it splits.

Question 20

How should you connect a voltmeter in a circuit to measure potential difference?

Answer 20

Voltmeters should be connected in parallel to the component that they are measuring the potential difference of

Question 21

Why is it advantageous to connect lamps in parallel?

Answer 21

If one lamp blows, the rest will be unaffected and can still receive current (I.e. The circuit is still complete)