P4 Electrical circuits

Question 1

Function of a fixed resistor

Answer 1

Limits the current in a circuit

Question 2

Function of a variable resistor

Answer 2

Allows the current to be varied

Question 3

Function of an indicator

Answer 3

An indicator, such as a bulb, is designed to emit light as a signal when a current passes through it

Question 4

Function of a diode

Answer 4

Allows current through in one direction only

Question 5

Function of an ammeter

Answer 5

Used to measure electric current

Question 6

Function of a fuse

Answer 6

Designed to melt and therefore ‘break’ the circuit if the current through it is greater than a certain amount

Question 7

Function of a switch

Answer 7

Enables the current in a circuit to be switched on or off

Question 8

Function of a cell

Answer 8

Necessary to push electrons around a complete circuit. A battery consists of two or more cells

Question 9

Function of a voltmeter

Answer 9

Used to measure potential difference i.e. voltage

Question 10

Function of a heater

Answer 10

Designed to transfer the energy from an electric current to heat the surroundings

Question 11

Function of an LED

Answer 11

A light-emitting diode emits light when a current passes through it

Question 12

What is an electric current?

Answer 12

A flow of charge

Question 13

What determines the size of an electric current?

Answer 13

The bigger the number of electrons that pass through a component each second, the bigger is the current passing through it

Question 14

How to calculate the size of an electric current from the charge flow and time taken

Answer 14

charge flow = current x time taken
charge flow (coulombs, C)
curent (amps, A)
time taken (seconds, s)

Question 15

Ohm’s Law

Answer 15

The current through a resistor at constant temperature is directly proportional to the potential difference across the resistor

Question 16

What is potential difference?

Answer 16

A measure of the work done or energy transferred transferred to the lamp by each coulomb of charge that passes through it, measured in volts

Question 17

How to calculate the potential difference across a component

Answer 17

potential difference = energy transferred / charge
potential difference (volts, V)
energy transferred (joules, J)
charge (coulombs, C)

Question 18

How to calculate the resistance of an electrical component

Answer 18

resistance = potential difference / current
resistance (ohms)
potential difference (volts, V)
current (amps, A)

Question 19

Why is a wire called an ohmic conductor?

Answer 19

Its resistance stays constant as the current changes, provided its temperature is constant

Question 20

What happens when you reverse the potential difference across a resistor?

Answer 20

Reversing the potential difference across a resistor reverses the current through it

Question 21

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

Answer 21

The resistance of a filament lamp increases as the filament temperature increases. The atoms in the metal filament vibrate more as the temperature increases. They resist the passage of electrons through the filament more. The resistance of any metal increases as the temperature increases

Question 22

What happens to the resistance of a temperature-dependent resistor if the temperature increases?

Answer 22

A thermistor’s resistance decreases if its temperature increases

Question 23

What happens to the resistance of a light-dependent resistor if the light intensity increases?

Answer 23

The resistance of an LDR decreases if the light intensity increases

Question 24

Describe the current, potential difference, and resistance for each component in a series circuit

Answer 24

In a series circuit:
- the same current passes through each component
- the total potential difference of the power supply is shared between the components
- the total resistance of two or more components in series is equal to the sum of the resistance of each component

Question 25

Describe the potential difference of several cells in series

Answer 25

The total potential difference of cells in series is the sum of the potential difference of each cell

Question 26

Why does adding more resistors in series increase the total resistance?

Answer 26

The total potential difference is shared between more resistors, as a result the potential difference across each of them is less than before. The current though the resistors is therefore less than before, and as the total potential difference is unchanged, the total resistance is greater

Question 27

Describe the current, potential difference, and resistance for each component in a parallel circuit

Answer 27

In a parallel circuit:
- the total current through the whole circuit is a sum of the currents through the separate branches
- for components in parallel, the potential difference across each component is the same
- the total resistance of two or mote components in parallel is less than the resistance of the resistor with the least resistance