10: Electricity and circuits

Alternating current

Alternating current

current flow consisting of charges that continually change direction

ammeter

a device connected in series with a component to measure the current that flows through it

amp ( A )

unit of current

Circuit breaker

A safety device that cuts off the power supply if a surge of current passes through it

coulomb

unit of charge

currents at a junction

the sum of current entering a junction must always equal the sum of currents leaving it

diode

a component that only allows current to flow in the forward direction

direct current

current flow consisting of charges flowing in a single direction only

earth wire

the green and yellow striped safety wire connected to metal casings that prevents an appliance from becoming live

electric current

the rate of flow of charge

energy transfers in circuits

electrical energy is transferred to thermal energy when current does work against a resistance

filament lamp

a light emitting component consisting of an enclosed metal filament

fuse

a safety device consisting of a thin metal filament that melts and cuts off a power supply if there is a surge in current

Light Dependent Resistor

A light sensitive component whose resistance decreases as its temperature increases

Light emitting diode

a device that gives out light when a current flows through it. Current can only flow in one direction and a minimum voltage must be applied across it before it lights up

live wire

the brown coloured wire that carries the alternating current from the supply in a mains power supply

mains electricity

An a.c supply that, in the UK, has a frequency of 50 Hz and a value of 230V

Neutral Wire

the blue coloured wire that completes the circuit in a mains power supply

Ohmic Conductor

A conductor whose current flow is directly proportional to the potential differance across it, when kept at a constant temperature

Ohms

Unit of resistance

Ohm’s law

the current flowing through an Ohmic conductor at a constant temperature is directly proportional to the potential differance across it

Parallel

Components connected in parallel had the same voltage across each component an the total current is equal to the total sum of currents flowing through each component

potential differance ( voltage )

the energy that is transferred per unit of charge between 2 points in a circuit

power

the rate at which an appliance transfers energy

power for a circuit component =

current passing through it x voltage across it

resistance

a measure of the opposition to current flow

resistors in parallel

the total resistance is less than the lowest individual resistance

resistors in series

the total resistance is equal to the sum of the resistances of the individual resistors

series

components connected in series have the same current passing through but share the total voltage of the power supply

thermistor

a temperature dependent component, whose resistance increases as temperature decreases

volt

the unit of potential differance

1 volt =

1 joule / coulomb

volt metre

a device that is connected in parallel with a component to measure the potential differance across it

watt

the unit of power

energy transferred =

charge moved x voltage

charge =

current x time

voltage =

current x resistance

resistors heat up when used because …

… the electrons carrying the charge collide with the ions in a lattice in the resistor, increasing the kinetic energy, increasing the temperature

energy transferred =

current x voltage x time

power =

energy transferred / time taken

electrical power =

current x voltage

electrical power =

current ^ 2 x resistance