Electricity

Circuits + Domestic electricity

fixed resistor

A resistor restricts or limits the flow of electrical current. A fixed resistor has a resistance that does not change.

variable resistor

Moving the position of the slider on this resistor, changes the resistance. A variable resistor is used in some dimmer switches and volume controls.

switch (open)

switch (closed)

cell

battery

diode

resistor

variable resistor

LED

lamp

fuse

voltmeter

ammeter

thermistor

LDR

Voltmeter

measures potential difference

always connected ‘across’ a component (known as parallel)

Ammeter

measures current
always connected ‘in line’ with a component (known as series)

What is current

rate of flow of electric charge.

current flows from

+ to -

current is

conserved - it has the same value everywhere in a single closed loop

current going into a junction = current going out of a junction

current through a component

depends on the resistance of the component and the p.d across the component

• More resistance = less current

• More p.d = more current

the current through a fixed resistor at constant temperature

is directly proportional to p.d across it

the difference between ohmic and non-ohmic reistors

Ohmic - obey ohms law

Non-Ohmic- do not obey ohms law

ohmic resistor

fixed resistance (at a constant temp)
current is directly proportional to pd

resistance of a filament lamp

increases as current and temp increase

diodes

only let current flow in one direction

because the resistance is so high in the reverse direction

resistance of a thermistor

decreases as temperature rises and this can be used in a thermostat

resistance of an LDR

decreases as it gets brighter and this can be used to automatically switch on lights

when components are connected in series:

• there is the same current through each component

• the total potential difference of the power supply is shared between the components

• the total resistance of two components is the sum of the resistance of each component.

when components are connected in parallel

the potential difference across each component is the same

the total current through the whole circuit is the sum of the currents through the separate components

the total resistance of two resistors is less than the resistance of the smallest individual resistor

adding resistors in series

increases resistance because the current is the same in every single component in the circuit, so the more resistors we add, the harder it is for current to flow.

adding resistors in parralell

decreases resistance because because there are more paths for the current to pass through.

What is pd

the difference in the amount of energy that charge carriers have between two points in a circuit

Alternative current

The electrons are constantly changing their direction of movement, the flow of charge is alternating

Direct Current

one-directional flow of electric charge

Energy is transferred by

circuit components

Power is the

rate of energy transfer

Step-up transformers

Increase pd (voltage)
Current goes down

Power lines have a

low current (avoid energy loss) and high voltage (dangerous)

For a circuit to work we need

we need a source of potential difference and a closed circuit in order for electrical charge to flow

that UK mains electricity

frequency of 50Hz and about 230V

Alternating vs Direct Potential Differences

In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forward" and then going "backward."

most appliances connect to the mains using

a three-core cable

earth wire

-green and yellow

-doesn’t normally carry a current

-safety wire: stops appliance becoming live

-only carries a current if there is a fault in the circuit
-pd = 0V

live wire

-brown

-electricity flows through this wire

-carries alternating pd at 230V from the mains

-starts off the flow of current through the circuit

neutral wire

-blue

-electricity flows out through this wire

-neutral wire completes the circuit

-0V

why is mains electricity dangerous?

the live wire is dangerous even if a switch is open and it is dangerous to provide a connection between the live wire and earth

a powerful device

transfers more energy per second and will have a larger p.d across it or a larger current through it

energy transfers in electrical appliences

-Every electrical appliance in the home is designed to transfer energy from one store to another.

energy transfer in a torch

the energy stored in the battery is used to heat up the filament of the bulb.

Work is done

when a charge flows in a circuit and energy is transferred

that the longer a device is used

the more energy will be transferred and the more powerful a device is the more energy will be transferred

The National grid

a system of cables and transformers that link power stations to consumers

step-up transformers

increase p.d for efficient transmission in power lines

increasing p.d reduces the current, so less energy is wasted as heat

step-down transformers

reduce the p.d to a much lower value for safe domestic use