Electricity

Page 1

Current and Circuit Symbols

Electrical current - flow of electrical charge

Electrical charge will only flow around a complete (closed) circuit if there is a potential difference

Unit of current is ampere (A)

In a single closed loop, current has the same value everywhere in circuit

Potential difference/voltage is driving force that pushes charge around

Unit is the volt (V)

Current flowing through component depends on pd and resistance

Greater the resistance, smaller the current (for a given potential difference)

Size of current is the rate of the flow of charge

When current flows past a point in a circuit for a length of time then the charge that has passed is given by formula:

Charge Flow = Current x Time

More charge passes around circuit when larger current flows

Page 2

Resistance and V=IR

Potential difference = Current x Resistance

P.d in volts

Current in amps

Resistance in ohms

For some components, as the current is changed, the resistance changes

Resistance of ohmic conductors doesn't change with the current

At a constant temperature, current flowing through an ohmic conductor is directly proportional to potential difference

R is constant in V = IR

Resistance of some resistors and components does change (diode or filament lamp)

When a electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament

Which is designed to heat up

Resistance increases with temperature, so as current increases, filament lamp heats up more and resistance increases

For diodes, resistance depends on direction on direction of current

They will let current flow in one direction, but have a high resistance is it's reversed

Page 3

Circuit Devices

An LDR is dependant on light intensity

In bright light, resistance falls

In darkness, resistance is highest

Have lots of applications including automatic night lights, outdoor lighting and burglar detectors

Thermistor is a temperature dependant resistor

In hot conditions, resistance drops

In cool conditions, resistance goes up

Thermistors make useful temperature detectors

Sensing circuits can be used to turn on or increase power to components depending on conditions they are in

Page 4

Series Circuits

In series circuits, different components are connecetd in a line

Between the negative and positive of power supply

Voltmeters are always connected in parallel

If you remove or disconnect one component, the circuit is broken and they all stop

Generally not very handy, and in practice very few things are connected in series

Can use rules to design series circuits to measure quantities and test components:

Potential Difference is shared

In series circuits, the p.d of the supply is shared between various components

So p.d around circuit always adds up to equal source's p.d

V total = V1 + V2...

Current is the same everywhere

In series circuits, the same current flows through all components

Size of the current is determined by the total p.d of the cells and total resistance of the circuit

R total = R1 + R2...

Adding a resistor in series, two resistors have to share total p.d

Potential difference across resistors is lower

In a series circuit, current is the same everywhere so total current in circuit is reduced when resistor is added - total resistance increases

Bigger a component's resistance, the bigger its share of total potential difference

Cell Potential Difference add up

There's a bigger p.d when more cells are in series, if they're all connected in the same way

Page 5

Parallel Circuits

Each component is separately connected to the positive and negative of the supply

Except ammeters, which are connected in series

If you remove or disconnected one of them, it will hardly affect others

This is obviously how most things must be connected

Have to be able to switch everything on and off separately

Everyday circuits often include mixture of series and parallel parts

In parallel circuits, all components get the full source potential difference, so it is the same across all components:

V1 = V2 =...

Means that identical bulbs connected in parallel will all be at the same brightness

In parallel circuits, total current flowing around circuit is equal to the total of all currents through separate components

I total = I1 + I2

There are junctions where the current either splits or re-joins

Total current going into a junction must equal the total current leaving

If two identical components are connected in parallel then the same current will flow through each component

If you have two resistors in parallel, their total resistance is less than resistance of the smallest of the two resistors

In parallel, both resistors have the same potential difference across them as the source

Means the "pushing force" making current flow is the same as the source potential difference for each resistor that you add

By adding another loop, current has more than one direction to go in

Increases total current that can flow around the circuit

An increase in current means a decrease in total resistance of the circuit

Page 6

Electricity in the home

Two different types of electricity supplies:

Alternating current

Direct current

In ac supplies, current is constantly changing direction

Are produced by alternating voltages in which positive and negative ends keep alternating

UK mains supply (around your home) is an ac supply at around 230v

Fequency of ac mains supply is 50 hertz

Cells and batteries supply direct current

Direct current - always flowing in the same direction

Created by a direct voltage

Most electrical appliances are connected to mains supply by three-core cables

Means they have three wires inside them, each with a core of copper and coloured plastic coating

Colour of insulation on each cable shows it purpose

Always the same for every appliance

This is so it's easy to tell the different wires apart

Live Wire - Brown

Provides alternating potential difference (about 230v) from mains supply

Neutral Wire - Blue

Completes circuit and carries away current

Electricity normally flows in through live wire and out through neutral wire

Around 0v

Earth Wire - Green and Yellow

Protecting wiring and for safety - stops appliance casing from becoming live

At 0v

Doesn't usually carry a current - only when there's a fault

Body is at 0v

Means that if you touch the live wire, a large potential difference is produced across your body and a current flows through you

Causes a large electric shock which could injure or even kill you

Even if a plug socket or a light switch is turned off there is still a danger of an electric shock

Current isn't flowing but there's still a potential difference in the live wire

If you made contact with live wire, your body would provide a link between supply and the earth, so a current would flow through you

Any connection between live and earth can be dangerous

If the link creates a low resistance path to earth, a huge current with flow, which could result in a fire

Page 7

Power of Electrical Appliances

A moving charge transfers energy

Because charge does work against the resistance of the circuit

Work done is the same as energy transferred

Electrical appliances are designed to transfer energy to components in the circuit when current flows

No appliance transfers all energy completely usefully

Higher the current, the more energy is transferred to the thermal energy stores of the components (and then surroundings)

Total energy transferred by an appliance depends on how long the appliance is on and its power

Power of an appliance is the energy that it transfers per second

So more energy it transfers in a given time, higher its power

Amount of energy transferred by electrical power is given by:

Energy transferred = Power x Time

E = Pt

Energy in Joules

Power in Watts

Time in seconds

Appliances are often given a power rating - labelled with maximum safe power that they can operate at

Can usually take this to be their maximum operating power

Power rating tells you the maximum amount of energy transferred between stores per second when appliance is in use

Helps customers choose between models - lower the power rating, the less electricity an appliance uses in a given time and so the cheaper it is to run

A higher power doesn't necessarily mean that it transfers more energy usefully

An appliance may be more powerful than another, but less efficient, meaning it might still transfer the same amount of energy to useful stores

When an electrical charge goes through a charge goes through a change in potential difference, then energy is transferred

Energy is supplied to the charge at the power source to "raise" it through a potential difference

Charge gives up this energy when it "falls" through any potential drop in components elsewhere in the circuit

Energy Transferred = Charge Flow x Potential Difference

E = QV

Means that a battery with a bigger potential difference will supply more energy to the circuit for every coulomb of charge which flows round it

Because charge is raised up "higher" at the start

As well as energy transferred in a given time, the power of an appliance can be found with:

Power = Potential Difference x Current

P = VI

Power in watts

Potential difference in volts

Current in amps

Can also find the power if you don't know the potential difference

Power = Current squared x resistance

Page 8

The National Grid

National Grid - giant system of cables and transformers that covers the UK and connects power stations to consumers (anyone who is using electricity)

Transfers electrical power stations anywhere on the grid (the supply) to anywhere else on the grid where it's needed

Throughout the day, electricity usage changes

Power stations must produce enough electricity for everyone to have it when they need it

Can predict when the most electricity will be used

Demand increases when people get up in the morning, come home from work/school and when it gets dark or cold

Popular events could also cause a peak in demand

Lots of smaller power stations that can start up quickly are kept in standby

To transmit huge amount of power needed, you need either a high potential difference or a high current

Problem with a high current is that you lose lots of energy as the wires heat up and energy is transferred to thermal energy store of the surroundings

Much cheaper to boost the p.d. up really high and keep the current as low as possible

For a given power, increasing the p.d. decreases the current

Which decreases the energy lost by heating the wires and the surroundings

Makes the national grid an efficient way of transferring energy

To get potential difference to 400,000v to transmit power requires transformers as well as big pylons with huge insulators - still cheaper

Transformers must step the potential difference up at one end, for efficient transmission, and then bring it back down to safe, usuable levels at the other end

Potential difference is increased using a step-up transformer

Then reduced again for domestic use using a step down transformer