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Mains Electricity

SyllabusLesson 1 Safe ElectricityLesson 2 PowerLesson 3 Heating

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Mains electricity identify common materials which are electrical conductors or

insulators, including metals and plastics (P2.2) recall the hazards of electricity including frayed cables, long cables,

damaged plugs, water around sockets, and pushing metal objects into sockets (P2.3)

describe the uses of insulation, double insulation, earthing, fuses and circuit breakers in a range of domestic appliances (P2.4)

know some of the different ways in which electrical heating is used in a variety of domestic contexts (P2.5)

understand that a current in a resistor results in the electrical transfer of energy and an increase in temperature (P2.6)

recall and use the relationship power = current × voltage (P = I × V)and apply the relationship to the selection of appropriate fuses (P2.7)

use the relationship between energy transferred, current, voltage and time:energy transferred = current × voltage × time (E = I × V × t) (P2.8)

recall that mains electricity is alternating current (a.c.) and understand the difference between this and the direct current (d.c.) supplied by a cell or battery (P2.9).

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Safe Electricity 22/04/23 AimExplain the safety features in electrical equipmentConnect a plug

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Do not stick anything other than a plug in a socket.

Never mix water and electricity.

Do not overload sockets.

How can electricity be used safely?

Wire plugs correctly, and check they are not damaged.

Regularly check cables for fraying.

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copper

insulating plastic

What is inside an electrical cable?

The wires are made of copper (a good conductor)

The wires are surrounded by plastic (an insulator)

In each cable there are two or three wires

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What does each wire do?

The neutral wire completes thecircuit. It is kept at a zero voltageby the electricity company.

The earth wire is a safety wire that is needed to earth appliances with a metal case. This makes it safer to touch the appliance if it develops a fault.

The live wire carries a current that alternates between a negative and positive voltage.

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How does earthing work?

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The fuse does two jobs.•It protects the wiring if something goes wrong.

•It can also protect us•A fuse has a rating in amps (ie 13A) the rating of the fuse must be greater than the current used by a device

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You choose which fuse to use!

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switch

electromagnet

How does a circuit breaker work?

Too much current makes the magnetic field produced by the electromagnet strong enough to open the switch.

The circuit breaker is said to ‘trip’ and switches off the current.

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What does ‘double insulated’ mean?

• Double insulated appliances have plastic cases

• Thus the case cannot become live

• So even if a wire inside the case breaks the appliance is still safe

symbol on double insulated appliance

Another way of improving the safety of electrical appliances is to make them double insulated.

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Safety device How it works / Notes

Fuse

• Melts when too much current flows.• Value of fuse must be higher than current

drawn by the device• Must be in live wire

Circuit Breaker• Breaks the circuit when too much current flows• Must be in live wire• Can be reset

Earth• Protects from shock with metal cased

appliances• Any charge on outside case will be dissipated

to earth (also causing the fuse to melt)Double Insulated

• Plastic exterior means that even if live wire touches case user will not receive a shock

PlasticInsulator

• Grips outer cable and prevents strain on the inner coloured wires.

• Is an insulator, stops current flowing from wire

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Earth Wire

Outer Insulation

Cable Grip

Neutral Wire Fuse

Live Wire

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Identifying wrongly-wired plugs

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Live wire Brown

Neutral wire Blue

Earth wire

Green and yellow stripes

Practical: Connect your plug up safely

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Electrical Safety1) An electric iron is fitted with an earth

and a plug containing a fusea) Describe briefly how a fuse worksb) Give two reasons why a fuse in a

plug is important for the safe use of mains-operated appliance

c) Why should the fuse in the plug be connected to the live wire?

d) Explain how earthing the iron protects the user from receiving an electric shock

2) A fault in an electrical circuit can cause too great a current to flow. Some circuits are switched off by a circuit breaker. One type of Circuit breaker is shown. A normal current is flowing. Explain, in detail, what happens when a current which is bigger than normal flows.

Worksheet

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Materials and properties of plug parts

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What does each wire do?

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Can you wire a plug?

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Earthing and fuses – true or false?

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Safe Electricity recap

AimConnect a plugExplain the safety features in electrical equipment

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Power 22/04/23Aimexplain alternating currentcompare ac and dc tracescalculate Power

Starter:a

b

de

c f

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1½ V

6V

12 V

230V

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1 ½ V

+-

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AC supply

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AC supply

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AC supply

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AC supply

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Comparing a.c and d.c.

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Mains ElectricityMains electricity is

Alternating Current.Small Circuits

(Batteries) use Direct Current

This means that the current changes from positive to negative.

Frequency (Hz) is number of waves per second

d.c.timevo

ltage

a.c.timevo

ltage

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Which type of current – a.c. or d.c.?

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Power• Power is “the rate of doing work” • Or how much energy does it use a second

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Calculating Power - Mechanical Method:

Whose the Most Powerful in the Class?

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Power

Energy = Power x time

( joule) (Watt ) (seconds)

E = Energy (J)

P = Power (W)

t = time (s)

E

tP

• Power is “the rate of doing work” (or how much energy it uses in a second)

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Some example questions1) What is the power rating of a light bulb that transfers

120 joules of energy in 2 seconds?2) What is the power of an electric fire that transfers

10,000J of energy in 5 seconds?3) Farhun runs up the stairs in 5 seconds. If he transfers

1,000,000J of energy in this time what is his power rating?

4) How much energy does a 150W light bulb transfer in a) one second, b) one minute?

5) Shaun’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a physics lesson?

6) Damien’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day?

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Calculating Power – Electrical Method

• What would happen if a greater voltage was applied

• What would happen if the bulb was replaced with a more powerful one?

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Power and fusesPower is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: P

VI

Power = current x voltage in W in A in V

Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A).

Peter Is a Veggie

Power and fusesCopy and complete the following table:ApplianceAppliance Power Power

rating (W)rating (W)Voltage (V)Voltage (V) Current Current

needed (A)needed (A)Fuse Fuse

needed (3, 5 needed (3, 5 or 13A)or 13A)

ToasterToaster 720720 240240

FireFire 20002000 240240

HairdryerHairdryer 300300 240240

HooverHoover 10001000 240240

ComputerComputer 100100 240240

StereoStereo 8080 240240

P

VI

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Mains Electricity recapAimexplain alternating currentcompare ac and dc tracescalculate Power

Heating 22/04/23AimTo understand the mechanism of electrical heatingTo name uses for resistors in homesTo calculate the energy transferred by a heater

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Resistance Simulation

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Electrical HeatingAs the electrons flow through a wire they

will occasionally collide with an atom.This results in a transfer of energy to the

wire causing it to heat.

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Electrical HeatingResistors are used in the home mostly for

heating:KettleElectric FireElectric OvenHeat BulbHair DryerElectric Radiators / Heaters

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Power Equations

Combining the equations:Energy = Powertime

Energy = voltage x currenttimeEnergy = voltage x current x time

E = V x I x t(or E = V I t) (J) (V) (A) (s)

Power = current x voltage

Energy = Power x time

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Example Question:A 12V immersion heater draws 2 A of current for

100 seconds how much energy is transferred?

What do we know?V = 12VI = 2At = 100sE = V x I x t

E = 12 x 2 x 100E = 2400 J

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Experiment: Measuring the Energy Transferred

Method:1. Measure 250 ml of water into a 250 ml beaker2. Connect up the circuit 3. Use a clamp to suspend the immersion heater

in the beaker4. Turn on the immersion heater5. Measure the temperature, Voltage and

Current and start your stopwatch6. Take readings every minute for 10 minutes

Temperature (°C) Time (s) Voltage (V) Current (A) Energy (J)060

Results:Plot a graph of Energy (X) against Temperature (Y)

V

A

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Heating recapAimTo name uses for heaters in homesTo understand the mechanism of electrical heatingTo calculate the energy transferred by a heater