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ElectricityElectricity

Lesson 4Lesson 4

• Revision of basics• Mains Electricity• AC/DC• Safety• Power• Cost of electricity• Electric Charge

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Basic ideas…Basic ideas…Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.

Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta.

Resistance is anything that resists an electric current. It is measured in _____.”

(Words: volts, amps, ohms, voltage, ammeter, voltmeter)

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More basic ideas…More basic ideas…

If a battery is added the current will ________ because there is a greater _____ on the electrons

If a bulb is added the current will _______ because there is greater ________ in the circuit

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SummarySummary

In a SERIES circuit:

Current is THE SAME at any point

Voltage SPLITS UP over each component

In a PARALLEL circuit:

Current SPLITS UP down each “strand”

Voltage is THE SAME across each”strand”

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Georg Simon Ohm 1789-1854

ResistanceResistance

Resistance is anything that will RESIST a current. It is

measured in Ohms, a unit named after me.

The resistance of a component can be calculated using Ohm’s Law:

Resistance = Voltage (in V)

(in ) Current (in A)

V

RI

Mains electricityMains electricity19/04/23

1.Mains electricity (230 volts in the UK) is dangerous and can kill if not used safely

2.Electrical appliances can be connected to the mains using;

• a cable (containing an inner core of copper, and an outer layer of plastic)

• a plug (containing a plastic case, brass pins, fuse, earth pin and cable grip)

3.Mains electricity is an a.c. supply

AC/DCAC/DC19/04/23

1.An alternating current (a.c.) constantly changes direction. 50Hz means the current changes direction 50 times in one second

2.Mains electricity is an a.c. supply3.A direct current (d.c.) is found in

battery/cell circuits. The current always flows in the same direction

Direct current (d.c.)Direct current (d.c.)Direct current (d.c.)Direct current (d.c.)

Direct current (d.c.)Direct current (d.c.)

Alternating current (a.c.)Alternating current (a.c.)Alternating current (a.c.)Alternating current (a.c.)

Alternating currentAlternating current

The difference between alternating current (a.c.) and direct current (d.c.) can be seen using an oscilloscope.

For each current, the oscilloscope trace is a graph showing how the voltage of an electricity supply varies with time.

The voltage of a d.c. supply is steady and always in the same direction.

The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zerochanges direction and so on.

d.c.d.c.

time

volt

ag

e a.c.a.c.

time

volt

ag

e peak forward voltage

peak reverse voltage

What is the difference between a.c. and d.c.?What is the difference between a.c. and d.c.?

The frequency of a.c. electricity is the number of complete cycles per second, which is measured in hertz (Hz).

a.c.a.c.

timevolt

ag

e peak forward voltage

peak reverse voltage

1 complete cycle

Frequency of alternating Frequency of alternating currentcurrent

The a.c. frequency can be determined from an oscilloscope by counting the number of complete waves per unit time.

frequency is doubled, the number of waves doubles.

If the frequency is increased, the number of complete waves shown on the screen increases. For example, if the

Comparing a.c and d.c.Comparing a.c and d.c.

Mains CircuitsMains Circuits

• Every mains circuits has a live wire and a neutral wire

• Neutral wire is Earthed at a substation

• Live wire alternates its voltage from + to – every cycle

• It goes alternates between 325V (+to-)

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230V230V

• Because mains electricity alternates from +325V to -325 V

• This is equivalent to a DC voltage 230V

• We say that the mains voltage is 230volts

• Changes 50 times per second (50Hz)

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PlugPlugPlugPlug

PlugPlug

PlugPlugPlugPlug

PlugPlug

A fuse is a safety device that protects an electric cable from overheating so that the insulation does not catch fire.

What is a fuse?What is a fuse?

A fuse also makes appliances with a metal case safer.

appliance.

If the live wire becomes loose and touches the metal case, a very large current flows along the earth wire and blows the fuse.

This makes it safe to touch the

A fuse is a built-in weak point in a circuit. It contains a thin wire with a higher resistance than normal wire.

thin wire with high

resistance

case made of insulating material

terminals

How does a fuse work?How does a fuse work?

When a large current flows the wire becomes hot. If too much current flows, the wire overheats and melts, whichbreaks the circuit.

A fuse is labelled with the maximum current that it will allow to flow through it.

To choose the correct fuse for an electrical device, always choose the one with the closest rating that is greater than the operating current of the device.Example:

If a kettle operates with an electrical current of 4.3 A, what fuse should it be fitted with? You can choose from fuses of 3 A, 5A and 13A.5 A

fuse

How do you choose the How do you choose the correct fuse?correct fuse?

Selecting a fuseSelecting a fuse

• Fuses may be 3, 5 or 13A• Calculate current of device, then

select a Fuse• 500w heater at 230V• 500/230 = 2.2A• Therefore use 3A fuse

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FusesFuses19/04/23

1. A fault in an appliance can cause the current to be too great and can lead to the wire overheating and possibly causing a fire.

2. A fuse is designed to melt if the current through a circuit is too high, thereby breaking the circuit

3. The fuse should be just higher than the normal working current

4. A circuit breaker can be reset rather than replaced

You choose which fuse to You choose which fuse to use!use!

FusesFusesFusesFuses

FusesFuses

FuseFuseFuseFuse

FuseFuse

EarthEarth19/04/23

1. If an appliance has a metal case it needs to be earthed.

2. A fault could cause the live wire to touch the metal case. If the appliance is earthed a large current flows down the earth wire and melts the fuse. If the appliance was not earthed there would be a risk of electrocution.

Earth wireEarth wireEarth wireEarth wire

Earth wireEarth wire

An electrical fault (1)An electrical fault (1)An electrical fault (1)An electrical fault (1)

An electrical fault (1)An electrical fault (1)

An electrical fault (2)An electrical fault (2)An electrical fault (2)An electrical fault (2)

An electrical fault (2)An electrical fault (2)

A house has several circuits and each one is protected by a separate circuit breaker.

The electrical wiring in a building must be protected from being overloaded so that it does not overheat.

What is a circuit breaker?What is a circuit breaker?

This is the job of a ‘fuse box’, which used to contain fuse wire but now contains circuit breakers to protect the wiring.

Circuit breakers do the same job as fuses but they are electromagnetic switches which are easy to reset.

Circuit breakers have different ratings as shown by the coloured dots.

The circuit breakers in a ‘fuse box’ are some of the most important safety mechanisms in your home.

switch

electromagnet

How does a circuit breaker How does a circuit breaker work?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.

Each circuit breaker is an electromagnetic switch which is designed to break the circuit when the current gets too high.

A residual current device (RCD), also called a residual current circuit breaker (RCCB), is another type of circuit breaker.

residual current device residual current device (RCD)(RCD)

This safety device compares the current in the live and neutral wires, which are the same when the appliance is working properly.

If the current is leaking, the RCD detects a difference between the live and neutral wires and instantly shuts down the power.

It prevents electric shocks when using extension cables to appliances like lawnmowers.

Electrical power is the rate at which an electrical appliance uses electrical energy. All appliances have a power rating.

electrical powerelectrical power

Power is measured in watts (W). 1000 watts = 1 kilowatt (kW). 1 watt of power means that 1 joule of energy is used every second.

Appliances that need to create heat, such as washing machines, cookers, hair dryers and kettles, usually use the most power.TVs, radios and computers usually use the least amount of power.

PowerPower19/04/23

1.Power = Current x Voltage2.Power is measure in Watts3.1 Watt means that 1 Joule of

energy was changed in 1 second

4.Energy transferred = Voltage x Charge

5.Charge = Current x Time

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Energy and PowerEnergy and PowerThe POWER RATING of an appliance is simply how much energy it uses every second.

In other words, 1 Watt = 1 Joule per second

E

TP

E = Energy (in joules)

P = Power (in watts)

T = Time (in seconds)

A filament bulb has a potential difference of 200 V across it and a current of 0.2 A running through it.

How is power calculated?How is power calculated?

At what power is the filament bulb operating?

= 40 W

= 0.2 A x 200 V

P = IV

PowerPowerPowerPower

PowerPower

Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in?

1 unit of electricity = 1 unit of electrical energy

Example:How many units of electricity is 17.6 kWh?17.6

units

The amount of electrical energy (i.e. the amount of electricity) used by an appliance depends on its power and how long the electricity is used for.

electrical energy = power x time

Calculating the units of Calculating the units of electricityelectricity

= 1 kilowatt hour (kWh)

Electricity costs money, which is why every home has an electricity meter.

The units of electrical energy are called kilowatt hours (kWh).

How is electricity paid for?How is electricity paid for?

The cost of an electricity bill is calculated from the number of units used.

The meter records how much electricity is used in a house in units of electrical energy.

The cost of electricity is the number of units of electrical energy multiplied by the cost per unit.

Example:How much would 10 units of electricity cost at a price of 9p per unit?

cost = 10 units x 9 p/unit

cost = number of units x cost per unit

How much does electricity How much does electricity cost?cost?

= 90 p

A kettle uses 45.2 kWh of energy.If electricity costs 10 p per unit, how much does it cost to use the kettle?

Number of units:

Cost of electricity:

Buying electricity – exampleBuying electricity – example

= 45.2 unitsnumber of units of electricity = number of kilowatt hours

= 452 p or £4.52

= 45.2 units x 10 p / unit

cost = number of units x cost per unit

An iron that operates at a power of 3 kW for 4 hours uses electricity that costs 8p per unit. How much does it cost for the electricity used by the iron in that time?

Number of units:

Cost of electricity:

Buying electricity – exampleBuying electricity – example

= 12 units

= 12 kWh

= 3 kW x 4 hnumber of units of electricity = number of kilowatt hours

= 96p

= 12 units x 8 p / unit

cost = number of units x cost per unit

Electric chargeElectric charge

• When a device is on, electrons are forced through the device by the P.D

• The P.D causes a flow of charge through the device carried by the electrons

• Unit of charge is a Coulomb (C).

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Charge QCharge Q

• Unit is Coulomb (C)• Equal to the charge flow when

current is 1 ampere for 1 second• Charge flow = I x t• Example – 5A in 200s is 1000 C• 1 A is 1 coulomb per second

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CalculationsCalculations

• Q=I x t• Calculate Q, (0.4A, 10s)• Calculate Q, (0.4A, 10 minutes)• Calculate I, (5s, 10C)• Calculate I, (15s, 15C)• Calculate t, (15C, 3A)• Calculate t, (10.5C, 9A)

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AnswersAnswers

• 4C• 240C• 2A• 1A• 5s• 1.17s

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Electrical power – Electrical power – calculationscalculations

Buying electricity – Buying electricity – calculationscalculations

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