4.2.1 Electric charge
• Distinguish between electrical conductors and insulators and give typical examples
• Recall and use a simple electron model to distinguish between conductors and insulators

Static Electricity

When an insulator or conductor has static electricity, it contains an imbalance of positive or negative charges.

*Insulator: A material that does not easily allow electricity to pass through it.
Examples) A i r, Cloth and Rubber

*Conductor: A material that readily permits the passage of an electric currentthrough it. A material which contains movable electric charges.
Examples) Metal, Graphite, Plasma
• Most of the conductors belong to the class of substances called....................

More information on Conductors, insulators and electron flow;

Explore the website below.

all about circuits

• State that there are positive and negative charges
• State that unlike charges attract and that like charges repel
• Describe simple experiments to show the production and detection of electrostatic charges
• State that charging a body involves the addition or removal of electrons.
• State that charge is measured in coulombs

Brainpop: Electricity, Current electricity,

What causes a charge? Friction

Consider an atom.
If the object is physically rubbed, the electrons can be transferred from one to the other. Charges can be +positive or - negative.

Electrostatic charges from thefreedictionary.com (Produced or caused by static electricity):

Simulation: JohnTravoltage from Phet
Make sparks fly with John Travoltage. Wiggle Johnnie's foot and he picks up charges from the carpet. Bring his hand close to the door knob and get rid of the excess charge. Answer the the questions below.
John Travoltage
Click to Run

  1. How does John Travoltage pick up charges? He picks up charges by gaining electrons from the carpet when rubbing his foot against it.
  2. What is his charge as he rubs his foot against the carpet? He becomes negatively charged.
  3. What is the carpet’s charge? The carpet is positively charged.
  4. What happens when John Travoltage touches the metal door knob? The electrons flow through the door knob.
  5. What is this term called? Discharge


Unlike charges attract.

+ve and -ve charges attract.


Like charges repel.

+ve and +ve charges repel.

-ve and -ve charges repel.

The same chargesrepel.

like repel unlike attract.JPG


• Give an account of charging by induction
• Recall and use the simple electron model to distinguish between conductors and insulators

Electrostatic Induction

This is when we cause a charge to be in a previously uncharged object.

Simulation from Phet Balloon and Static Electricity
Balloons and Static Electricity
Click to Run

1. Draw the diagram. What happened to the charged balloon?

(balloon, -ve)--> l + _ +_ + + wall
-ve charges are repelled from the surface of the wall.
-ve balloon is attracted to +ve on the wall.

2. Draw a charged rod and a piece of paper. Explain why the pieces of paper jiggling around between them.

Static Electricity from the Physics Classroom
Uses of electrostatics

• Describe an electric field as a region in which an electric charge experiences a force
• State that the direction of an electric field at a point is the direction of the force on a positive charge at that point
• Describe simple field patterns, including the field around a point charge and the field around a charged conducting sphere and the field between two parallel plates ( not including end effect )

Electric fields

An electric field is a region of space where a charged object experiences a force due to its charge.
Electric field lines can be used to show the direction and strength of the field. We take the direction of the field that a small +ve charge would accelerate if place in the field.
Electric field induced by a positive electric charge.png

Electric field induced by a negative electric charge.png
Electric field line induced by a positive charge

Electric field line induced by a negative charge

Dipole field from http://abyss.uoregon.edu
Electric Field Hockey simulation from Phet
Electric Field Hockey
Click to Run

Field lines due to parallel charged plates. Field between plates is uniform.
Electric field lines.PNG
Images from www.pstcc.edu/

Electric field lines for two oppositely charged parallel plates are evenly spaced except at the very ends. The field between plates is uniform and the electric field strength is the same anywhere between the plates.
Charged Parallel Plates from http://www.regentsprep.org
Watch Paintsprayer. mpg/Electric Fence. mpg
Stactic electricity

Brainiac - Electric Fence Published on 7 Nov 2006 Clip taken from UK Show, Brainiac, Season 1 Episode 4.


How an electric current may exist within a solid material such as a metal wire?
How do electrons "flow" through a solid material such as copper?
How does anything flow through a solid material, for that matter?

4.2.2 Current
• State that current is related to the flow of charge
• Use and describe the use of an ammeter, both analogue and digital
• State that current in metals is due to a flow of electrons
• Show understanding that a current is a rate of flow of charge and recall and use the equation I = Q/t
• Distinguish between the direction of flow of electrons and conventional current

Electric current (Electricity) is the rate of the flow of invisible particles called electrons. They go round a track of wire. Current is measured in ampere, the unit of amp (A).
1 ampere of electric current is the rate of electron motion equal to 1 coulomb per second:
I = Q/t
I = Electric current (amperes)
Q = Charge in motion (coulombs)
t = Time (seconds)
Coulomb: a unit of electrical charge equal to the amount of charge transferred by a current of 1 ampere in 1 second.
The brighter the bulb, the greater the current flowing.

Do you remember this!!!
A usage of current : When electric charges flow through wires, it produces heat. Some kinds of wire get so hot that they emit .....................
One kind of light bulb, called a filament lamp, uses this effect to produce light. They are expensive to run because they give off more ................... energy than ..................... energy.

Voltage and Current
  • We always place the ammeter in series with the component we are testing.
  • We always measure the Voltage in parallel with the component we are testing.
Voltage and Current
  • In a SERIES circuit the voltage is divided but the current remains the same. This is why we measure current in series.
  • In PARALLEL circuits the current is divided but the voltage remains the same. This is why we measure voltage in parallel.
Complete questions on Combination circuits from www.physicsclassroom.com


Think of a way to test whether water is a conductor or an insulator. Try your idea out.

• Most of the conductors belong to the class of substances called ....................

• I think that the hard shiny object that felt cold would .................... electricity, because it is probably made of a ....................

• Pure water is an .................... However,if there are any impurities in it, such as salt, or chlorine, then the water is a ....................

• Air is an .................... which explains why we do not get an electric shock when we stand near a mains electricity socket.

• Find out as much as you can about low-energy light bulbs. Explain to your partner why it is a good idea to use these instead of ‘normal’ light bulbs (the ones that get very hot!)

4.2.5 Resistance
  • State that resistance = p.d./current and understand qualitatively how changes in p.d. or resistance affect current
  • Recall and use the equation R = V/I
  • Describe an experiment to determine resistance using a voltmeter and an ammeter
  • Relate (without calculation) the resistance of a wire to its length and to its diameter

Homework reading!

Explain what Ohm's law is and find the properties of resistivity.

Using a tap, we can change the flow of water from fast to slow. With electricity, we change the flow using a resistor.
Resistance is the opposition to the electricity. Resistance is measured in ohm, the unit (Ω) is ohm(s). A resistor limits the flow of electricity. The bigger the resistance, the smaller the electric current. The rate of the flow of electric charges can be reduced by adding more resistance to a series circuit.

The unit is named after German physicist Georg Simon Ohm. The Ohm’s law states that the electrical charges flowing through an electrical element(a resistance) between two points on the element( the resistor) is directly proportional to the potential difference(voltage) across those points. The unit ohm can be defined as the resistance offered by an element when a voltage of 1 volts applied to the element produces a current of 1 ampere in the element.
What are three variables affecting electrical resistance from tutorvista

Investigation project on Ohm's Law

Support material for Physics investigation lab report
Aim: To find out the resistance of an unknown resistor using Ohm's law.
To construct a Voltage against Current graph to analyze the resistance of an unknown resistor using a variable resistor (rheostat).
Introduction/Background information :
Ohm's law resistance from BBC Bitesize
Ohm's Law from ftp://ftp.pasco.com


What is it? Name it.
Methods of management or measurement.
Independent varialble (unit)

Dependent variable (unit)

Controlled variable 1

Controlled variable 2

Controlled variable 3


Ohms law 1.PNG

Method: Numbered step measuring resistance using voltmeter and ammeter
1) Construct your circuit the same as the diagram above.
2) With the rheostat placed to the far RIGHT, take readings of both current (Ammeter) and potential difference (Voltmeter) across the unknown resistor.
( In IB DP Physics, ensure you add uncertainties in your readings.)
3) Slide the rheostat a small distance to the right and repeat step 2. Repeat this process until you have AT LEAST 6 different readings for both potential difference and current.
4) Make a graph of potential difference (y-axis) against current (x-axis).
5) Draw a line of best fit through your graph ensuring it passes through the origin.

1. Explain why do we need to ensure the line of best fit passing through the origin.
2. Draw error bars for all data points.
3. Do the same experiment using a filament lamp and find the resistance of it.

Results:(mA means milliamp……..1000 mA = 1 A)
Raw data table
Voltage (V)
Current (A)
Voltage (V)
Current (A)
Trial 1
Trial 1
Trial 2
Trial 2
Brightness / Colour / Change in Temperature / Change in shape / Smell / etc.

Processed data table (You need to prepare some values to plot a graph. Think of what values you need to plot.)
Any calculated data

Sample calculation ( i.e. one example of each calculation you used in your processed data table)

Graph ( V against I graph )
  1. Plot V against I graph. Add the line of best fit.
  2. Calculate the gradient if the line of best fit is showing a linear relationship and find the value of your resistance. (Gradient and resistance a warning from www.furryelephant.com)
  3. Calculate an ‘average’ resistance of your resistor and compare it to your graphical answer.

Some questions to consider !
1. Describe what happens to V when I increases.
2. Describe the value of R. Are there any change in R as I increases?
3. Describe and explain if your resistance is Ohmic or Non-ohmic (To answer this, you will need to read a lot about Ohm’s law).
4. Identify possible causes of uncertainty, in data or in a conclusion

Evaluation: Improvements to be made.
1. Were your graphical and calculated results the same? What could have caused any differences?
2. Is your experiment valid?
3. Is your procedure reliable?
4. Identifies strengths of the investigation.
5. Discuss how the weakness/limitation might have impacted the results and think of any improvements you can suggest in your methods if you do the experiment again.
6. Think of any investigations you would like to do further. (i.e. Finding a resistance using other devices such as a filament bulb.)

1. Restate your results. Were they what you expected?
2. Describe whether your hypothesis is supported or not?

Don't forget to include Bibliography! (MLA format)

In report writing,
  • Use specific scientific notation and terminology but, otherwise, use simple sentence structure and language to express your ideas.
  • Include evidence supporting your ideas.
  • Do not start your discussion/analysis/evaluation/conclusion with ‘I’. Start with one of the following suggesstions. ( this result, this data, the method used in this investigation, the limination, it,... )
  • Human error occurs all the time and it is one of the biggest sources so indicate what you’ve done to reduce the error but do not use it as an evaluation as it is random error which means it is hard to improve or control. Focus your evaluation on ‘systemic errors’ as it is easier to improve and find a solution.
  • Give specific examples of interpretations and applications.
  • Make sure to include if your investigation is valid and reliable and explain the reasons why.

Further information you can consider including in a Physics investigation report taken from IGCSE 0625 Syllabus
Homework: Complete Ohm's law lab report (Due on MB: 22nd September 2017) Bring your printed lab report on 29th September 2017 for peer editing.
Compare your graph with the sample report below.

• Sketch and explain the current-voltage characteristic of an ohmic resistor and a filament lamp
Ohm's law investigation if a filament lamp is Ohmic or non-Ohmic.(Thank to Mihn Tri for sharing the original copy of this lab report.)

• Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire
Resistance in a wire from Phet

Answer to the questions below.
  1. What variables affect the resistance in the wire?
  2. What must happen for the wire’s resistance to be at its greatest?
  3. What must happen for the wire’s resistance to be at its least?
Electricity from the Physics Classroom
Watch resistance.flv


Fuses are safety devices that we use in electricity. Fuses are included in circuits to stop excess currents from flowing. If for any reason the current becomes too high the fuse gets hot and melts. The circuit is broken and stops working.
The current rating of the fuse should be just above the value of the current that flows when the appliance is operating normally.
Read the worked example 3 on your text page 213.
The fine fibre of steel wool gets hotter than the other wires. In fact, it gets so hot that it melts. Try to find out what temperature steel melts at. Once the fibre melts, there is an air gap in the circuit. No electric current can flow.

A fuse contains a fine metal wire. When the flow of electricity gets too big, this metal gets so hot that it ...................., and breaks. This creates an air .................... in the circuit, which stops the flow of electricity. This stops the other .................... in the circuit from getting too hot, and causing a fire.

Watch Voltage, Current and Resistance then, Series and Rarallel Circuits video on iLearn.

Short circuit: A circuit where the electricity can get from one side of the power supply to the other without going through a electronic device.

1. What is the difference between direct current (d.c.) and alternating current (a.c.).

a) Why do we have a fuse in an appliance or a plug?

b) How does a fuse work?

3. This question 3 is about an oven.
a) If the normal working current of the oven is 9A. What size fuse should be fitted?

b) If a fault occurs and a current of 15A flows, explain how the fuse would protect the appliance from overheating and causing a fire.

c) What is the advantage of having a circuit breaker rather than a fuse to protect an appliance?

4.2.3 Electro-motive force
• State that the e.m.f. of a source of electrical energy is measured in volts
• Show understanding that e.m.f. is defined in terms of energy supplied by a source in driving charge round a complete circuit

The volt is defined as the energy transfer per coulomb of charge as charges move between two points in a circuit.
V = ΔW / ΔQ
i.e. energy change per unit charge (so that 1 V = 1 J C-1)
Introduce the terminology of electromotive force (voltage across a source of electrical energy) and potential difference (voltage across a component that uses electrical energy). Despite its name, emf is not a force but a voltage, measured in volts.
from Teaching Advanced Physics Episode 105

4.2.4 Potential difference
• State that the potential difference(p.d.) across a circuit component is measured in volts
• Use and describe the use of a voltmeter, both analogue and digital

Mains Electricity from BBC Bitesize
The electricity which comes into your house through the wall sockets is called Mains Electricity.
It is different from the electricity which comes from cells in two important ways.
1. It is alternating current (a.c.)
2. It is at a much higher voltage.

Mains electricity in the UK is supplied at 240 Volts, China is at 230 Volts, Vietnam is 220 Volts and 110 Volts in the USA. The battery provides the electrical push (energy per charge) measured in volts. Mr Volt pushes the electrons around the circuit.
  • The Electricity we get from a plug is very dangerous. We must NEVER play with this type of electricity. Cells, Batteries and power packs are safe and all of our experiments will use these.

• State that charge is measured in coulombs
• Recall that 1V is equivalent to 1 J/C


Electric Potential Difference (Electrical "pressure") between two different terminals.
Voltage is the energy transferred per unit charge passed. The volt is a joule per coulomb.
Voltage is measured in the unit of the volt (V).
1 volt is equal to 1 joule of energy imparted to 1 coulomb of charge (6.25 ×1018 electrons):
V = Voltage (volts)
W = Work, or potential energy (joules)
q = Charge (coulombs)
Coulomb: a unit of electrical charge equal to the amount of charge transferred by a current of 1 ampere in 1 second.

DC is an acronym meaning Direct Current: that is, electrical current that moves in one direction only. AC is an acronym meaning Alternating Current: that is, electrical current that periodically reverses direction (alternates).

Watch potential difference.flv

4.2.6 Electrical energy
• Understand that electric circuits transfer energy from the battery or power source to the circuit components then into the surroundings
• Recall and use the equations P=IV and E=IVt



Power is the rate at which energy is transferred or transformed.
Power (W)
Energy transformed (J) / time taken (s)
1 W
1 J / s
The rate of at which energy is transferred in the circuit depends on both the e.m.f. V of the supply and the current I that it pushes round the circuit.
Power (W) = current (A) X potential difference (V)
P = VI
P = E/t, E = Pt, E = VIt

Homework: Do questions on page from 194 to 196.

Remember the equations and the units below.

E = VIt

P = E/t

P = VI

V = IR

P = I2R

V = W/Q = E/Q

Q = IT


Electricity consumption in your household

V = W/Q, Q = It, P = W/t,
W = VQ = VIt and W = Pt
thus, P = VI

Example) 3V, 0.4A is written on the packet of torch bulbs.
1. Calculate the power conversion for the bulb in normal use.
P = VI = 3V x 0.4A = 1.2 W
2. The life of the bulb is approximately 100 hours. How much energy will it have dissipated in its life time?
Energy consumption: E = Pt = 1.2 W x 100 hours = 1.2 W x 360000 s = 432000 J

Calculate estimated electricity cost of your household for a month.

1. What are your daily, weekly and monthly electricity consumption?

2. Estimate your electricity cost.

3. What is the biggest electricity consuming device?

Power of electronic device in watts
Number of device in your house
Daily time estimation using the device in hour
Daily Electricity Consumption in kWh
Weekly E.C. in kWh
Monthly E.C. in kWh
Electricity cost per hour in Shanghai
Monthly Electricity Cost in your household in RMB


4. What is your actual monthly electricity cost?

5. What are your strategies to reduce your electricity consumption?

4.3.1 Circuit diagrams
• Draw and interpret circuit diagrams containing sources, switches, resistors (fixed and variable), heaters, thermistors, light-dependent resistors, lamps, ammeters, voltmeters, galvanometers, magnetising coils, transformers, bells, fuses and relays
• Draw and interpret circuit diagrams containing diodes
Refer to your text page 198, 207


Standard electrical circuit symbols from BBC Bitesize
Try out the quiz on circuit symbols from www.sporcle.com
An electrical circuit is any continuous path for electrons to flow away from a source of electrical potential (voltage) and back again.

4.3.2 Series and parallel circuits
  • Understand that the current at every point in a series circuit is the same
  • Give the combined resistance of two or more resistors in series
  • State that, for a parallel circuit, the current from the source is larger than the current in each branch
  • State that the combined resistance of two resistors in parallel is less than that of either resistor by itself
  • State the advantages of connecting lamps in parallel in a lighting circuit
  • Calculate the combined e.m.f. of several sources in series
  • Recall and use the fact that the sum of the p.d.s across the components in a series circuit is equal to the total p.d. across the supply
  • Recall and use the fact that the current from the source is the sum of the currents in the separate branches of a parallel circuit
  • Calculate the effective resistance of two resistors in parallel

Simulation: Circuit Construction Kit (DC only) from Phet
Build circuits from schematic drawings. Use an ammeter and voltmeter to take readings in circuits.
Warning: There must be an unbroken path of conductors going from the power supply to the bulb/device and then back to the power supply.
Circuit Construction Kit (DC Only)
Click to Run

  • Provide reasoning to explain the measurements and relationships in circuits.
  • Discuss basic electricity relationships in series and parallel circuits.
  • Provide reasoning to explain the measurements in circuits.

Type of Circuit
Voltage splits
Voltage is the same
Current is the same
Current splits

4.3.3 Action and use of circuit components

  • Describe the action of a variable potential divider (potentiometer)

V2 out = [ R2 / (R1 +R2 ) ] V


V IN = V1 + V2 out

V IN = (IR1 + V2 )

V IN = [ R1 ( VIN / RTotal ) + V2 out

V2 out = [ VIN - VINR1 / (R1 +R2 ) ]

V2 out = [ R2 / (R1 +R2 ) ] VIN

Image from www.webanswers.com
Potential dividers divide up the voltage within a circuit, so that parts of a circuit only receive the voltage they require. Potential dividers usually consist of two or more resistors arranged in series across a power supply.
Potential divider from BBC Bitesize

Potentiometer: A variable resistor with a third adjustable terminal. The potential at the third terminal can be adjusted to give any fraction of the potential across the ends of the resistor.
  • Describe the action of thermistors and light- dependent resistors and show understanding of their use as input transducers
Thermistor: An electrical device whose resistance decrease as its temperature increases
How fire alarm(circuiteasy.com/fire-alarm) works? Image from text p.208
Fire alarm circuit.jpg
LDR: Light Dependent Resistors

How a lamp switched on in the dark automatically? Image from your text p 208.
Auto light on in dark.jpg
Thermistors and LDRs from BBC Bitesize
Thermistors and LDRs from schoolphysics.co.uk
  • Describe the action of a relay and show understanding of its use in switching circuits
Relay: Type of switch, typically incorporating an electromagnet, that is activated by a current or signal in one circuit to open or close another circuit.

Image above from BBC GCSE Bitesize
Image left from itp.nyu.edu

A relay is a switch that’s controlled by a small electric current. Relays take advantage of the fact that when you pass an electric current through a wire, a magnetic field is generated surrounding the wire as well. This is called induction.

Transducer: a device that converts variations in a physical quantity, such as pressure or brightness, into an electrical signal. A transducer converts a signal in one form of energy to a signal in another.
Examples) Thermistor, LDR(Light Dependent Resistors), Potentiometers(They convert the change in displacement into change in the resistance, which can be measured easily.), Bridge circuits( (These convert the physical quantity to be measured into the voltage.), Wheatstone bridge (It converts the displacement produced by the physical quantity to the current in the circuit.)

  • Describe the action of a diode and show understanding of its use as a rectifier
Diode: ​A semiconductor device with two terminals, typically allowing the flow of current in one direction only.
​Diode from BBC Bitesize
LED (Light emitting Diode) from BBC Bitesize

  • Describe the action of a transistor as an electrically operated switch and show understanding of its use in switching circuits
Transistor: ​ Components which do not conduct electricity unless they are turned on by different electrical current. It can act as an amplifier as a transistor can control its output in proportion to the input signal. Alternatively, the transistor can be used as a switch to turn current on or off in a circuit.
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit.
Transistor switching circuit from BBC Bitesize

How does a Transistor work? Let's watch and figure it out!

Published on 9 Jul 2013 by Veritasium

  • Recognise and show understanding of circuits operating as light sensitive switches and temperature-operated alarms (to include the use of a relay)
Light sensitive switches and temperature operated alarms by Desaqila ramlee Prezi
Transistor, LDR operating as light sensitive switches to turn on a LED in a circuit from educypedia

  • Describe the action of a capacitor as an energy store and show understanding of its use in time- delay circuits - Removed 2017/2018 ~
Capacitor: A device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator.
Capacitors from www.technologystudent.com
Capacitors from BBC Bitesize
A capacitor by Adam Britton
4.4 Digital electronics
  • Explain and use the terms analogue and digital in terms of continuous variation and high/low states
The difference between a digital signal and an analogue signal;
Digital signal: A signal which can be represented as a series of 1s and 0s. It can be perfectly copied.
Analogue signal: A continuously varying signal. It degrades as it is copied.
Digital and analogue signals from BBC Bite size
What could they look like graphically?
Digital signals
Analogue signals

  • State that logic gates are circuits containing transistors and other components
Transistor in a circuit from text p.208
  • Describe the action of NOT, AND, OR, NAND and NOR gates
  • Recall and use the symbols for logic gates
  • Design and understand simple digital circuits combining several logic gates
  • Use truth tables to describe the action of individual gates and simple combinations of gates

Logic gates are digital electronic devices that allow an electronic system to make a decision based on a number on its inputs.
Each logic gate (NOT, AND, NAND, OR, NOR etc) acts in a different way, and the action of any logic gate is shown by a Truth Table.

Logic gates and the symbols from BBC Bitesize
Circuit symbols for logic gates.jpg

IGCSE Electric Circuits Revision notes by physicsatweb.com

1) What do we call signals where only an on or off option is possible?
2) The device used for rectification
3) Changing a ac signal to a dc signal
4) Gates which give a high output if neither inputs are high
5) Gates which give a high output if either input is high
6) The number of leads on a transistor
7) An electronic switch or amplifier
8) A device for storing charge
9) Gates which give a high output if both inputs are high
10) A temperature dependent input device
11) An analogue to digital converter
12) A device which converts analogue signals such as sound into a digital signal
13) What do we call signals that can have a range of values?
1) Digital
2) Diode
3) Rectification
5) Or
6) Three
7) Transistor
8) Capacitor
9) And
10) Thermistor
11) Logic gate
12) Transducer
13) Analogue

Logic Gates 04: NAND, NOR, XOR and XNOR Published on 1 Apr 2011

4.5 Dangers of electricity
  • state the hazards of
    • – damaged insulation
    • – overheating of cables
    • – damp conditions
Dangers of electricity from physicsatweb.com

  • State that a fuse protects a circuit
  • Explain the use of fuses and circuit-breakers and choose appropriate fuse ratings and circuit-breaker settings
Fuses and circuit breakers from BBC Bitesize

• Explain the benefits of earthing metal cases
Electrical current finds the easiest path to go through so electricity flows down to earth instead, in case there is huge resistance, such as you in the circuit. Earth wire effectively bypass the current (short-circuit) to the ground and not damaging anybody or device.
Earth wire and fuse.JPG

Image from www.edexcel-cie.com
external image Earthing3.png

Article taken from Electrical safety from BBC BITESIZE
You would get an electric shock if the live wire inside an appliance, such as a cooker, came loose and touched the metal casing.
However, the earth terminal is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock. A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse and disconnects the appliance.

Double insulation

Some appliances - such as vacuum cleaners and electric drills - do not have an earth wire. This is because they have plastic casings, or they have been designed so that the live wire cannot touch the casing. As a result, the casing cannot give an electric shock, even if the wires inside become loose. These appliances have double insulation.
external image small
Symbol for double insulation

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