Electrostatics

ELECTROSTATICS

• Electrostatics is the study of static electric charges.
• Each atom has negatively charged electrons that orbit round a nucleus.
• A nucleus is made up of
• positively charged particles called protons
• neutral particles called neutrons
• In an uncharged atom,the number of electrons is equal to the number of protons.
• If the atom gains electrons, it becomes negatively charged.
• If the atom loses electrons, it becomes positively charged.
• We can charge certain materials by rubbing them against each other.
• Some electrons from the surface atoms of one material will be transferred to the other material.
• Charge cannot be created or destroyed during friction. It can only be transferred from one object to another.
• Table below shows the charges produced on some materials during friction.
• The force between two charged objects brought close together is called electrostatic force.
• Like charges always repel each other. Unlike charges always attract each other.

Electroscope

• An electroscope is a device used to
• detect charges
• test the kind of charge
• Figure below shows the structure of a typical electroscope.

Detection of Charges

• In an uncharged electroscope,the gold leaf is close to the brass plate. It is said to be in a collapsed state.
• The brass cap, brass rod, brass plate and the gold leaf are electrically neutral. This means that there is no excess charge.
• When a charged object, for example a positively charged glass rod, is placed near the brass cap, free electrons from the brass and gold parts of the electroscope are attracted to the brass cap.
• This results in the brass plate and gold leaf to be positively charged.
• Being positively charged, the gold leaf will diverge since like positive charges repel each other.
• Hence, we can conclude that the glass rod is charged.

Everyday Phenomena Related to Static Electrical Charges

Spray painting

• In car production, electrostatic spray painting is frequently used.
• The car's body and the spray nozzle are given opposite charges.
• The paint can be applied uniformly throughout the car. 

Lightning

• Usually, flashes of lightning can be seen before and during a thunderstorm.
• This is caused by the large amount of electrical charges built up in the thunderclouds.
• The thunderclouds are charged due to the friction between water molecules in them and the surrounding air molecules.
• When the charge on the thunderclouds gets large enough, it ionizes the air.
• The ionized air creates a conducting path for the large amount of charge to discharge in the form of lightning to the nearest or sharpest object on the ground.

Fires and explosions

• Too much electric charges produced by friction may cause fires and explosions.
• For example, an aeroplane during a flight accumulates electric charges due to friction with the air molecules.
• An oil tanker becomes charged by friction with air when it moves.

Combing hair

Combing hair on a dry day produce charges. Hair becomes charged and will be attracted to the charged comb.

Safety Measures Taken to Deal with Static Electricity

• During a thunderstorm, do not swim in the open sea or swimming pool, play in an open field or at the beach, or take shelter under a tree.
• Soldiers who build trenches on mountainous areas must not use zinc sheets as shelter from thunderstorms.
• Use lightning conductors on tall buildings to prevent damages caused by lightning.The conductor becomes a discharge path for the electrons to flow from the top of the building down to earth.
• Use slightly conductive rubber for tyres of an aircraft to allow excess electrons to discharge harmlessly during landing.
• A metal chain hanging from the back of the oil tanker allows charges to flow to the ground.

Electricity

• Electricity is a form of energy that is used in electrical appliances such as computers, radios and electric fans.
• Electricity is a useful form of energy because it can be changed into many other forms.
• Sources of electricity for use in everyday life are
• Electric cells
• Small amounts of electrical energy are produced through chemical reactions.
• (Examples of electric cells are dry cells and accumulators.
• (They are used in many portable devices.
• Generators
• They convert mechanical energy into electrical energy.
• (Power stations use generators that are usually run by water, steam or gas.
• Solar cells that convert light energy from the sun into electrical energy.

Current

• Electric charges are made up of
• positive charges such as protons and positive ions.
• negative charges such as electrons and negative ions.
• When these electric charges flow, they produce current.
• Current is the rate of flow of electric charges.
• Figure 24.5 and Figure 24.6 below show the movement of free electrons in a conductor and the flow of electrons through a conductor, forming electricity.
• In an electric circuit, the direction of electric current is from the positive terminal to the negative terminal of a battery.
• However, electrons flow in the opposite direction, that is, from the negative terminal to the positive terminal of the battery.

Van de Graaff Generator

• The flow of electric current can be shown using a device known as the Van de Graaff generator.
• This device is a high voltage electrostatic generator which is able to produce a potential difference of millions of volts.
• It is used to create static electricity used for experimentation.
• It is made up of
• a motor
• two rollers
• a belt
• two brush assemblies and an output terminal (usually a metal or aluminum sphere).

Voltage

• Energy is needed to move electric charges through a conductor, in order to produce electricity.
• Voltage is the electrical force needed to move electric charges from one point to another in a conductor.

Resistance

• Resistance is the measure of how much an electrical component restricts current flow.
• A good conductor of electricity has a lower resistance and allows a larger electric current to flow through it.
• For example, copper is used in connecting wires as it has low resistance.
• Tungsten, on the other hand, has higher resistance. Some of the electrical energy that flows through it is transformed into light and heat.
• This energy loss slows down the movement of electric charges and reduces current flow.

Measuring Electricity

Ammeters

• An ammeter is the device used to measure electric currents.
• It has to be connected in series with the other electrical components in the circuit.
• By connecting in series, the current will flow into the ammeter by the positive (red) terminal and leaves by the negative (black) terminal.
• The deflection of the pointer of the ammeter shows the amount of current flowing through the circuit.
• The SI unit for electric current is the ampere (A).
• If it is connected wrongly, the ammeter
• may suffer from damages due to wrong connections
• does not measure the current of the required component
• does not show the correct reading as the pointer moves the wrong way.
• Figure X shows the correct and incorrect ways
• To measure the correct amount of current flowing in a circuit, an ideal ammeter should have a very low resistance so that the amount of electrical energy wasted is very small.

Voltmeters

• A voltmeter is the device used to measure the voltage across an electrical component.
• It has to be connected in parallel across the other electrical components in the circuit.
• A negligible amount of current enters the voltmeter by the positive (red) terminal and leaves by the negative (black) terminal.
• The deflection of the pointer of the voltmeter shows the voltage across the component.
• The SI unit for voltage is the volt (V).
• If it is connected wrongly, the voltmeter
• does not measure the voltage of the required component.
• does not show the correct reading as the pointer moves the wrong way.
• To avoid drawing a large amount of current from the circuit, an ideal voltmeter should have a resistance much larger than the component it is connected across.

Measuring the Current and Voltage of an Electric Circuit

• Set up the circuit as shown in Figure X.
• 'i' indicates the current flow in the circuit.
• The ammeter will show the amount of current flowing through the circuit.
• The voltmeter will show the voltage across the resistor R.
• By measuring the current and voltage, we can obtain the resistance of resistor R.
• The SI unit for resistance is the ohm (Ω).

Relationship Between Current and Voltage

Relationship between Current and Resistance

• Resistors are electrical component used to reduce the current flow in a circuit.
• A fixed resistor has a resistance that cannot be changed.
• A variable resistor, or rheostat, has a resistance that can be changed.
• The larger the resistance, the smaller the current flowing through the circuit.

Relationship between Voltage and Current

• If the resistance is fixed, we can change the current flow by changing the voltage of the electrical supply in the circuit.
• The larger the voltage, the larger the current flow through the circuit.

Ohm's Law

• Current flowing through a circuit is dependent on the voltage and resistance in the circuit.
• Ohm's law states that current flowing through a conductor is directly proportional to its voltage.
• This is true if the temperature and other physical conditions remain constant.

Electric Circuit

• An electric circuit is the path where electrical charges flow.
• It is made up of a source of electrical energy (such as batteries),connecting wires and electrical components (such as switches, resistors, ammeters, voltmeters or bulbs).
• These common components are represented by electric symbols when drawing circuit diagrams.
• Some of these symbols are shown in Table 24.4.
• Current will flow only through a complete circuit. & A complete circuit is a circuit that has no gaps.
• A gap occurs when a switch is open.This stops the current from flowing through the circuit.
• Figure 24.15 shows the difference between a complete and open circuit.
• There are two main types of electric circuits:
• (a) series circuit
• (b) parallel circuit

Series Circuits

• A series circuit connects an electrical source with its
• components, one after another, in a single loop.
• Current flowing through each component is the same throughout the circuit.
• A disadvantage of this circuit is that if there is a break in any part of the circuit, current flow throughout the whole circuit is cut off.
• Light bulbs connected in series are less bright than those connected in parallel.
• If one bulb is disconnected, the other bulbs do not light up because the circuit becomes incomplete.

Parallel Circuits

• In a parallel circuit, the current source is split into two or more branches.
• Current flowing through each branch in the parallel circuit may be the same or different.
• However, it is certainly less than the current flowing out of the electrical source.
• The current entering any junction in the circuit is also equals to the current leaving that junction.
• Light bulbs connected in parallel are brighter than those connected in series.
• If one bulb is disconnected, the other bulbs can still light up because current can still flow through the other paths.

Current, Voltage and Resistance in a Circuit

Series Circuit

Current in a Series Circuit

• The current at every point in a series circuit is the same.
• In the circuit in Figure X, I = I1 = 12 = I3.

Voltage in a Series Circuit

• The sum of voltage in a series circuit is equal to the voltage across the whole circuit.
• In the circuit in Figure X, V = V1 + V2 + V3.

Resistance in a Series Circuit

• As more resistors are added to the circuit, resistance will increase.
• The sum of the resistances of the bulb is equal to the total resistance of the circuit.
• For n resistors in a series, the total resistance is R = R1 + R2 + R3 ... + Rn
• where R = total resistance and n 2.
• In the circuit in Figure X, R = R1 + R2 + R3. Example:
• Bulbs X and Y with resistances 2Ω and 3Ω respectively are connected in series as shown in Figure X.
• What is the reading on the ammeter?
• Calculate the voltage across each bulb.

Advantages and Disadvantages of a Series Circuit

Advantages

• As there are no wire junctions in the circuit and all components are placed consecutively on the same path, current flow will decrease if more components are added.Thus, we can control the amount of current flow in the circuit by changing the number of components.
• The current flowing through the circuit can be increased by connecting more cells in series.
• If a component in the circuit fails, the whole system will shut down rather than run dangerously. This reduces electrical hazards (such as electric shock).

Disadvantages

• If a component in the circuit fails, then all the components in the circuit fail because the circuit has been broken.
• The more components there are in the circuit, the greater the resistance in the circuit.
• Bulbs in series will be dimmer because the electrical energy supplied is shared among more bulbs.

Parallel Circuit

Current in a Parallel Circuit

• The current from the source is the sum of the currents in the separate branches. 
• In the circuit in Figure X, I = I1 + 12.

Voltage in a Parallel Circuit

• The voltage across two or more components connected in parallel to each other is the same as the voltage across the circuit.
• In the circuit in Figure X, V = V1 = V2.

Resistance in a Parallel Circuit

• As more resistors are added to the circuit, total resistance will decrease.
• Each resistor acts as an alternative path for the current.
• Thus, the more resistors connected in parallel, the more alternative paths there are for current flow.
• For n resistors in parallel, the total resistance is 1/R = 1/R1 + 1/R2 where R. = total resistance

Example:
Bulbs X and Y with resistances 2 Ω and 3Ω respectively are connected in parallel as shown in Figure X .The ammeter shows a reading of 5 A.
(a) What is the total resistance in the circuit?
(b) Calculate the voltage across the bulbs.
(c) Calculate the current flowing through each bulb.

Advantages and Disadvantages of a Parallel Circuit

Advantages

• If a component in the circuit fails, the other components can still work.This is because a parallel circuit consists of more than one branch and each component can be controlled
• When more identical bulbs are added to the circuit in parallel, the brightness of the bulbs remains the same. This is because the bulbs receive the same amount of voltage from the cells.

Disadvantages

• Cells connected in parallel will result in the same amount of voltage as one cell.
• Power supplied to the circuit will be consumed much faster if more components are connected in parallel.

Similarities and Differences between Series and Parallel Circuits

In Terms of Current

• In a series circuit, current at any point in the circuit is the same. However, current in a parallel circuit is the sum of current in its individual branches.
• From Figure X, bulbs A and B have the same brightness, that is, I = I1 = /2.
• From Figure X, if I1 > I2, bulb C is brighter than bulb D, and I = I1 + I2
• If bulbs A, B, C and D are identical, bulbs C and D will be brighter then bulbs A and B.

In Terms of Voltage

• In series circuits, voltage can be increased by adding more cells to the circuit. However, adding more cells to parallel circuits does not increase voltage throughout the circuit.
• From Figure X, bulb V3 is the brightest, that is, Vi < V2 < V3.
• From Figure X, all three bulbs have the same brightness, that is, Vi = V2 = V3.

In Terms of Resistance

• The total resistance in a series circuit is the sum of the individual resistences, given by: R = R1 + R2
• The total resistance in a parallel circuit is given by:  1/R = 1/R1 + 1/R2

Magnetism

Magnet

• A magnet is an object that has the ability to attract magnetic materials such as iron, cobalt and nickel.
• A bar magnet has two ends called magnetic poles. They are
• (a) north-seeking pole (north pole)
• (b) south-seeking pole (south pole)
• When the magnet is hung freely from a thread, the north pole will always point to the north of the Earth while the south pole points to the south.
• This property of a bar magnet makes it suitable for use in a compass.
• A compass has a magnetised needle hanging freely inside it.
• The compass is used in the navigation of ships at sea, aeroplanes in the sky and on land vehicles.
• A magnetic field is the region around a magnet in which magnetic forces act.
• The magnetic force of a magnet is strongest at its poles.
• If the magnetic field is strong, the magnetic force is big. If the field is weak, the force is small.

Magnetic Field Lines

• A magnetic field consists of magnetic field lines.
• The stronger the magnetic field, the closer the magnetic field lines.
• Magnetic field lines differ according to the arrangement of the bar magnets.
• Two bar magnets placed with opposite poles (unlike poles) facing each other form different field lines than those with same poles (like poles) facing each other.
• The magnetic field lines always begin from the north pole and end at the south pole.
• If there are many field lines which are close together, this means that the field is strong.
• No two field lines can cross. Each has its own path. a When like poles are near each other, the field lines repel.
• When unlike poles are near each other, the field lines attract.
• When two fields cancel out, there are no field lines. This is called the neutral point. Refer to the spots marked X on Figure X.
• When field lines are spaced equally with all lines pointing to the same direction, it is called a uniform field.

Electromagnetism

• When current passes through a wire,a magnetic field is produced around the wire.
• The wire is said to have magnetised.
• This magnetic effect is called electromagnetism.
• The magnetic field lines form concentric circles around the wire.
• If a compass is placed near the wire, the magnetic field can cause the compass needle to deflect.
• A larger current in the wire will produce a stronger magnetic field.
• The magnetic field lines in a stronger magnetic field will be closer together.
• In a stronger magnetic field, the deflection of the compass needle will be greater.

Right-Hand Grip Rule

• To predict the direction of magnetic current around a wire, the right-hand grip rule is used.
• To use this rule, grip the wire with the right hand.
• The thumb should be pointing along the direction of the current.
• The fingers will point to the direction of the magnetic field around the wire.

Electromagnets

• An electromagnet is a magnet produced by the flow of electric current.
• It loses its magnetism when the current stops flowing.
• A coil of wire, or solenoid, carrying electric current can also show magnetic properties.
• The magnetic field lines around the coil are similar to the field lines around a bar magnet.
• The strength of an electromagnet can be increased by
• (a) increasing the number of turns of the coil
• (b) increasing the amount of current flowing through the coil
• (c) inserting a soft iron core into the coil.
• Electromagnets are used in many devices such as electric bells, microphones, telephones, loudspeakers, in cranes at dockyards and steelworks, and in hospitals to remove iron or steel splinters from our body.

Generation of Electrical Energy

Generators

• Power stations use various types of generators to produce electricity.
• Among these are
• thermal generators
•  hydroelectric generators
• diesel generators
• gas turbine  generators
• nuclear generators

Thermal generators

• Fuels such as coal and petroleum are burned in a boiler to produce steam.
• The steam produced,under high pressure, rotates the turbines at high speed.
• These turbines are connected to a large generator which produces electricity.
• The steam is then cooled until it condenses in a condenser and is channelled back to the boiler to repeat the process.
• The change of energy is as follows:
• Chemical energy from the fuel is converted to heat energy during burning.
• Heat energy is converted to kinetic energy as the steam turns the turbines and generator.
• Kinetic energy is converted to electrical energy as the generator produces electricity.

Hydroelectric generators

• Hydroelectric generators use water to produce electricity.
• Dams are built to hold water at a high altitude.
• When water from this dam is released in a tunnel, the water will cause the turbine which is connected to a generator in the tunnel to spin, generating electricity.
• The change of energy is as follows:
• Stored energy from the water is converted to mechanical energy as the water flows through the tunnel.
• Mechanical energy from the flowing water is converted to kinetic energy as the water turns the turbines.

Gas turbine generators

• Air is sucked in, filtered and compressed in a compressor.
• The compressed air is then mixed with natural gas.
• The mixture of compressed air and natural gas is ignited by a spark plug in a combustion chamber.
• The burnt mixture produces hot gas that expands with great force.
• This high-pressure, high-velocity gas from the combustion chamber turns the turbines and generator.
• The change of energy is as follows:
• Chemical energy from natural gas is converted to heat energy as the gas is burnt.
• Heat energy is converted to mechanical energy as hot gas from the burning mixture expands.
• Mechanical energy is then converted to kinetic energy as the gas turns the turbines.
• Kinetic energy from the turbines is converted to electrical energy as the generator produces electricity.

Diesel generators

• This generator works the same way as a diesel engine that uses diesel as fuel.
• Burning the diesel in an engine would produce a great thrust.
• This thrust would move the piston and turn the axle to rotate the generator.
• The change of energy is as follows:
• Chemical energy from the diesel is converted to heat energy as it is burnt after mixing with air.
• Heat energy is converted to mechanical energy as hot gas from the burning mixture expands and creates a thrust in the engine.
• Mechanical energy is then converted to kinetic energy as the gas turns the turbines.
• Kinetic energy from the turbines is converted to electrical energy as the generator produces electricity.

Nuclear generators

• A nuclear power station produces nuclear energy in a reactor.
• Heavy nucleus of an atom such as uranium and plutonium are split into fragments, releasing enormous amounts of energy.
• Energy from the nuclear fission reaction changes water to steam.
• The steam under high pressure turns the turbines, which in turn rotates the generator to produce electricity.
• The change of energy is as follows:
• Nuclear energy in the atoms is converted to heat energy when the atoms split.
• Heat energy is converted to kinetic energy as the steam turns the turbines.
• Mechanical energy is converted to electrical energy as the dynamo connected to the turbines produces electricity.

Alternative Sources of Energy

• Solar energy
• Energy from the Sun can be converted to electricity by solar cells (or photovoltaic cells)
• Some drawbacks of using solar energy include the high cost of infrastructure, the unavailability of solar energy at night and during seasonal periods such as winter.
• Wind
• Wind power is a more realistic and economically viable form of energy.
• Low operating costs and efficient generation of energy means that more countries are developing wind generators.
• Geothermal
• Geothermal energy comes from the structure of the Earth and its interior heat source and circulation.
• (b) There is a continual flow of heat energy towards the surface of the Earth through volcanoes and hot springs.
• Biomass
• Wood and animal waste can be burnt to produce energy.
• The decomposition of dead organisms produces fuels such as alcohol, methane and biogas (mixture of methane and carbon dioxide).

Transformer

• A transformer is a device that is used to change the voltage of an alternating current.
• An alternating current is one that flows back and forth, continually reversing its direction.
• It is useful for
• transmitting electrical power from power stations to the consumers such as households and factories.
• regulating voltages for the proper use of electrical appliances such as a radio or a mains-operated television.
• It consists of two coils of wire:
• primary coil connected to the irnput voltage
• secondary coil connected to the output voltage
• These coils are wound around a laminated soft iron core at an appropriate number of turns.
• The soft iron core is laminated to reduce heat loss in the core.
• The transformer transfers electrical energy from the primary coil to the secondary coil by electromagnetic induction between both coils.
• At the primary coil, an alternating current (a.c.) is
• applied which sets up a changing magnetic field.
• This induces a voltage in the secondary coil.

Step-up and Step-down Transformers 

• A step-up transformer
• has more turns in the secondary coil than in the primary coil.
• has a higher voltage in the secondary coil than in the primary coil. 
• A step-down transformer
• has more turns in the primary coil than in the secondary coil.
• has a higher voltage in the primary coil than in the secondary coil.
• Usually, a transformer is not 100% efficient.
• This means that the power supplied to the primary coil is not transferred completely to the secondary coil.
• This is due to the resistance in the wire and heat loss in the core.
• The efficiency of the transformer can be increased by using copper wire (low resistance) and the laminated iron core.

Role of Transformers

• Electricity is generated in the power stations as large values of current.
• If transmitted through cables, a lot of energy may be lost as heat due to the resistance in the cables.
• To reduce power loss due to resistance, very thick cables can be used. However, the cables would be very heavy and uneconomical to use.
• A step-up transformer is used to increase the voltage so that less energy is lost in the form of heat in the transmission cables.
• The step-down transformer is then used to reduce the voltage at different stages before reaching homes and factories.

Electricity Transmission and Distribution System

• The components of the electricity transmission and distribution system are:
• National Grid Network
• switch zone
• transformer station
• main substation
• branch substations
• Electrical energy is distributed from power stations to consumers through a network of cables that is called the National Grid Network. In this network, electrical energy is combined and then distributed to the consumers in an orderly and efficient mannner.
• The advantages of the national grid network include:
• Extra electrical energy from a power station in a particular area can be channeled to an area with higher demand.
• It ensures that electricity supply to consumers is not disrupted when a power station breaks down.
• Some power stations can be closed when demand is low and re-opened when demand is high.This will help save running costs and prevents wastage.
• Power stations produce electricity at a voltage of 11 kV.
• Step-up transformers are used to raise the voltage to 132 kV, 275 kV or 500 kV to reduce power loss while transmitting the current through cables.

Switch Zones

• Switch zones are located at several locations in the network.
• They are used to control the electrical energy in the network and channel it to designated areas.
• Each switch zone is equipped with 'switches' electrical energy leaving the power station can be controlled.

Main Substation and its Branches

• From the grid network, electricity is passed to the main substation.
• Here, the electric voltage is reduced to 11 kV 33 kV.
• The electricity is then sent to the branch substations or straight to heavy industrial areas.
• At the branch substation, the voltage is further reduced to 240V or 4I5V before it is distributed to consumers.

Electrical Supply and Wiring

Electrical Supply at Home

• There are two types of electric:
• direct current
• alternating current
• The electric current that is supplied to consumers at home through the mains is an alternating current.
• The voltage of the supply is 240V.
• The electrical energy used is calculated in kilowatt-hours (kWh) or joules (J).

Electrical Wiring System

• The electricity supply in homes are distributed through electric cables.
• Electric cables are  divided into two types:
• the live wire
• the neutral wire
• The live wire carries current at a voltage of 240 V from the local substation to homes.
• The neutral wire returns the current to the substation from the homes to be earthed.
• The voltage of the neutral wire is close to zero.
• The neutral wire completes the circuit.
• The electrical wiring system consists of the main fuse, the electric meter, the main switch, circuit breakers, live wire, neutral wire and earth wire.
• The main fuse acts as a safety device. If there is a large current flowing through it, this fuse will melt and the circuit will break.
• The electric meter will record the amount of electrical energy that has been used up.
• The consumer unit controls the current that flows through the different parts of the house. It consists of the main switch and circuit breakers.
• The main switch, when turned off, will cut off the current that flows through the circuit in the house.
• The circuit breakers will cut off the electric current flowing through the circuit under abnormal conditions.
• There are two types of wiring circuits at home
• lighting circuit
• power circuit.
• In the lighting circuit, all the lights are connected in parallel to prevent the circuit from breaking when there is one light that is faulty.
• The power circuit is the circuit connected to electrical appliances such as irons and cookers.
• There are two types of wiring systems:
• single-phase wiring system usually used in homes
• three-phase wiring system usually used in commercial and industrial areas

Single-phase and Three-phase Distribution Lines

• Electrical power is usually distributed from substations on three-phase distribution lines.
• Single-phase distribution line
• Sufficient for consumers in residential areas who need a low voltage such as 240V.
• It is common to tap single-phase lines off a three-phase line.
• Three-phase distribution line
• Runs along major streets, and commercial and industrial areas.
• Used in places in need of high voltage such as 415V.

Three-pin Plug

• Electrical appliances are connected to wall sockets through three-pin plugs.
• Figure X shows the wiring inside a three-pin plug.
• The international colour code is used to identify the different wires in the three-pin plug.
• Table X shows the colours of insulation and the functions of the wires in the three-pin plug.
  Live wire:
  Covered with brown insulation
  Carries current from the mains to the electrical appliance
  Neutral wire:
  Covered with blue insulation
  Carries current from the electrical appliance to the mains
  Earth wire:
  Covered with green and yellow insulation
  Carries any leaking current to the earth
  Fuse:
  Wire with high resistance and low melting point
  Connected to the live wire
  Melts and breaks the circuit if there is large current flow or a short circuit