Physics

Heat energy makes the particles in a solid, for example, vibrate faster until the intermolecular bonds between them are broken and the particles start to move around - this is called melting. When a substance is melting or boiling, it is still taking in heat energy, but the energy is being used for breaking intermolecular bonds rather than raising the temperature. When a substance is condensing or freezing, bonds are forming between particles, which releases energy. This means the temperature doesn't go down until all the substance has turned into a liquid (condensing) or a solid (freezing).

Specific Latent Heat (SLH) is the volume of energy needed to change the state of 1kg of material. For example, the SLH of melting is the amount of energy required to melt 1kg of material without changing its temperature. SLH is different for various mediums and it's different for boiling and melting. A high SLH means a lot of heat energy is required to change the state of the material i.e. melt or evaporate. The equation for SLH is:

Energy (J) = Mass (kg) x Specific Latent Heat (J/kg)

Conduction of heat is the process where vibrating particles in a solid pass on extra kinetic energy to neighbouring particles. Metals conduct heat well because some of their electrons are free to move inside the metal. Heating makes the electrons move faster and collide with other free electrons, transferring energy. These then pass on their extra energy to other electrons. Because the electrons move freely, energy transfer is much more efficient than slowly passing it between jostling neighbouring atoms.

Most non-metals don't have free electrons, so warm up more slowly, making them good for insulation. This is why metals are used for saucepans, but non-metals are used for saucepan handles. Liquids and gases conduct heat more slowly than solids - the particles aren't held so tightly together, which prevents them colliding with one another as often. In turn, this means liquids and gases are better insulators.

When particles are heated, they gain kinetic energy (KE). This energy makes the particles move around faster. In a solid, the particles vibrate more rapidly, which is what eventually causes solids to melt and liquids to boil. Heat energy is measured on an absolute scale which means it cannot go lower than zero because there is a limit to how slow particles can move. Heat is measured in Joules (J).

Temperature is a measure of hotness - it measures the average kinetic energy of the particles in a substance. The hotter something is, the higher its temperature, and the higher the average KE of particles. Temperature is usually measured in degrees Celcius (*C) but it can also be measured in degrees Fahrenheit (*F). These are not absolute scales as they can go below zero.

Energy flows from hot to cold objects. For example, warm radiators heat a cold room as energy flows from the radiator into the room. If there's a difference in temperature between two places, then energy will flow between them. The greater the difference in temperature, the faster the rate of cooling will be.

Specific Heat Capacity (SHC) is the energy needed to raise the temperature of a 1kg block of material by 1*C. It takes more heat energy to increase the temperature of some materials than others. For example, 4200J of energy is required to heat 1kg of water by 1*C, but only 139J to heat 1kg of mercury.

SHC is a measure of how much energy a substance can store. A high SHC means the material requires a lot of energy in order to be heated up, but must also lose a lot of energy to cool down; however, it also means the material can store a larger volume of energy.

Water has a high SHC. Once water is heated, it can store a lot of energy, which makes it good for central heating systems. Also, water is a liquid so it can easily be pumped around a building. Water has a SHC of 4200 J/kg/*C. The equation for SHC is:

Energy (J/kg/*C) = Mass (kg) x Specific Heat Capacity (J) x Temperatre Change (*C)

Convection occurs in liquids and gases. When liquids and gases are heated, the particles move faster and the fluid expands, becoming less dense. The warmer, less dense fluid rises above its colder, denser surroundings. As the warm fluid rises, cooler fluid takes its place. As this process continues, a circular motion of fluid is created (convenction currents), with the warmer fluid constantly rising and the cooler fluid being heated at the bottom and then rising. Convection occus when the more energetic particles move from a hotter region to a cooler region, and take their heat energy with them.

Radiators in the home rely on convection to make the warm air circulate the room. Convection cannot happen in solids because the particles can't move - they just vibrate on the spot. To reduce convection, the fluid needs to be prevented from moving. Clothes, blankets and cavity wall foam insulation all work by trapping pockets of air. The air can't move so the heat has to conduct very slowly through the air pockets, as well as the materials in between.

The sun gives out energy in the form of heat and light. Some of that energy is stored on earth as fossil fuels (coil, oil and gas). Wind power can also be traced back to the sun as the sun heats the air, the hot air rises and cold air whooshes in to take its place, creating wind.

Wind farms are places where lots of wind turbines are set up the generate electricity from the wind. Wind power involves putting up lots of turbines in exposed areas such as moors, the coast or out at sea. Wind turbines work by converting the kinetic energy of moving air into electricity. The wind turned the blades, which turn a generator and therefore generates electricity.

Wind power is a renewable source as it will never run out. The advantages of wind power include; wind turbines are quite cheap to run, and they are very tough and reliable. Wind power also does not create any waste pollutants and its renewability makes it a sustainable source. However, the disadvantages of wind power is that you need 1500 turbines in order to replace one coal-fird power station. Some people think wind farms are unsightly (visual pollution) and the spinning blades cause noise pollution. Another issue is that little or no wind is not sufficient to generate electricity, meaning the amount generated depends only on the strength of the wind each day. It may also be difficult to find a place to set up a wind farm.

Photocells generate electricity directly from sunlight. They generate a direct current (DC) - the same as a battery. Direct current means the current always flows the same way round the circuit.

Photocells are ususally made of silicon - a semiconductor. When sunlight falls on the cell, the silicon atoms absorb some of the energy, knocking loose some electrons; and these electrons then flow round a circuit - which is electricity.

The current and power output of a photocell depends on; its surface area, the intensity of the light and the distance from the light source. A larger surface area means more electricity is generated as the photocell can absorb a greater volume of light energy. Increasing the intensity of the light means the beams are more concentrated, and therefore more electricity can be produced. Also, the closer the photocell is to the light source, the more intense the light hitting it will be, hence generating more electricity.

Advantages include; there are no moving parts, so they're sturdy, low maintainence and last a long time. Power cables or fuel is not required and solar power will not run out as it's a renewable source. Also, no pollution is given off into the environment. However, a major disadvantage is that if there is no sunlight, then no electricity can be generated.

Heat can be transferred using radiation. Heat is radiated as infra-red waves - these are electromagnetic waves that travel in staight lines at the speed of light. Radiation is different from conduction and convention because it can occur in a vacuum - this is how we receive heat from the sun as space has no particles for other methods of heat transfer. Radiation can only occur through transparent substances such as glass, air and water.

The amount of radiation emitted or absorbed by an object depends largely on its surface colour and texture. All objects are continually emitting and absorbing heat radiation. The hotter an object gets, the more heat radiation it emits. Cooler objects will absorb the heat radiation emitted by hot objects, so their temperature increases. Light-coloured, smooth and shiny objects are very poor absorbers and emitters of radiation. They effectively reflect heat radiation, whereas matt black surfaces are very good absorbers and emitters of radiation.

Heat radiation is important in cooking. Grills and toasters heat food by infrared radiation. The heat radiated by a grill is absorbed by the surface particles of food, increasing their kinetic energy. The heat is then conducted or convected to more central parts.

Objects that emit energy are called sources e.g. radiators and materials that transfer and waste and lose energy are called sinks e.g. windows. To save energy, people insulate their homes so the sinks drain less energy. Sources and sinks can also be made more efficient, so they waste less energy. It costs money to buy insulation or more efficient supplies, but it also saves money because energy bills become lower. Eventually, the money you have saved on energy bills will equal the initial cost - the time this takes is called the payback time.

Loft insulation: fibreglass 'wool' laid across the loft floor reduces conduction through the ceiling into the roof space. Initial cost is £200; annual saving is £100, so payback time is 2 years.

Double glazing: two layers of glass with an air gap between reduces convection. Initial cost is £2400; annual saving is £80, so payback time is 30 years.

Cavity wall insulation: two layers of bricks with a gap between them reduce conduction. By squirting foam into the gap, air pockets are trapped and this minimises convection. Initial cost is £150; annual saving is £100, so payback time is 18 months.

Passive solar heating is when energy from the sun is used to heat something directly. You can reduce the amount of energy needed to heat a building if it is built sensibly to utilise passive solar heating e.g. the direction of the windows can have a significant effect on the amount of sunlight that enters a building. Glass lets in heat and light from the sun, which is then absorbed by objects in the room, heating them up.

The light from the sun has a short wavelength, so it can pass through glass and into a room. However, the heated items in the room emit infrared radiation of a longer wavelength which cannot escape back through the glass. It's reflected back into the room instead, just like a greenhouse. This 'greenhouse effect' continues to work to heat the building and also means heat is kept inside.

Solar water heaters also use passive heating. The glass allows light and heat from the sun in and this is then absorbed by black pipes, heating up the water. The heated water can then be used for washing or pumped to radiators to heat the building.

When an unstable nucleus decays, it gives off nuclear radiation. The three kinds of radiation are alpha, beta and gamma. The three types of radiation cause ionisation - atoms lose or gain electrons, turning them into ions. Background radiation is caused by rocks, medical uses, cosmic rays, building materials and food.

Alpha particles are relatively large, heavy and slow moving particles. They consist of two protons and two neutrons. Due to their size, they are easily stopped by a sheet of paper and therefore cannot penetrate into materials. However, this means they are strongly ionising - they collide with many atoms and knock off loose electrons before they are absorbed.

Beta particles are electrons. They're small and can move quite quickly. Beta particles penetrate moderately before colliding, so they're moderately ionising. But they can still be stopped by a thin sheet of metal such as aluminium.

Gamma rays are very high frequency electromagnetic waves. Gamma rays have no mass and no charge and can penetrate a long way into materials without being stopped, which means they are weakly ionising.They can be stopped by thick concrete or lead.

Alpha radiation can be used in smoke detectors. The detectors have a weak source of a-radiation close to two electrodes. The radiation ionises the air and a current flows between the electrodes. But if there is a fire, the smoke absorbs the radiation, the current stops and an alarm sounds.

Beta radiation is used in tracers and thickness gauges. Radioactive substances can be injected into a patient and tracked using a tracer. Readings can be taken to determine if the body is funtioning properly. Beta or gamma radiation is used as it passes out of the body and only remains radioactive for a few hours. Beta is also used in thickness control; radiation is directed through the product being made and a detector is placed on the other side and connected to a control unit. If the amount detected decreases, it means the material is too thick, so the control unit pinches the rollers up to make it thinner, and vice versa.

Gamma radiation can be used to treat cancer. The rays are carefully directed onto the tumour in order to kill cancerous cells whilst leaving healthy cells unharmed. Gamma is also used to steralise medical equipment by killing all the microbes on the instruments. Strongly radioactive sources must be used so that it lasts a long time and doesn't need regular replacement.

Radioactive sources need to be safely stored and handled. Industrial nuclear workers wear full protective suits to prevent any tiny radioactive particles being inhaled or lodging on the skin. Lead-lined suits, lead/concrete barriers and thick lead screens shield workers from gamma rays in highly radioactive areas. Workers sometimes use remote-controlled robot arms to carry out some tasks.

Radioactive waste can be difficult to dispose of safely. Low-level waste such as paper, clothing, gloves and syringes can be safely buried in a landfill. High-level waste may be sealed in a glass container, then in a metal canister ,and buried deep underground as it can remain radioactive for hundreds of years. However, it is difficult to find locations to dispose of waste. The location has to eb geologically stable as any earthquakes or volcanic eruptions may release radioactive materials that could contaminate the air or sources of water. Even when suitable sites are found, many people object - so the majority of nuclear power stations keep their waste on site.

Not all radioactive waste has to be disposed of. Some of it is reprocessed to reclaim useful materials e.g. when uranium fuel is recycled, more uranium and some plutonium can be harvested. Nuclear power plants are generally very secure as they have clear warnings outside about the dangers of exposure to radioactive materials.

In a nuclear power station, nuclear fission is used to provide heat to make steam, which turns a turbine and a generator to produce electricity.

The advantages of nuclear power stations include; they are able to make lots of energy without releasing high volumes of CO2 into the atmosphere, which contributes to global warming. Nuclear reactions release more energy than chemical reactions, so it takes much less uranium to produce the same amount of power as a fossil fuel. Nuclear fuels e.g. uranium fuel is relatively cheap. Also, there is still plenty of uranium sources underground - although it can be expensive to extract them and make it suitable for a reactor.

However, the disadvantages of nuclear power include; the power stations are expensive to build and maintain and it takes longer to start up a nuclear power station compared to a fossil fuel power station. Processing uranium before use also creates pollution, and there is always the risk of radioactive material leaking into the environment or a major catastrophe like the Chemobyl disaster. Another issue is the amount of radioactive waste generated and difficulties with its disposal. Despite the large reserves of uranium, it is not a renewable source and will therefore run out eventually.

Accelerating means an object is changing speed and acceleration also tells us how quickly the change in speed is taking place. It is measured in m/s  if the speed is in m/s and the time taken is measured in seconds. A vehicle can also accelerate whilst changing direction, regardless of whether or not the speed is increasing or decreasing. A decrease in speed is deceleration or negative acceleration. The equation for acceleration is:

Acceleration = Change in Speed / Time Taken

On a speed-time graph, the gradient is the acceleration and the flat areas represent a consistent speed. The steeper the gradient, the greater the acceleration or deceleration. The area under any section of the graph is the distance travelled in that time interval. A curve means non-uniform acceleration or deceleration is taking place.

Velocity describes both the speed and direction of an object. Relative velocity refers to the speed an object appears to travel at if you are travelling parallel to it - it is calculated by working out the difference between the two velocities. If the objects are travelling in different directions, one object will have a negative velocity.

Force is measured in Newtons (N) when mass is in kg and acceleration in m/s . Gravity is the force of attraction between all masses. On Earth, gravity is equal to 9.8m/s  and causes all objects to accelerate towards the ground equally. Gravity also gives an object weight - which is different to its mass. Mass is simply the volume of the object and it would be the same value anywhere in the universe whereas weight is caused by the pull of gravity. Weight is a force and is therefore measured in Newtons whereas mass is measured in kilograms (kg).

Stationary objects have all their forces in balance. The force of gravity is acting downwards on all objects; this causes a reaction force from the surface which pushes up on the object to balance the downward push of gravity. In a diagram, balanced forces are shown using arrows that are equal in size. Steady horizontal forces such as the uniform speed of a car and steady vertical forces such as a skydiver falling at terminal speed are examples of forces being balanced. In a car, thrust pushes the car forward and this is equal to drag or air resistance which is slowing it down. Similarly, the skydiver's weight is equal to the force of drag slowing them down as they fall.

Forces are not always balanced. For example, acceleration can only occur if there is an overall unbalanced resultant force. The bigger this resultant force, the greater the acceleration. On a diagram, the unequal forces would be represented using arrows that were not equally sized. In acceleration, the thrust is greater than drag; and in deceleration, the drag or air restistance is greater than the thrust. However, the vertical forces of weight and drag remain equal on the car. Similarly, just after a skydiver drops out of a plane, they accelerate vertically until terminal speed is reached. In this case, the force of weight is greater than the force of drag and therefore the skydiver accelerates downwards due to gravity. However, the horizontal forces in this example remain equal as the skydiver is travelling downwards as opposed to sidewards as a car does.

Friction is a force that will always slow an object down. Frictional forces will match the size of the force trying to move it, up to a point - after this, friction will be less than the other force and the object will move. Friction will act to make a moving object slow to a stop, so to travel at a steady speed or accelerate, the driving force must always be greater than friciton. Friction occurs in three main ways: friction between solid surfaces which are gripping, friction between solid surfaces which are sliding past one another and resistance or drag from fluids.

The longer it takes for a vehicle to stop after spotting a hazard, the higher the risk of crashing and injury is. The distance it takes to stop a car is divided into thinking distance and braking distance.

Thinking distance is the distance travelled in the time taken between the driver noticing the hazard and applying the brakes. It is affected by two main factors; the driver's reaction times (this is affected by drugs, alcohol, tiredness and distraction) and the speed of the car (regardless of reaction times, the faster you are travelling, the greater the thinking distance).

Braking distance is the distance taken to stop the vehicle once the brakes have been applied. It is affected by four main factors; the speed of the vehicle, the weight of the vehicle, how effective the brakes are and the road conditions. Speed affects braking distance more than thinking distance as the relationship between speed and braking distance is a squared relationship; as speed doubles, braking distance quadruples. Furthermore, a heavier vehicle will decrease braking distance as the object has a greater weight and therefore more friction is acting on it. The quality of brakes affects braking distance because older, more worn brakes won't be as effective as new ones and therefore will result in an increase in braking distance. Road conditions affect braking distance because wet conditions, the road surface and the tyre depth impact how long the braking distance must be in order to stop safely.

When a force makes an object move, energy is transferred and work is done. 'Work' refers to the effort needed to move the object a particular distance by transferring the energy. Whether this energy is transferred in a useful or wasteful way, 'work' is still done.

Work Done (Joules) = Force (Newtons) x Distance (metres)

Kinetic energy is the energy of movement. Kinetic energy depends on mass and speed. The greater the mass and the faster the object is moving, the greater its kinetic energy will be.

Kinetic Energy = 1/2 x Mass (kg) x Speed  (m/s)

If mass is doubled, kinetic energy is doubled; however, if speed is doubled, kinetic energy is quadrupled due to squared relationships between the forces. When a car brakes, kinetic energy is lost and therefore work is being done by the brakes. The kinetic energy has to be converted to heat energy at the brakes and tyres in order to stop the vehicle.

Kinetic Energy Transferred = Work Done by Brakes1/2 x Mass (kg) x Speed  (m/s) = Force (N) x Distance (m)

Power is a measure of how quickly work is being done. A powerful machine is not necessarily one which can exert a strong force, a powerful machine is one which transfers a lot of energy in a short space of time. The formula for power is:

Power (Watts) = Work Done (Joules) / Time Taken (seconds)

One watt is equal to one joule of energy transferred per second. Power refers to the energy transfer per second, so watts is the same as joules per second. However, power can also be expressed in this formula:

Power (Watts) = Force (Newtons) x Speed (m/s)

This is because work done is force multiplied by distance, and distance over time is equal to speed; therefore power can be expressed as force times speed as well as work done divided by time taken.

The size and design of car engines determines how powerful they are. The larger or more powerful an engine, the more energyt it transfers from its fuel every second, so the higher the fuel consumption. The fuel consumption of a car is usually stated at the distance travelled using a certain amount of fuel; it is often given in miles per gallon (mpg) or litres per 100km (l/100km). Cars that use a lot of fuel are more expensive to run because more fuel is required to travel the same distance.

A car's fuel consumption depends on different factors - for example, the size of the engine, how the car is driven, the mass of the car, the speed it's driven at and the road conditions. The heavier the car and the faster you drive increase the fuel consumption. Cars work more efficiently at some speeds compared to others - the most efficient speed is usually between 40-55 mph. Driving style will affect fuel consumption; faster accelerations need more energy and use more fuel. Frequent braking and acceleration will increase fuel consumption. Driving in different road conditoins can also affect how often you need to brake. The energy from the fuel is also needed to do work against friction e.g. between the tyres and the road. So, opening the window, for example, creates more air resistance and therefore increases fuel consumption. Cars are now designed to be more fuel efficient as they have better engines and are more streamlined.

Two objects with opposite electric charges are attracted to one another. However, two things with the same electric charge will repel one another. These forces become weaker as the two objects are pulled apart. Atoms or molecules that become charged are known as ions.

Electrostatic phenomena are caused by the transfer of electrons when two insulating materials are rubbed together. Build up of static electricity is caused by friction. The loss of electrons results in a positive charge as there are fewer negatively charged electrons on one object, whereas the other object has gained electrons and therefore becomes negatively charged. Which way the electrons are transferred depends on the materials involved. For example, polyethene rods rubbed with a cloth duster causes the rods to become negatively charged whereas acetate rods rubbed with a cloth duster causes the rods to become positively charged.

If enough static charge is built up, it can cause a sudden movement of electrons - this can cause sparks and shocks that can be dangerous if there is a large gap between the two objects or a lot of charge has been built up.

To create sparks in a Van der Graaf, a rubber belt is turned and a comb is inserted to rub against it, creating friciton between the two materials. The belt becomes negatively charged due to the movement of electrons from the comb to the belt. The belt is connected to a dome, and therefore this dome also becomes negatively charged. An earthed conducter is placed near the dome, but does not come into contact with it. The positive protons in the earthed conductor are attracted to the negative electrons in the dome. The charge builds up and eventually the electrons 'jump' the gap between the dome and the conductor, creating a spark.

Static electricity can cause dust particles to become charged. The particles will be attracted to anything with the opposite charge. Unfortunately, many objects around a house are made of insulators, therefore the charged dust particles will be attracted to them and become 'stuck' on the surface of the material.

When synthetic clothes are dragged over one another or over your head, electrons are transferred, leaving static charges in both materials. This causes attraction, meaning the clothes stick to each other and to the wearer; small sparks or shocks may be created as the charges rearrange themselves.

Shocks from door handles can also occur if you walk on a nylon carpet wearing shoes with insulating soles. Charge builds up on your body and if you then touch a metal door handle or water pipe, the charge flows via the conductor and you receive a little shock.

A large amount of static charge can build up on clothes made from synthetic materials. Eventually this charge may become large enough to make a spark which can cause an explosion if it occurs near any inflammable gases or fuel fumes.

Also, as fuel flows out of a filler pipe, when paper drags over rollers, or grain shoots out of pipes, static charge can build up. This can easily lead to a spark that causes an explosion in dusty or fumey places as the small particles are ignited - like when filling up a car at a petrol station.

Dangerous sparks can be prevented by connecting a charged object to the ground using a conductor; this is called earthing and it provides an easy route for static charge to travel safely into the ground. Ths means no charge can build up and cause dangerous sparks. Static charge is a big issue where sparks could ignite inflammable gases, or where there's high concentrations of oxygen. Fuel takers are earthed to prevent any sparks that might cause the fuel to explode - refuelling aircraft are bonded to their fuel tankers using an earthed cable to prevent sparks. Anti-static sprays are liquids that work by making the surface of a charged object conductive; providing an easy path for charges to move away. Anti-static cloths are conductive and carry charge away from objects they're used to wipe. Insulating mats and shoes with insulating soles prevent static electricity from moving through them so you cannot receive a shock.

Bikes and cars are painted using electrostaic paint sprayers. The spray gun is chargesd which in turn charges up the small drops of paint. As all the paint drops have the same charge, they repel one another, so a very fine paint is produced as a result.

The object that is going to be painted is given the opposite charge to the gun and the paint drops. Therefore, the object is attracted to the fine paint particles. When the paint is sprayed, it allows an even coat to be given and also means hardly any paint is wasted.

This method is effective because areas of the object that are facing away from the sprayer are also painted as the particles are still attracted to these areas - no area is left unpainted. For example, a bike may be positively charged meaning it has lost electrons and the paint would therefore be negatively charged, so the electrons in the liquid would be attracted to the protons on the bike.

The beating of your heart is controlled by tiny electrical pulses inside your body. If the heart stops, an electic shock can make it restart and continue beating. Hospitals and ambulances have machines called defibrillators which can be used to shock a stopped heart back into operation.

The defibrillator consists of two paddles connected to a power supply. The paddles of the defibrillator are placed firmly on the chest of the patient to ensure a good electrical contact. The defibrillators are then charged up. Everyone is warned to move away from the patient except from the defibrillator operator who holds the insulated handles. This makes sue the patient is the only person who receives the shock as the charge would seriously injur or kill anyone whose heart has not stopped beating. The charge is passed through the paddles to the patient's heart to make it contract and start beating again.

Factories and power stations produce lots of smoke, which is made up of tiny particles. Fortunately, smoke can be removed using a dust precipitator. As smoke particles reach the bottom of the chimney, they meet a wire grid or rods with a high voltage and negative charge. The dust particles gain electrons from the grid and become negatively charged. The dust particles then induce a charge on earthed metal plates as the negatively charged particles repel electrons on the plates, meaning the plates become positively charged.

The dust particles are attracted to the metal plates as they now have the opposite charge, and here they stick together to form larger particles. When they are heavy enough, the particles fall off the plates or are knocked off by a hammer. The dust falls to the bottom of the chimney and can be removed. This means the gases coming out of the chimney have very few smoke particles in them.

Charge flows around a circuit. Current is the flow of electrical charge around a circuit - essentially the flow of electrons. It is measures in Amps (A). More charge passes around a circuit when a higher current flows. Current will only flow through a component if there is a voltage across that component. Voltage is the driving force that pushes the current around the circuit. Voltage is measured in Volts (V). Resistance is anything in the circuit which slows the flow of electrons down. Resistance is measured in Ohms (  ). Longer, thinner wires have more resistance and therefore allow less current to flow.

There is a balance: the voltage is trying to push the current round the circuit, and the resistance is opposing it - the relative sizes of the voltage and resistance decide how big the current will be. If you increase the voltage, then more current will flow. If you increase the resistance, then less current will flow - or more voltagwe will be needed to keep the same current flowing.

If you break the circuit, then the current will stop flowing. Current can only flow if there is a comnplete loop for it to flow around. Wire fuses and circuit breakers are safety features that break a circuit if there is a dangerous fault.

All the wires in a plug are colour coded and the correct coloured wire is connected to each pin, and firmly screwed in place so no bare wires show. The live wire is brown and it carries the voltage. It alternates between a high positive and negative voltage of about 230V. The neutral wire is blue and it completes the circuit. Electricity normally flows in through the live wire and out through the neutral wire. The neutral wire is always at 0V. The earthed wire and fuse (or circuit breaker) are for safety and work together to carry charge away if a fault causes the casing to become live. All appliances with metal cases must be attached to an earthed wire to reduce the danger of an electric shock. An earthed conductor can never become live as the earth wire prevents this from happening. If the appliance has a casing that is not conductive it is said to be double-insulated.

If a fault develops in which the live wire touches the metal case, then because the appliance is earthed, a big current flows in through the live wire, through the case and down to earth. The surge in current blows the fuse and causes the wire inside it to melt. This cuts off the live supply because it breaks the circuit. This isolates the whole appliance, making it impossible to get an electric shock fron the case. It also stops the flex overheating, which could cause a fire, and it prevents further damage to the appliance. A circut breaker works like a fuse but can be replaced after it trips and used again whereas fuses must be replaced.