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Models of the solar system

Our understanding of the universe has changed over time. Different civilisations have created different models to explain what the universe is and how the universe began. The Greek astronomer Ptolemy (c90-168AD) used measurements of the sky to create his geocentric model. This had the earth at the centre and all the planets and the sun orbiting around it.

The geocentric model lasted a long time. It wasn’t until the mid 18th century that Nicolaus Copernicus (1473-1543) came up with a different model. His heliocentric theory put the sun at the centre if the universe. It was based on observations with the telescope – work pioneered by the Italian astronomer Galileo Galilei (1564-1642).

Discovering new planets

As telescopes have improved scientists have discovered new planets. Copernicus’ model of the universe didn’t include Uranus, Neptune or the dwarf planet Pluto because telescopes at the time weren’t good enough to see them.

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Observing the Universe

Distant stars and galaxies are too far away for us to reach. We cannot go to them to study them. So everything we know about distant stars and galaxies comes from analysing the radiation they produce.

Telescopes are devices used to observe the universe. There are many different types and some are even sited in space.

Optical telescopes

Optical telescopes observe visible light from space. Small ones allow amateurs to view the night sky relatively cheaply but there are very large optical telescopes sited around the world for professional astronomers to use.

Optical telescopes on the ground have some disadvantages:

  • they can only be used at night
  • they cannot be used if the weather is poor or cloudy.
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Observing the Universe

Radio telescopes detect radio waves coming from space. Although they are usually very large and expensive, these telescopes have an advantage over optical telescopes. They can be used in bad weather because the radio waves are not blocked by clouds as they pass through the atmosphere. Radio telescopes can also be used in the daytime as well as at night.

X-rays are partly blocked by the Earth's atmosphere and so X-ray telescopes need to be at high altitude or flown in balloons.

Space telescopes

Objects in the universe emit other electromagnetic radiation such as infrared, X-rays and gamma rays. These are all blocked by the Earth's atmosphere, but can be detected by telescopes placed in orbit round the Earth.

Telescopes in space can observe the whole sky and they can operate both night and day. However, they are difficult and expensive to launch and maintain. If anything goes wrong, only astronauts can fix them.

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Converging lenses

A lens is transparent block that causes light to refract (changes the direction the light travels in). A converging lens (or convex lens) is curved on both sides. This means the light rays coming out of it come together at a point – they converge.

Diagram of how a lens works (

The point at which the light rays meet is called the focal point. The focal length is found by focussing a distant object on a piece of paper through the lens. The focal length is the distance between the centre of the lens and the image.

A converging lens is used in a refracting telescope to focus the image. Galileo’s telescope would have been a refracting telescope.

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Investigating converging lenses

Converging lenses are often used to produce images that are magnified. The amount of magnification depends on:

  • how curved the surface of the lens is
  • how close the lenses are placed.

There are two types of image that can be seen. A real image is the image formed where the light rays are focussed. A virtual image is one from which the light rays appear to come but don’t actually come from that image like in a mirror.

Converging lens (

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Refracting telescopes

Refracting telescopes

A refracting telescope works bending light through a lens so that it forms an image. There are a few problems with refracting telescopes:

  • some of the light reflects off the lens so the image is very faint
  • large lenses are needed to improve the magnification – this can be difficult to do perfectly.

Refracting telescope (

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Reflecting telescopes

Reflecting telescopes

In a reflecting telescope the image is formed by reflection from a curved mirror. It is then magnified by a secondary mirror. Reflecting telescope (

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What are waves?

Waves are vibrations that transfer energy from place to place without matter - solid, liquid or gas - being transferred. Think of a Mexican wave in a football crowd. The wave moves around the stadium, while each spectator stays in their seat only moving up then down when it's their turn.

Some waves must travel through a substance. The substance is known as the medium, and it can be solid, liquid or gas. Sound waves and seismic waves are like this. They must travel through a medium. It is the medium that vibrates as the waves travel through.

Other waves do not need to travel through a substance. They may be able to travel through a medium, but they do not have to. Visible light, infrared rays, microwaves and other types of electromagnetic radiation are like this. They can travel through empty space. Electrical or magnetic fields vibrate as the waves travel through.

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Transverse and longitudinal waves

Light and other types of electromagnetic radiation are transverse waves. Water waves and S waves (a type of seismic wave) are also transverse waves. In transverse waves, the vibrations are at right angles to the direction of travel.

Longitudinal waves

Sound waves and waves in a stretched spring are longitudinal waves. P waves (relatively fast moving longitudinal seismic waves that travel through liquids and solids) are also longitudinal waves. In longitudinal waves, the vibrations are along the same direction as the direction of travel.

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Amplitude, wavelength and frequency


The amplitude of a wave is its maximum disturbance from its undisturbed position. Take care, the amplitude is not the distance between the top and bottom of a wave.


The wavelength of a wave is the distance between a point on one wave and the same point on the next wave. It is often easiest to measure this from the crest of one wave to the crest of the next wave, but it doesn't matter where as long as it is the same point in each wave.


The frequency of a wave is the number of waves produced by a source each second. It is also the number of waves that pass a certain point each second. The unit of frequency is the hertz (Hz). It is common for kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) to be used when waves have very high frequencies.

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Amplitude, wavelength and frequency

wave speed (metre per second) = frequency (hertz) × wavelength (metre)(

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Refraction and reflection


Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.


Sound waves and light waves reflect from surfaces. Remember that they behave just like water waves in a ripple tank. The angle of incidence equals the angle of reflection

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The visible spectrum

Using a prism, you can split up white light to form a spectrum. (A prism is a block of glass with a triangular cross-section.) The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.

The electromagnetic spectrum

Visible light is just one type of electromagnetic radiation: there are various types of electromagnetic radiation with longer wavelengths of light than red light and with shorter wavelengths than violet light. All the different types of electromagnetic waves travel at the same speed through space.

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Infrared and Ultraviolet

The discovery of Infrared

British astronomer William Herschel (1738-1822) was making observations of the sun when he put coloured filters over his telescope in order to make his observations safer. He noticed that different coloured filters heated up his telescope by different amounts. Using a prism to break up visible light he put a thermometer in the different colours. He found that the temperature rose as he moved the thermometer from violet to red. He then measured the temperature where there was no visible light, at the red end of the spectrum. The temperature was the highest: he had discovered infrared.

The discovery of Ultraviolet

Following Herschel’s work, Johann Ritter (1776-1810) tried to find invisible rays at the violet end of the spectrum. As part of the experiment he used silver chloride, which turns black when exposed to light. This happened fastest when exposed to the invisible rays at the violet end of the spectrum.

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Electromagnetic radiation

X-rays, visible light and radio waves are all types of electromagnetic radiation.

The main types of electromagnetic radiation

frequencytype of electromagnetic radiationwavelength highest gamma radiation shortest X-rays ultraviolet visible light infrared microwaves lowest radio waves longest

All types of electromagnetic radiation:

  • are transverse waves
  • travel at the same speed in a vacuum - empty space.
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Electromagnetic uses and dangers

Gamma radiation – Killing cancer cells -Causes cell damage/mutation and cancer

X- rays – Medical images of bones -Causes cell damage/mutation and cancer

Ultraviolet – Detecting forged bank notes -Sunburn and damage to cells

Visible light – Seeing -Temporary blindness

Infrared – Optical fibre/communication -Causes skin burns

Microwaves – Cooking - Internal heating of body tissues

Radiowaves – Television signals -Little effect

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Ionising radiation

Radioactive substances give out radiation all of the time. There are three types of nuclear radiation: alpha, beta and gamma. Alpha is the least penetrating, while gamma is the most penetrating. Nonetheless, all three are ionising radiation: they can knock electrons out of atoms and form charged particles.

Radiation can be harmful, but it can also be useful - the uses of radiation include to:

  • detect smoke
  • gauge the thickness of paper
  • treat cancer
  • sterilise medical equipment.

Types of radiation

  • alpha
  • beta
  • gamma.
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Ionising radiation

Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air, while gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out.

Alpha radiation

Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper.

Beta radiation

Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation

Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.

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Ionising radiation

alpha radiation cannot pass through humans, beta is stopped by aluminium, gamma by lead  (

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Origin of Universe

Scientists believe that the universe began in a hot 'big bang' about 13,600 million years ago. The universe continues to expand. Stars do not remain the same but change as they age. The evidence for the Big Bang Theory includes the existence of a microwave background radiation, and red-shift.

The Big Bang Theory

Scientists have gathered a lot of evidence and information about the universe. They have used their observations to develop a theory called the Big Bang. The theory states that about 13,700 million years ago all the matter in the universe was concentrated into a single incredibly tiny point. This began to enlarge rapidly in a hot explosion, and it is still expanding today.Gravity is slowing down the rate of expansion. It is possible that the universe may expand for ever, or it may stop expanding. It may even contract and become very small again - the 'Big Crunch'.

The Steady state theory

The Steady State Theory suggests that as the universe expands new matter is created, so that the overall appearance of the universe never changes.

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The birth of a star

Dust and gas

Stars form from massive clouds of dust and gas in space.

Gravity pulls the dust and gas together.As the gas falls together, it gets hot. A star forms when it is hot enough for nuclear reactions to start. This releases energy, and keeps the star hot. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. Our sun is at this stable phase in its life. Gravity pulls smaller amounts of dust and gas together, which form planets in orbit around the star.

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Red giant stars

Our sun is a type of star called a yellow dwarf. It has been shining for nearly five billion years, and has enough hydrogen fuel to last another five billion years. The sun and other stars eventually begin to run out of hydrogen. Gravity makes the core of the star smaller and hotter, which results in the outer layers expanding. They eventually expand so much that the star becomes a red giant star. In around 6.2 billion years from now the Sun will have become a red giant star

White dwarf stars

After a star becomes a red giant, what happens next depends on how massive the star is. If its mass is relatively small, gravity eventually leads to the star contracting to form a white dwarf. It fades and changes colour as it cools. The matter in a white dwarf is millions of times denser than the matter on Earth.

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Red shift


Our sun contains helium. We know this because there are black lines in the spectrum of the light from the sun, where helium has absorbed light. These lines form the absorption spectrum for helium. spectrum of light with black verticle lines  ( Spectrum of the sun When we look at the spectrum of a distant star, the absorption spectrum is there, but the pattern of lines has moved towards the red end of the spectrum, as you can see below. as before, but the lines are shifted towards the left and right ends of the spectrum  ( Spectrum of a distant star This is called red-shift, a change in frequency of the position of the lines. Astronomers have found that the further from us a star is the more its light is red-shifted. This tells us that distant galaxies are moving away from us, and that the further a galaxy is the faster it is moving away. Since we cannot assume that we have a special place in the universe this is evidence for a generally expanding universe. It suggests that everything is moving away from everything else. The Big Bang Theory says that this expansion started billions of years ago with an explosion.

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Cosmic Microwave Background radiation

Scientists discovered that there are microwaves coming from every direction in space. Big Bang Theory says this is energy created at the beginning of the universe, just after the Big Bang, and that has been travelling through space ever since.

A satellite called COBE has mapped the background microwave radiation of the universe as we see it. Big Bang theorists are still working on the interpretation of this evidence.

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EvidenceInterpretation The light from other galaxies is red-shifted. The other galaxies are moving away from us. This evidence can be used to explain both the Big Bang theory and Steady State universe. The further away the galaxy, the more its light is red-shifted. The most likely explanation is that the whole universe is expanding. This supports the theory that the start of the universe could have been from a single explosion. Cosmic Microwave Background (CMB) The relatively uniform background radiation is the remains of energy created just after the Big Bang.

Red-shift is used to explain both the Steady State and Big Bang theories of the universe. Cosmic Microwave Background radiation is evidence for the Big Bang theory only. This discovery has led to the Big Bang theory becoming the currently accepted model.

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Larger stars

Heavy-weight stars

Stars that are much heavier than our Sun have a different fate. A heavy-weight star will still become a red giant, but then:

  • it blows apart in a huge explosion called a supernova
  • the central part left behind forms a neutron star, or even a black hole, if it is heavy enough
  • black holes have a large mass, and a large gravity - even light cannot escape them because their gravitational field is so strong
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Sound waves

Sound waves are longitudinal waves that must pass through a medium. Ultrasound waves have a frequency above the normal range of human hearing. They can be used to scan for birth defects in unborn babies and for defects in manufactured equipment. Infrasound has a frequency below normal hearing. Infrasound can be used to track animals and monitor seismic activity.

Sound summary

Sound waves

Sound waves are longitudinal waves. Their vibrations occur in the same direction as the direction of travel. Sound waves can only travel through a solid, liquid or gas.


When an object or substance vibrates, it produces sound. The bigger the vibrations, the greater the amplitude and the louder the sound.

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Ultrasound waves have frequencies above about 20kHz (20,000Hz). This is above the normal hearing range for humans, so we cannot hear ultrasound.


When ultrasound waves reach a boundary between two substances with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through. A detector placed near the source of the ultrasound waves is able to detect the reflected waves. It can measure the time between an ultrasound wave after leaving the source to reach the detector. The further away the boundary, the longer the time between leaving the source and reaching the detector:

distance (metre, m) = speed (metre/second, m/s) × time (second, s)

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3-D ultrasound scan of a human foetus

Computers are able to create detailed images by combining many ultrasound reflection readings. This is used in medicine for pre-natal scanning (checking unborn babies).

Ultrasound can be used in industry for quality control procedures to check manufactured objects, such as railway tracks and oil pipelines, for damage or defects. The diagram shows how a piece of metal may be tested for cracks or other flaws using ultrasound.

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Sonar is used on ships and submarines to detect fish or the sea bed. A pulse of ultrasound is sent out from the ship. It bounces off the seabed or shoal of fish and the echo is detected. The time taken for the wave to travel indicates the depth of the seabed or shoal of fish.

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Sound with a frequency less than 20 hertz, Hz, is known as infrasound. This is below the range that humans can hear (usually between 20-20 000Hz). Infrasound is detected using a microphone. Infrasound has many uses.

Elephants and giraffes use infrasound to communicate between herds over long distances. Scientists can now use microphones to track the herds even if they are hidden in dense forests. This helps with the conservation and protection of these animals.

Infrasound can be used to detect volcanic eruptions. As a volcano erupts it produces infrasound, which can be detected even if the volcano is in a remote location a long way away. Scientists also use infrasound to track the passage of meteors through the atmosphere. Meteors are lumps of rock from space. Most burn up in the atmosphere but some hit earth. If a meteor hit a populated area it could cause considerable damage.

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Structure of the Earth

  1. crust - relatively thin and rocky
  2. mantle - has the properties of a solid, but can flow very slowly
  3. outer core - made from liquid nickel and iron
  4. inner core - made from solid nickel and iron
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Plate tectonics

The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.

The plates move because of convection currents in the Earth's mantle. These are driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth.

Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges

Seismic waves can be reflected and refracted at boundaries between the crust, mantle and core. This means that they change direction.

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Detecting Earthquakes

Earthquakes are detected using a seismometer – a piece of equipment that picks up the vibrations in the earth. A scientist can work out the location of an earthquake by calculating the time difference between the arrival of the S and P waves. Information from three different seismometers is compared to work out the exact location of the earthquake – the epicentre.

Three seismometers can determine an earthquake's epicentre

The seismometers will only work out how far the wave has travelled and not the direction. By using information from three seismometers, you can calculate the epicentre, which is where all three distances meet: this process is called triangulation.

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S and P waves

The speed of P waves and S waves increases as they travel deeper into the mantle. They travel through the Earth in curved paths, but they change direction suddenly when they pass through the boundary between substances in different states. The diagrams show what happens when P waves and S waves pass through the Earth.

S waves

  • transverse
  • slow moving
  • travel through solids only

P waves

  • longitudinal
  • fast moving
  • travel through liquids and solids only

S waves cannot pass through the liquid outer core, but P waves can. When P waves pass from solid to liquid, then from liquid to solid, there are sudden changes in direction – they are reflected and refracted. Seismic waves are also reflected and refracted as they pass into different rock types.

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Types of current

There are two types of electric current:

Direct current (DC)

If the current flows in only one direction it is called direct current, or DC. Batteries and solar cells supply DC electricity. A typical battery may supply 1.5V.

Alternating current (AC)

If the current constantly changes direction it is called alternating current, or AC. Mains electricity is an AC supply. The UK mains supply is about 230V. It has a frequency of 50Hz (50 hertz), which means that it changes direction and back again 50 times a second.

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An electric current is produced when a magnet is moved into a coil of wire in a circuit. We say that the electric current has been induced, and the process is called induction. The direction of the induced current is reversed when the magnet is moved out of the coil again. It can also be reversed if the other pole of the magnet is moved into the coil. Check your understanding of this with this simulation.

Increasing the induced current

To increase the induced current:

  • move the magnet faster
  • use a stronger magnet
  • increase the number of turns on the coil
  • increase the area of the coil.
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Turning generators directly

Generators can be turned directly, for example, by:

  • wind turbines
  • hydroelectric turbines
  • wave and tidal turbines.

When electricity is generated using wave, wind, tidal or hydroelectric power (HEP) there are two steps:

  1. The turbine turns a generator.
  2. Electricity is produced.
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Generators can be turned indirectly using fossil or nuclear fuels. The heat from the fuel boils water to make steam, which expands and pushes against the blades of a turbine. The spinning turbine then turns the generator.

These are the steps by which electricity is generated from fossil fuels:

  1. Heat is released from fuel and boils the water to make steam.
  2. The steam turns the turbine.
  3. The turbine turns a generator and electricity is produced.
  4. The electricity goes to the transformers to produce the correct voltage.
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Renewable energy

Our renewable energy resources will never run out: their supply is not limited, and there are no fuel costs either. They typically generate far less pollution than fossil fuels.

Renewable energy resources include:

  • wind energy
  • water energy, such as wave machines, tidal barrages and hydroelectric power
  • geothermal energy
  • solar energy
  • biomass energy, for example energy released from wood.

However, there are some disadvantages to generating energy from renewable sources: for example, wind farms are noisy and may spoil the view of people who live near them. The amount of electricity generated depends on the strength of the wind. If there is no wind, there is no electricity.

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Non-renewable energy

There is a limited supply of non-renewable energy resources, which will eventually run out. They include:

  • fossil fuels, such as coal, oil and natural gas
  • nuclear fuels, such as uranium.

Fossil fuels release carbon dioxide when they burn, which adds to the greenhouse effect and increases global warming. Of the three fossil fuels, coal produces the most carbon dioxide, for a given amount of energy released, while natural gas generates the least.

The fuel for nuclear power stations is relatively cheap. But the power stations themselves are expensive to build. It is also very expensive to dismantle old nuclear power stations or store radioactive waste, which is a dangerous health hazard.

In an exam question you may be asked to discuss the advantages and disadvantages of methods of large-scale electricity production. You should list both renewable and non-renewable resources with advantages and disadvantages for both.

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Current and potential difference

A current flows when an electric charge moves around a circuit – measured as the rate of flow of charge. No current can flow if the circuit is broken, for example, when a switch is open. Click on the animation to see what happens to the charge when the switch is opened or closed.

Measuring current

  • current is measured in amperes - often abbreviated to amps or A
  • The current flowing through a component in a circuit is measured using an ammeter
  • the ammeter must be connected in series with the component.

A potential difference, also called voltage, across an electrical component is needed to make a current flow through it. Voltage is the electrical pressure that gives a measure to the energy transferred. Cells or batteries often provide the potential difference needed.

Measuring potential difference

  • Potential difference is measured in volts, V
  • Potential difference across a component in a circuit is measured using a voltmeter
  • The voltmeter must be connected in parallel with the component.
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Power and energy

Power is a measure of how quickly energy is transferred. The unit of power is the watt (W). You can work out power using this equation:

Power is equal to energy divided by time. Divide joules by seconds to find watts (

The more energy that is transferred in a certain time, the greater the power. A 100W light bulb transfers more electrical energy each second than a 60W light bulb.

The equation below shows the relationship between power, potential difference (voltage) and current:

power (watts) = current (amps) x potential difference (volts)

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Saving energy

Electricity can be used to heat homes and offices. If some of the heat escapes from the house, it costs money and wastes resources. There are several ways that heat can escape from a house, and different ways to reduce these losses. In deciding how cost-effective an energy-saving measure is, we need to know what its payback time is:

payback time = cost of energy-saving measure ÷ money saved each year


payback time = cost of energy-saving measure ÷ money saved each year

                   = 2,500 ÷ 100

                   = 25 years

When buying an energy-saving device, it is important to consider the advantages and disadvantages.

Some disadvantages would be:

  • initial cost
  • use of extra resources to manufacture new device
  • cost of disposal of old device.

Some of the advantages would be:

  • cost efficiency
  • saving energy and resources.
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