Heat can be transferred from place to place by conduction, convection and radiation. Dark matt surfaces are better at absorbing heat energy than light shiny surfaces. Heat energy can be lost from homes in many different ways and there are ways of reducing these heat losses.
All objects emit (give out) and absorb (take in) thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits.
Infrared radiation is a type of electromagnetic radiation, which involves waves rather than particles. This means that, unlike conduction and convection, radiation can even pass through the vacuum of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the Earth.
Some surfaces are better than others at emitting and absorbing infrared radiation.
The kinetic particle theory explains the properties of the different states of matter. The particles in solids, liquids and gases have different amounts of energy. They are arranged differently and move in different ways.
Heat energy can move through a substance by conduction. Metals are good conductors of heat but non-metals and gases are usually poor conductors of heat. Poor conductors of heat are called insulators. Heat energy is conducted from the hot end of an object to the cold end.
Heat conduction in metals
The electrons in piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now charged metal ions. The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons. These move through the structure of the metal, colliding with ions as they go.
Liquids and gases are fluids. The particles in these fluids can move from place to place. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy. Heat energy is transferred from hot places to cooler places by convection.
Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size.
The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up.
Evaporation and condensation
Evaporation and condensation are changes of state:
- evaporation involves a liquid changing to a gas
- condensation involves a gas changing to a liquid.
Evaporation is the reason why damp clothes dry on a washing line. Condensation is the reason why windows become foggy on a cold day.
The particles in a liquid have different energies. Some will have enough energy to escape from the liquid and become a gas. The remaining particles in the liquid have a lower average kinetic energy than before, so the liquid cools down as evaporation happens. This is why sweating cools you down. The sweat absorbs energy from your skin so that it can continue to evaporate.
The particles in a gas have different energies. Some may not have enough energy to remain as separate particles, particularly if the gas is cooled down. They come close together and bonds form between them. Energy is released when this happens. This is why steam touching your skin can cause scalds: not only is the steam hot, but energy is released into your skin as the steam condenses.
Factors affecting the rate of condensation and evaporation
The rate of condensation increases if the temperature of the gas is decreased. On the other hand, the rate of evaporation increases if the temperature of the liquid is increased. It is also increased if:
- the surface area of the liquid is increased
- air is moving over the surface of the liquid.
Keeping warm or cool
The bigger the difference in temperature between an object and its surroundings, the greater the rate at which heat energy is transferred. Other factors also affect the rate at which an object transfers energy by heating. These include the:
- surface area and volume of the object
- material used to make the object
- nature of the surface that the object is touching.
Small animals like mice have a large surface area compared to their volume. They lose heat to their surroundings very quickly and must eat a lot of food to replace the energy lost. Large animals like elephants have a different problem. They have a small surface area compared to their volume. They lose heat to their surroundings more slowly and may even have difficulty avoiding overheating.
Elephants have large ears with a large surface area compared to their volume. These allow heat to be transferred from the elephant to its surroundings, helping to keep the animal cool.
In general, similar animals have different ear sizes depending on the climate in which they live. The arctic fox has much smaller ears than the fennec fox, which lives in the desert. The arctic fox must conserve its heat energy in the cold climate, while the fennec fox must avoid overheating in the hot climate.
Engineers design heat transfer devices so that they gain or lose heat energy efficiently. For example, car radiators are flat, with many small fins to provide a large surface area. Similarly, household radiators are thin and flat, and may have fins so that heat energy is transferred to the room quickly.
U-values measure the effectiveness of a material as an insulator in buildings. Solar panels use heat energy from the Sun to provide hot water or to heat buildings. The specific heat capacity of a substance allows us to calculate the amount of energy needed to heat it up.
U-values measure how effective a material is an insulator. The lower the U-value is, the better the material is as a heat insulator. For example, here are some typical U-values for building materials:
- a cavity wall has a U-value of 1.6 W/m²
- a solid brick wall has a U-value of 2.0 W/m²
- a double glazed window has a U-value of 2.8 W/m².
The cavity wall is the best insulator and the double glazed window is the worst insulator. Note that you do not need to remember any U-values for the exam or know the equation used to calculate U-values.
Homeowners may install double glazing or extra insulation to reduce heat energy losses and so save money. However, these energy-saving solutions cost money to buy and install. The payback time of an energy-saving solution is a measure of how cost-effective it is. Here is the equation to calculate payback time:
payback time (years) = cost of installation (£) ÷ savings per year in fuel costs (£)
The payback time will be shortest if the cost of installation is low compared to the savings made each year.
Solar panels do not generate electricity, but rather they heat up water. They are often located on the roofs of buildings where they can receive heat energy from the sun.Cold water is pumped up to the solar panel where it heats up and is transferred to a storage tank.A pump pushes cold water from the storage tank through pipes in the solar panel. The water is heated by heat energy from the sun and returns to the tank. In some systems, a conventional boiler may be used to increase the temperature of the water.
Specific heat capacity
Temperature and heat are not the same thing:
- temperature is a measure of how hot something is
- heat is a measure of the thermal energy contained in an object.
Temperature is measured in °C, and heat is measured in J. When heat energy is transferred to an object, its temperature increase depends upon the:
- the mass of the object
- the substance the object is made from
- the amount energy transferred to the object.
Specific heat capacity
The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1 kg of the substance by 1°C. Different substances have different specific heat capacities.
Calculating specific heat capacity
Here is the equation relating energy to specific heat capacity:
E = m × c × θ
- E is the energy transferred in joules, J
- m is the mass of the substances in kg
- c is the specific heat capacity in J / kg °C
- θ (‘theta’) is the temperature change in degrees Celsius, °C
For example, how much energy must be transferred to raise the temperature of 2 kg of water from 20°C to 30°C?
E = m × c × θ (θ = 30 – 20 = 10°C)
E = 2 × 4181 × 10 = 83,620 J or 83.62 kJ
Forms of energy
You should be able to recognise the main types of energy. One way to remember the different types of energy is to learn this sentence:
Each capital letter is the first letter in the name of a type of energy:
- kinetic (movement energy)
- heat (thermal energy)
- gravitational potential
- elastic potential
Magnet energy in magnets and electromagnets
kinetic the energy in moving objects – also called movement energy
heat also called thermal energy
light also called radiant energy
gravitational potential stored energy in raised objects
chemical stored energy in fuel, foods and batteries
sound energy released by vibrating objects
electrical energy in moving charges or static electric charges
elastic potential stored energy in stretched or squashed objects
nuclear stored in the nuclei of atoms
Energy transfer diagrams
Different types of energy can be transferred from one type to another. Energy transfer diagrams show each type of energy, whether it is stored or not, and the processes taking place as energy is transferred.
This energy transfer diagram shows the useful energy transfer in a car engine. You can see that a car engine transfers chemical energy, which is stored in the fuel, into kinetic energy in the engine and wheels.
Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved.
This Sankey diagram for an electric lamp shows that most of the electrical energy is transferred as heat rather than light.
The efficiency of a device such as a lamp can be calculated:
efficiency = useful energy out ÷ total energy in (for a decimal efficiency)
efficiency = (useful energy out ÷ total energy in) × 100 (for a percentage efficiency)
The efficiency of the filament lamp is 10 ÷ 100 = 0.10 (or 10 percent).
This means that 10 percent of the electrical energy supplied is transferred as light energy (90 percent is transferred as heat energy).
The efficiency of the energy-saving lamp is 75 ÷ 100 = 0.75 (or 75 percent). This means that 75 percent of the electrical energy supplied is transferred as light energy (25 percent is transferred as heat energy).
Note that the efficiency of a device will always be less than 100 percent. You might be given the power in W instead of the energy in J. The equations are the same – just substitute power for energy.
Electrical energy calculations
The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh.
E = P × t
- E is the energy transferred in kilowatt-hours, kWh
- P is the power in kilowatts, kW
- T is the time in hours, h.
Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000.
For example, 2,000 W = 2,000 ÷ 1,000 = 2 kW.
Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600.
For example, 7,200 s = 7,200 ÷ 3,600 = 2 h.
Joules, watts and seconds
You also use the equation E = P × t when:
- E is the energy transferred in joules, J
- P is the power in watts, W
- T is the time in seconds, s.
Cost of electricity
Electricity meters measure the number of units of electricity used in a home or other building. The more units used, the greater the cost. The cost of the electricity used is calculated using this equation:
total cost = number of units × cost per unit
For example, if 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.
Remember that the number of units used can be calculated using this equation:
units (kWh) = power (kW) × time (h) … so …
total cost = power (kW) × time (h) × cost per unit
Disadvantages of using fossil fuels
Fossil fuels are non-renewable energy resources: their supply is limited and they will eventually run out. Fossil fuels do not renew themselves, while fuels such as wood can be renewed endlessly.
Coal and oil release sulfur dioxide gas when they burn, which causes breathing problems for living creatures and contributes to acid rain.
Fossil fuels release carbon dioxide when they burn, which adds to the greenhouse effect and increases global warming. Of the three fossil fuels, for a given amount of energy released, coal produces the most carbon dioxide and natural gas produces the least.
Carbon capture and storage is a way to prevent carbon dioxide building up in the atmosphere. It is a rapidly evolving technology that involves separating carbon dioxide from waste gases. The carbon dioxide is then stored underground, for example in old oil fields or gas fields such as those found under the North Sea.
The main nuclear fuels are uranium and plutonium. These are radioactive metals. Nuclear fuels are not burnt to release energy. Instead, nuclear fission reactions (where the nuclei in atoms are split) in the fuels release heat energy.
The rest of the process of generating electricity is then identical to the process using fossil fuels. The heat energy is used to boil water. The kinetic energy in the expanding steam spins turbines, which then drive generators to produce electricity.
Advantages of nuclear fuels
Unlike fossil fuels, nuclear fuels do not produce carbon dioxide or sulfur dioxide.
Fossil fuels, nuclear fuels are non-renewable energy resources. If there is an accident, large amounts of radioactive material could be released into the environment. In addition, nuclear waste remains radioactive and is hazardous to health for thousands of years. It must be stored safely.
The wind is produced as a result of giant convection currents in the Earth's atmosphere, which are driven by heat energy from the sun. This means that the kinetic energy in wind is a renewable energy resource: as long as the sun exists, the wind will too.
Wind turbines use the wind to drive turbines directly. They have huge blades mounted on a tall tower. The blades are connected to a 'nacelle', or housing, which contains gears linked to a generator. As the wind blows, it transfers some of its kinetic energy to the blades, which turn and drive the generator. Several wind turbines may be grouped together in windy locations to form wind farms.
Wind is a renewable energy resource and there are no fuel costs. No harmful polluting gases are produced.
Wind farms are noisy and may spoil the view for people living near them. The amount of electricity generated depends on the strength of the wind. If there is no wind, there is no electricity.
Like the wind, water can be used to drive turbines directly. There are several ways that water can be used, including waves, tides and falling water in hydroelectric power schemes.
The water in the sea rises and falls because of waves on the surface. Wave machines use the kinetic energy in this movement to drive electricity generators.
Huge amounts of water move in and out of river mouths each day because of the tides. A tidal barrage is a barrier built over a river estuary to make use of the kinetic energy in the moving water. The barrage contains electricity generators, which are driven by the water rushing through tubes in the barrage.
Like tidal barrages, hydroelectric power (HEP) stations use the kinetic energy in moving water. But the water comes from behind a dam built across a river valley. The water high up behind the dam contains gravitational potential energy. This is transferred to kinetic energy as the water rushes down through tubes inside the dam. The moving water drives electrical generators, which may be built inside the dam.
Water power in its various forms is a renewable energy resource and there are no fuel costs. No harmful polluting gases are produced. Tidal barrages and hydroelectric power stations are very reliable and can be easily switched on.
It has been difficult to scale up the designs for wave machines to produce large amounts of electricity. Tidal barrages destroy the habitat of estuary species, including wading birds. Hydroelectricity dams flood farmland and push people from their homes. The rotting vegetation underwater releases methane, which is a greenhouse gas.
Hot water and steam from deep underground can be used to drive turbines: this is called geothermal energy.
Several types of rock contain radioactive substances such as uranium. Radioactive decay of these substances releases heat energy, which warms up the rocks. In volcanic areas, the rocks may heat water so that it rises to the surface naturally as hot water and steam. Here the steam can be used to drive turbines and electricity generators.
Geothermal power station exists in places such as Iceland,California and Italy.
In some places, the rocks are hot, but no hot water or steam rises to the surface. In this situation, deep wells can be drilled down to the hot rocks and cold water pumped down. The water runs through fractures in the rocks and is heated up. It returns to the surface as hot water and steam, where its energy can be used to drive turbines and electricity generators. The diagram below shows how this works.
Geothermal energy is a renewable energy resource and there are no fuel costs. No harmful polluting gases are produced.
Most parts of the world do not have suitable areas where geothermal energy can be exploited.
Solar cells are devices that convert light energy directly into electrical energy. Do not confuse solar cells with solar panels, which use heat energy from the Sun to heat up water.
You may have seen small solar cells in calculators. Larger arrays of solar cells are used to power road signs in remote areas, and even larger arrays are used to power satellites in orbit around Earth.
Solar energy is a renewable energy resource and there are no fuel costs. No harmful polluting gases are produced. Solar cells provide electricity in remote locations, such as roadside signs.
Solar cells are expensive and inefficient, so the cost of their electricity is high. Solar cells do not work at night.
You should be able to compare and contrast the different energy resources used to produce electricity.
Power stations fuelled by fossil fuels or nuclear fuels are reliable sources of energy, meaning they can provide power whenever it is needed. However, their start-up times vary according to the type of fuel used.
This list shows the type of fuel in order of start of time going from short to long:
- gas-fired station (shortest start-up time)
- oil-fired station
- coal-fired station
- nuclear power station (longest start-up time).
Renewable resources of fuel do not cost anything, but the equipment used to generate the power may be expensive to build. Certain resources are reliable, including tidal barrages and hydroelectric power. Others are less reliable, including wind and solar energy.
The National Grid
Electricity is distributed from power stations to consumers through the National Grid, which allows distant power stations to be used. It also allows a mix of different energy resources to be used efficiently to supply the country’s electricity, whatever the local demand.
Electricity is transferred from power stations to consumers through the wires and cables of the National Grid. When a current flows through a wire some energy is lost as heat. The higher the current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a low current. This needs a high voltage.
Transformers are used in the National Grid. A transformer is an electrical device that changes the voltage of an alternating current (ac) supply, such as the mains electrical supply. A transformer that:
- increases the voltage is called a step-up transformer
- decreases the voltage is called a step-down transformer.
Electricity from a power station goes to:
- step-up transformers
- high voltage transmission lines
- step-down transformers
- consumers, for example homes, factories and shops.
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, and 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 and magnetic fields vibrate as the waves travel.
Longitudinal and transverse waves
You should be able to describe the characteristics of transverse and longitudinal waves.
In transverse waves, the oscillations (vibrations) are at right angles to the direction of travel and energy transfer
Light and other types of electromagnetic radiation are transverse waves. All types of electromagnetic waves travel at the same speed through a vacuum, such as through space.
In longitudinal waves, the oscillations are along the same direction as the direction of travel and energy transfer.
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.
Longitudinal waves show area of compression and rarefaction. In the animation, the areas of compression are where the parts of the spring are close together, while the areas of rarefaction are where they are far apart.
Amplitude, wavelength and frequency
You should understand what is meant by the amplitude, wavelength and frequency of a wave.
As waves travel, they set up patterns of disturbance. 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. For example, most people cannot hear a high-pitched sound above 20 kHz, radio stations broadcast radio waves with frequencies of about 100 MHz, while most wireless computer networks operate at 2.4 GHz.
The speed of a wave is related to its frequency and wavelength, according to this equation:
v = f × λ
- v is the wave speed in metres per second, m/s
- f is the frequency in hertz, Hz
- λ (lambda) is the wavelength in metres, m.
All waves obey this wave equation. For example, a wave with a frequency of 100 Hz and a wavelength of 2 m travels at 100 × 2 = 200 m/s.
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.
There is one special case you need to know. Refraction doesn't happen if the waves cross the boundary at an angle of 90° (called the normal) - in that case they carry straight on.
When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is called diffraction.
The extent of the spreading depends on how the width of the gap compares to the wavelength of the waves. Significant diffraction only happens when the wavelength is of the same order of magnitude as the gap. For example:
- a gap similar to the wavelength causes a lot of spreading with no sharp shadow, eg sound through a doorway
- a gap much larger than the wavelength causes little spreading and a sharp shadow, eg light through a doorway.
Sound waves and light waves reflect from surfaces. When waves reflect, they obey the law of reflection:
the angle of incidence equals the angle of reflection
- The normal is a line drawn at right angles to the reflector
- The angle of incidence is between the incident (incoming) ray and the normal
- The angle of reflection is between the reflected ray and the normal.
Smooth surfaces produce strong echoes when sound waves hit them, and they can act as mirrors when light waves hit them. The waves are reflected uniformly and light can form images The waves can:
- appear to come from a point behind the mirror, for example a looking glass
- be focused to a point, for example sunlight reflected off a concave telescope mirror.
Rough surfaces scatter sound and light in all directions. However, each tiny bit of the surface still follows the rule that the angle of incidence equals the angle of reflection.
Constructing a ray diagram
In a ray diagram, the mirror is drawn a straight line with thick hatchings to show which side has the reflective coating. The light rays are drawn as solid straight lines, each with an arrowhead to show the direction of travel. Light rays that appear to come from behind the mirror are shown as dashed straight lines.
Make sure that the incident rays (the solid lines) obey the law of reflection: the angle of incidence equals the angle of reflection. Extend two lines behind the mirror. They cross where the image appears to come from.
The image in a plane mirror is:
- virtual (it cannot be touched or projected onto a screen)
- upright (if you stand in front of a mirror, you look the right way up)
- laterally inverted (if you stand in front of a mirror, your left side seems to be on the right in the reflection).