Heat transfer and efficiencyHeat can be transferred from place to place by conduction [conduction: The transfer of heat energy through a material - without the material itself moving. ], convection [convection: The transfer of heat energy through a moving liquid or gas. ] and radiation [infrared radiation: Electromagnetic radiation emitted from a hot object. ]. 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.There are several different types of energy, and these can be transferred from one type to another. Energy transfer diagrams show the energy transfers in a process. More efficient devices transfer the energy supplied to them into a greater proportion of useful energy than less efficient devices do.Heat transfer by conduction and convectionHeat is thermal energy. It can be transferred from one place to another by conduction, convection and radiation. Conduction and convection involve particles, but radiation involves electromagnetic waves.Conduction

Thermogram of a pan being heated on a stove 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.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.Heat transfer by conduction

Heat transfer by conduction

Heat transfer by conduction

ConvectionLiquids 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.< p>Your web browser does not have JavaScript switched on at the moment. For information on how to enable JavaScript please go to the Webwise site.< p>You will not be able to see this content until you have JavaScript switched on. Heat transfer by radiationRadiationAll objects give out and take in thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits.

Light from the sun reaching earth Infrared radiation is a type of electromagnetic radiation that involves waves. No particles are involved, unlike in the processes of conduction and convection, so radiation can even work 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 reflecting and absorbing infrared radiation. Comparison of surfaces abilities to reflect and absorb radiationcolourfinishability to emit thermal radiationability to absorb thermal radiation

darkdull or mattgoodgood

lightshinypoorpoor

If two objects made from the same material have identical volumes, a thin, flat object will radiate heat energy faster than a fat object. This is one reason why domestic radiators are thin and flat. Radiators are often painted with white gloss paint. They would be better at radiating heat if they were painted with black matt paint, but in fact, despite their name, radiators transfer most of their heat to a room by convection. Reducing heat lossYou should be able to describe how heat energy is lost from buildings and to explain how these losses can be reduced.Heat escape routesTake a look at this image showing heat loss from a house.

Most household heat is lost through the windows and roofHeat is lost: through the roof through windows through gaps around the door through the walls through the floor Heat energy is transferred from homes by conduction [conduction: The transfer of heat energy through a material - without the material itself moving. ] through the walls, floor, roof and windows. It is also transferred from homes by convection [convection: The transfer of heat energy through a moving liquid or gas. ]. For example, cold air can enter the house through gaps in doors and windows, and convection currents can transfer heat energy in the loft to the roof tiles. Heat energy also leaves the house by radiation through the walls, roof and windows.Ways to reduce heat lossThere are some simple ways to reduce heat loss, including fitting carpets, curtains and draught excluders.Heat loss through windows can be reduced using double glazing. The gap between the two panes of glass is filled with air. Heat loss through conduction is reduced, as air is a poor conductor of heat. Heat transfer by convection currents is also reduced by making the gap is very narrow.WatchYou may wish to view this BBC News item from 2006 about energy efficiency in homes. WatchHeat loss through walls can be reduced using cavity wall insulation. This involves blowing insulating material into the gap between the brick and the inside wall, which reduces the heat loss by conduction. The material also prevents air circulating inside the cavity, therefore reducing heat loss by convection.Heat loss through the roof can be reduced by laying loft insulation. This works in a similar way to cavity wall insulation.Forms of energyYou should be able to recognise the main types of energy. One way to remember the different types of energy is to learn this sentence:Most Kids Hate Learning GCSEEnergy NamesEach capital letter is the first letter in the name of a type of energy.Types of energyType of energyDescription

Magnetic

MagnetEnergy in magnets and electromagnets

Kinetic

A bullet cutting a playing cardThe energy in moving objects. Also called movement energy.

Heat

Burning matchAlso called thermal energy

Light

SunlightAlso called radiant energy

Gravitational potential

Sky diversStored energy in raised objects

Chemical

Organic foodStored energy in fuel, foods and batteries

Sound

GuitarEnergy released by vibrating objects

Electrical

LightningEnergy in moving or static electric charges

Elastic potential

CatapultStored energy in stretched or squashed objects

Nuclear

Nuclear fuel assemblyStored in the nuclei of atoms

Energy transferYou should recall from your Key Stage 3 studies how to draw and interpret an energy transfer diagram.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 it is transferred. Sankey diagrams also show the relative amounts of each type of energy.Energy transfer diagramsThis 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.

Process of using chemical energyThis diagram shows the energy transfer diagram for the useful energy transfer in an electric lamp. You can see that the electric lamp transfers or converts electrical energy into light energy.

Process of using electrical energyNotice that these energy transfer diagrams only show the useful energy transfers. However, car engines are also noisy and hot, and electric lamps also give out heat energy.Sankey diagramsSankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved. The Sankey diagram for an electric lamp below shows that most of the electrical energy is transferred as heat rather than light.

Sankey diagram for a filament lampEfficiencyYou should know that energy can be 'wasted' during energy transfers, and you should be able to calculate the efficiencyefficiency: The fraction of the energy supplied to a device which is transferred in a useful form. of a device.'Wasted' energyEnergy cannot be created or destroyed. It can only be transferred from one form to another or moved. Energy that is 'wasted', like the heat energy from an electric lamp, does not disappear. Instead, it is transferred into the surroundings and spreads out so much that it becomes very difficult to do anything useful with it.Electric lampsOrdinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is transferred as heat energy instead of light energy. This is the Sankey diagram for a typical filament lamp.

Sankey diagram for a filament lampModern energy-saving lamps work in a different way. They transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energy-saving lamp.

Sankey diagram for a typical energy-saving lampFrom the diagram, you can see that much less electrical energy is transferred, or 'wasted', as heat energy.Calculating efficiencyThe efficiency of a device such as a lamp can be calculated using this equation:efficiency = ( useful energy transferred energy supplied ) 100The efficiency of the filament lamp is (10 100) 100 = 10%.This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy).The efficiency of the energy-saving lamp is (75 100) 100 = 75%. This means that 75% of the electrical energy supplied is transferred as light energy (25% is transferred as heat energy).Note that the efficiency of a device will always be less than 100%.

Thermal Insulation Prevents Heat From Escapingby Ron Kurtus (revised 29 April 2006)Thermal insulation is the method of preventing heat from escaping a container or from entering the container.In other words, thermal insulation can keep an enclosed area such as a building warm, or it can keep the inside of a container cold. Heat is transferred by from one material to another by conduction, convection and/or radiation. Insulators are used to minimize that transfer of heat energy. In home insulation, the R-value is an indication of how well a material insulates.Questions you may have include: Where is thermal insulation used? How does insulation work? What is the R-value?This lesson will answer those questions. Useful tool: Units Conversion

Where thermal insulation is usedIf you have an object or area that is at a certain temperature, you may want to prevent that material from becoming the same temperature as neighboring materials. This is usually done by employing a thermal insulation barrier.For example: If the air outside is cold, you may want to protect your skin by wearing clothes that keep the cold out and the body warmth in. If your house has cool air inside during the summer, you may want to prevent the temperature from becoming the same as the hot air outside by having the house well insulated. If you have a hot drink, you may want to prevent it from becoming room temperature by putting it in a thermos bottle.In any location where there are materials of two drastically different temperatures, you may want to provide an insulating barrier to prevent one from becoming the same temperature as the other. In such situations, the effort is to minimize the transfer of heat from one area to another.How insulation worksInsulation is a barrier that minimizes the transfer of heat energy from one material to another by reducing the conduction, convection and/or radiation effects.Insulating materialsMost insulation is used to prevent the conduction of heat. In some cases radiation is a factor. A good insulator is obviously a poor conductor.Less dense materials are better insulators. The denser the material, the closer its atoms are together. That means the transfer of energy of one atom to the next is more effective. Thus, gases insulate better than liquids, which in turn insulate better than solids.An interesting fact is that poor conductors of electricity are also poor heat conductors. Wood is a much better insulator than copper. The reason is that metals that conduct electricity allow free electrons to roam through the material. This enhances the transfer of energy from one area to another in the metal. Without this ability, the material--like wood--does not conduct heat well.Insulation from conductionConduction occurs when materialsespecially solidsare in direct contact with each other. High kinetic energy atoms and molecules bump into their neighbors, increasing the neighbor's energy. This increase in energy can flow through materials and from one material to another.Solid to solidTo slow down the transfer of heat by conduction from one solid to another, materials that are poor conductors are placed in between the solids. Examples include: Fiberglass is not a good conductor nor is air. That is why bundles of loosely packed fiberglass strands are often used as insulation between the outer and inner walls of a house. Conductive heat cannot travel though a vacuum. That is why a thermos bottle has an evacuated lining. This type of heat cannot be transferred from one layer to the other through the thermos bottle vacuum.Gas to solidTo slow down the heat transfer between air and a solid, a poor conductor of heat is placed in between.A good example of this is placing a layer of clothing between you and the cold outside air in the winter. If the cold air was in contact with your skin, it would lower the skin's temperature. The clothing slows down that heat loss. Also, the clothing prevents body heat from leaving and being lost to the cold air.Liquid to solidLikewise, when you swim in water, cold water can lower your body temperature through conduction. That is why some swimmers wear rubber wet suits to insulate them from the cold water.Insulation from convectionConvection is transfer of heat when a fluid is in motion. Since air and water do not readily conduct heat, they often transfer heat (or cold) through their motion. A fan-driven furnace is an example of this.Insulation from heat transfer by convection is usually done by either preventing the motion of the fluid or protecting from the convection. Wearing protective clothing on a cold, windy day will inhibit the loss of heat due to convection.Insulation from radiationHot and even warm objects radiate infra-red electromagnetic waves, which can heat up objects at a distance, as well as lose energy themselves. Insulation against heat transfer by radiation is usually done by using reflective materials.A thermos bottle not only has an evacuated lining to prevent heat transfer by conduction, but it also is made of shiny material to prevent radiation heat transfer. Radiation from warm food inside the thermos bottle is reflected back to itself. Radiation from warm outside material is reflected to prevent heating cold liquids inside the bottle.R-valueThe R-value of a material is its resistance to heat flow and is an indication of its ability to insulate. It is used as a standard way of telling how good a material will insulate.The higher the R-value, the better the insulation.DefinitionThe R-value is the reciprocal of the amount of heat energy per area of material per degree difference between the outside and inside. Its units of measurement for R-value are:(square feet x hour x degree F)/BTU in the English system and(square meters x degrees C)/watts in the metric systemTableInsulation for the home has R-values usually in the range of R-10 up to R-30.The following is a listing of different materials with the English measurement of R-value:MaterialR-value

Hardwood siding (1 in. thick)0.91

Wood shingles (lapped)0.87

Brick (4 in. thick)4.00

Concrete block (filled cores)1.93

Fiberglass batting (3.5 in. thick)10.90

Fiberglass batting (6 in. thick)18.80

Fiberglass board (1 in. thick)4.35

Cellulose fiber (1 in. thick)3.70

Flat glass (0.125 in thick)0.89

Insulating glass (0.25 in space)1.54

Air space (3.5 in. thick)1.01

Free stagnant air layer0.17

Drywall (0.5 in. thick)0.45

Sheathing (0.5 in. thick)1.32

Reference Hyperphysics Georgia State UniversityThe R-value is proportional to the thickness of the material. For example, if you doubled the thickness, the R-value doubles.SummaryThermal insulation is used minimize the heat transfer in many everyday situations. It is done by reducing conduction, convection and/or radiation effects. The R-value is a standard of measurement of this insulation. Page 1 of 18