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School Physics notes: Heat transfer by conduction, convection and radiation

Introduction to heat energy transfer by conduction (including thermal conductivity), convection and radiation

Doc Brown's physics revision notes: GCSE physics, IGCSE  physics, O level & ~US grades 9-10 school science courses or equivalent for ~14-16 year old students

See also: Electromagnetic spectrum, sources, types, properties, uses, including infrared dangers and

1. Introduction to heat transfer

2a. Heat transfer by conduction - movement of fluid

3.

1. Introduction to heat transfer

• Energy can be transferred from one place to another by work or by heating processes.

• You  need to know and understand how this energy is transferred and which heating processes are most important in a particular situation.

•  When energy is transferred to an object by some means or other, the energy is stored in the object's energy stored. This energy store may be thermal (dealt with here), chemical, kinetic, magnetic, gravitational potential, elastic potential or nuclear.

•  Here we are interested in energy transfer by heating (conduction, convection) and infrared (radiation).

•  Heat energy must always flow from hotter material at a higher temperature to cooler material at a lower temperature and the bigger the temperature difference the bigger the rate of heat energy transfer.

•  eg the greater the temperature difference between a body and its surroundings like a hot object (eg mug of coffee) standing in a cold room, the faster the heat energy is transferred from the hotter material to the cooler material (eg surrounding air).

• Systems, thermal energy stores and states of matter:

• System is a word that means a particular object or objects that is being looked at in a particular context eg boiling water in a kettle.

• When a system changes, energy is transferred into or out of the system, this may be between different objects/materials in the system or perhaps between different energy stores (same of different).

• Closed systems are systems that do not allow energy to leave or enter (lose or gain) so the ne change in the total energy is zero.

• When an object/material is heated to raise its temperature, the thermal energy store of the object is increased.

• This thermal energy is stored throughout all the material by increasing the kinetic energy (KE) stores of the material's individual particles eg the KE particle vibration in a solid and the KE of the rapid movement of the freely moving particles in a liquid or gas.

• The thermal KE is distributed by either the particles vibrating against each other in a solid or the collisions between the freely moving particles in a gas or liquid. Higher KE particles will on average lose energy to lower KE particles - that's the way energy 'flows'.

• The higher the temperature of particles the greater their average kinetic energy so they will vibrate more violently in a solid and move faster in gases and liquids.

• Once heat energy has stopped being supplied to an object, it will distribute itself evenly to give a uniform temperature throughout the material by conduction or convection. However, if object's/material's surroundings are at a lower temperature, then heat energy will drain from this thermal store until its temperature has fallen to that of the surroundings - that's the way heat flows!

• -

• The amount of thermal energy transferred can be calculated from the formula

See also Particle theory models, internal energy, heat transfer in state changes, latent heat and particle motion in gases

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2. Heat energy can be transferred by conduction, convection and radiation

2a. Heat transfer by conduction - vibration of particles

• Conduction involves heat transfer by particles vibrating against each other in a solid or collisions between particles in a gas or liquid. Conduction is the main mode of heat transfer in a solid.

• Particle theory: In a solid the hotter particles vibrate more strongly, having a greater kinetic energy (KE) store and bang into neighbouring cooler lower KE particles and so transfer KE to them, so heat energy is transferred from a higher temperature region to a cooler region in any solid material. This kinetic energy of vibration is referred to as thermal energy store. In other words the higher the temperature of a material the more 'thermal energy' it contains.

• There is no effective heat transfer by convection or radiation within a solid material.

• The more dense a material, generally speaking the better the conductor - the faster heat is conducted.

• In materials where the particles are further apart the rate of heat transfer (rate of conduction) is reduced eg gases like air are much poorer conductors than solids like stone.

• Most non-metallic materials are poor conductors (good heat insulators) eg water, fat (in animals), wood, stone, concrete, plastics - particularly foams where poorly heat conducting gases are trapped giving even better insulation that the bulk solid plastic - and cheaper by using less material.

• Metals are particularly good conductors because of free moving electrons - a different heat transfer mechanism to that described here, which applies to all solids. Because the electrons are free to move in the solid metal, they can rapidly transfer kinetic energy by particle movement. The 'hot' electrons in the higher temperature region collide with neighbouring cooler electrons and so rapidly transferring heat energy (KE) - much faster than vibrating atoms in non-metals which are held in fixed positions.

• Incidentally if you pick up a cold poor conductor like a stone and then pick up an equally cold metal object at the same cool temperature, the metal object feels colder (but it isn't) because it conducts heat from your fingers faster than the stone!

• Water in an electric kettle is a two part system, but even though the kettle contains the water, it is NOT a closed system because electrical energy is coming in, and being changed to heat energy by the electrical resistance of the heating element.

• The heat energy conducts through the heating element and into the water whose temperature rises as its thermal energy store increases. However, the heat is then transferred to all the water by convection currents coming from the hottest least dense water by the heating element which then rises and circulates around (see convection below).

• BUT, you cannot stop some thermal energy escaping from the kettle by conduction through the case, convection through top opening and radiation from the kettle's surface. These heat losses will be minimised by the design of the kettle e.g.

• Plastic is a poor conductor.

• A silver surface is a poor radiator.

• In a toasted sandwich maker the heat is transferred to the bread by conduction. In the toaster the electrical energy is converted into heat and the thermal energy stores of the toaster and the sandwich are increased to effect the cooking.

• A measure of good/bad a material is at conducting heat is called its thermal conductivity.

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2b. Heat transfer by convection - movement of fluid

• Convection also involves heat transfer via particles but this involves bulk movement of particles in liquids or gases (fluids) and cannot take place in solids.

• Heating a gas or liquid increases its thermal energy store which will distributes itself through the kinetic energy stores of the particles. Although from the point of heating, conduction will be slow, most heat will be transferred to the bulk of the fluid by convection.

• Convection occurs when hotter/warmer less dense fluid (gas/liquid) flows and rises, is replaced by cooler more dense fluid moving (flowing) downwards.

• This cycle of events is called a convection current and its 'mechanism' is explained below.

• When a material is heated the particles have more KE, move faster and tend to push each other further apart, ie the material expands, becoming less dense. It is this change in density that cause convection to happen as gases and liquids are fluids - they can flow in convection currents.

• This is how hot water is produced in the hot water tank in the home, or the heating of water in a kettle where convection currents flow from the heating element so enabling all the water to be heated up. Note that the heating element must be near the bottom of the tank or kettle to produce the convection circulation to heat up all the water! If you put the heating element at the top there is no convection and all you do is heat up the top layer of water!

• Despite the name, radiators on the walls heat rooms up mainly by convection (there is some radiation too). Heat is transferred to the air particles when they collide with the radiator, and warm less dense air rises from the radiator towards the ceiling. The elevated air cools as the heat is distributed to the cooler air, which falls on the other side of the room. At the same time cooler air is drawn in at the base of the radiator to replace it - hence you get a convections current situation that gradually warms up all of the room.

• The hot gases from a gas fire will always rise due to the immediate formation of a rising convection current (recycling air flow) which carries the heat around the room. The chemical energy store of the fuel gas increases the thermal energy of the contents of the room.

• The energy is transferred in several ways.

• (i) The hot flame gases will heat the immediate surrounding air by conduction.

• This will cause lots of hot air to rise towards the ceiling.

• Heat is dissipated to the contents of the room and surroundings and the risen air begins to cool and become less dense.

• The more dense air falls and eventually circulate back round near the fire where cool air is drawn in to the fire.

• The process repeats itself continuously as long as the fire is lit - hence the formation of the convection current flow of air.

• (ii) The heat is also transferred by infrared radiation emitted from the hot flame - from the higher temperature flame to the lower temperature regions of the room.

• Hot water or electrical radiators work in exactly the same way.

• The surrounding air next to the radiator is heated by conduction.

• The thermal energy from the hot water or electrical element heats the particles of radiator case.

• The increased kinetic energy of the hot metal/concrete particles is transferred to the air particles in contact with the radiator i.e. those that collide with the radiator.

• The air particles move faster and spread out to decrease the density.

• The less dense air rises to start the convection current and are replaced by cooler air coming up and passing by the radiator surface.

• Again, some energy will be transferred to the room from the radiator surface by infrared radiation - you can feel this for yourself by placing your hand near, but not touching the radiator.

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Thermal (heat) radiation is emitted by all materials, gases, liquids or solids and the hotter the material the more strongly it gives out heat radiation which is called infrared radiation (IR).

• (a) All objects continuously emit and absorb infrared radiation from their surface, whatever their temperature - increasing or decreasing their thermal energy stores.

• Infrared radiation is a form of electromagnetic radiation and can travel through a vacuum.

• (b) The hotter an object is the more infrared radiation it radiates in a given time, the higher the temperature of the material, the more intense is the infrared radiation.

• An object that is hotter (higher temperature) than its surroundings will emit more radiation than it absorbs and an object that is cooler than its surroundings will absorb more radiation than it emits.

• You notice this effect in bright sunlight by feeling the warmth on your hand or standing near a fire.

• When an object cools down to the same temperature as its surroundings emitted infrared radiation equals the absorbed heat radiation.

• (c) Dark, matt surfaces are good absorbers and good emitters of infrared radiation eg rough black surfaces.

• Solar panels for hot water comprise of pipes carrying water to be heated set under a black surface to efficiently absorb the infrared radiation from the Sun. You can even just use matt black painted water pipes. You may even have a silvered surface under the pipes so more infrared ins reflected onto the black surface rather than becoming waste heat radiation. The pipes are made of copper which allows efficient conduction of the surface heat energy to the incoming cold water., so the hot water can be used as part of the households domestic heating or washing etc.

• (d) Light, shiny surfaces are poor absorbers and poor emitters of infrared radiation eg white gloss paint, silver surface used in vacuum flask. See 'thermos flask' in section (c).

• (e) Light, shiny surfaces are good reflectors of infrared radiation, this maybe to keep heat in to keep things warm or to minimise heat radiation in to keep things cool eg a vacuum flask. See 'thermos flask' in section (c).

(e) Another domestic case of infrared radiation! Unlike 'modern' LED bulbs, 'old fashioned' filament bulbs emit quite a bit of IR heat radiation. You can detect this with a frosty car where the central portion of the ice melts first on the transparent headlamp cover. Filament bulbs only convert ~10% of the electrical energy into visible light energy, most of the rest is converted into infrared radiation. The ice on the headlamp cover absorbs infrared equivalent to the latent heat of fusion (melting) and changes the ice to liquid water.

Energy store changes: The chemical energy store of the battery decreases as it is converted into electrical energy. The electrical energy increases the thermal energy store of the metal filament of the bulb. The thermal energy store of the filament decreases as it emits visible and infrared EM radiation. The absorbed EM radiation increases the thermal energy store of the headlamp cover and ice - causing the latter to melt. Eventually all the energy involved from the battery increases the thermal energy store of the surroundings.

A 'green' note: If there is, and its happening now in the UK and other countries, a change from very inefficient filament light bulbs to very efficient low energy LED light bulbs, there will be quite a reduction in the domestic demand for electricity. This reduced demand will help, on closure fossil fuel power stations, reduce CO2 emissions, reducing the greenhouse effect, and allow renewable energy resources to take over more of our electricity generation.

(f) The electrical resistance elements of a cooker ring or a toaster become hot enough to emit a strong beam of infrared radiation to heat the contents of a pan or grill the toast. Electrical energy is converted into heat energy which increases the thermal energy store of the elements and then increases the thermal energy store of the pan and contents or bread being toasted.

and

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3. More on applications of heat transfer science

See

Thermal conductivity of good insulators OR good conductors

The thermal conductivity of a material is a measure of efficiently heat is transferred through a material by conduction.

Materials like metals are very good heat conductors and transfer thermal energy very quickly.

Materials like stone, brick, wood and concrete etc. are poor heat conductors and have low thermal conductivities.

Thermal conductivity data is important when considering the material required to fulfil a particular application e.g. in heating systems when in one situation you might want good insulation (e.g. in loft) and in another rapid heat transfer (copper piping inside a hot water tank).

The design of a vacuum flask and other examples of a 'thermos flask'

The 'thermos' vacuum flask is a container designed to keep hot liquids hot and cold liquids cold. Diagram on the right.

It is designed to minimise thermal energy (heat) transfer by conduction, convection or radiation, both in an out of the container.

The flask is double walled with a vacuum (of nothing!) between the walls.

The vacuum ensures there is no thermal energy transfer by conduction - no material to vibrate.

If the double-walled flask is made of glass, the inner surfaces exposed to the vacuum are silvered to reflect back any thermal radiation (infrared).

If the flask is steel, the surfaces are shiny and reflect infrared in the same way.

The top cap should be a poor conductor of heat, and is often made of plastic or incorporates a plastic seal.

Both double walls and the cap help minimise losses by convection. The design ensures no air can pass over any surface that is in contact with the fluid.

Insulated cups, flasks and jugs come in all sizes - illustrated by the pictures below.

Left picture:

A double walled plastic jug

Right picture:

Plastic insulated coffee cup, sometimes with an added cardboard hand holder for extra insulation.

Your knowledge of examples of heat transfer situations should include the ...

how humans and other animals cope with low temperatures

Homeostasis - thermoregulation, control of temperature

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Heat Transfer and explaining physical changes and physical properties using a particle model

The density of materials and the particle model of matter gcse physics revision notes

gcse physics revision notes

Particle theory models, internal energy, heat transfer in state changes and latent heat and particle motion in gases (written more from a 'physics' point of view)

IGCSE revision notes heat transfer by conduction, convection, radiation KS4  physics Science notes on heat transfer by conduction, convection, radiation GCSE  physics guide notes on heat transfer by conduction, convection, radiation for schools colleges academies science course tutors images pictures diagrams for heat transfer by conduction, convection, radiation science revision notes on heat transfer by conduction, convection, radiation for revising  physics modules  physics topics notes to help on understanding of heat transfer by conduction, convection, radiation university courses in technical science careers in physics jobs in the industry technical laboratory assistant apprenticeships technical internships in engineering physics USA US grade 8 grade 9 grade10 AQA  physics science GCSE notes on heat transfer by conduction, convection, radiation Edexcel  physics science notes on heat transfer by conduction, convection, radiation for OCR 21st century  physics science OCR GCSE Gateway  physics science notes WJEC gcse science CCEA/CEA gcse science

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