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

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

  •  1. INTRODUCTION

  • 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!

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

  • 2a. Conduction: 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 more kinetic energy (KE) 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 of thermal store. In other words the higher the temperature of a material the more 'heat energy' it contains.

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

  • The more dense the solid, generally speaking the better the conductor. 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).

  • 2b. Convection: 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.

  • See also notes on density

  • 2c. Radiation: 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.

    • (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 ('thermos flask').

    • (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.


  • 3. Applications of heat transfer science

  • Thermal conductivity

    • 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 and in another rapid heat transfer.

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


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