IGCSE Physics Heat Transfer

Heat Transfer

Heat energy can be transferred by RADIATION, CONDUCTION or CONVECTION. Thermal (infrared) radiation is the transfer of heat energy by electromagnetic waves. Conduction and Convection involve the transfer of energy by PARTICLES.Conduction is the main form of heat transfer by solids. Convection is the main form of heat transfer in liquids and gases.Emission of thermal radiation occurs in solids, liquids and gases. Any object can both absorb and emit heat radiation. The bigger the temperature difference, the faster heat is transferred between a body and its surroundings. 

Radiation

Thermal radiation involves emission of electromagnetic waves. Heat radiation can also be called infrared radiation and it consists purely of electromagnetic waves of a certain range of frequencies.  All objects are continually emitting and absorbing heat radiation.  An object that's hotter than its surroundings emits more radiation than it absorbs (as it cools down.) An object that is cooler than its surroundings absorbs more radiation than it emits (as it warms up.) You can feel this heat radiation if you stand near something hot like a fire or if you put your hand above the bonnet of a recently parked car. 

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Caption: : Fire is hotter than its surroundings therefore it emits more radiation than it absorbs thus giving out heat.

Conduction

Occurs mainly in solids. In a solid the particles are held closely together. When one particle vibrates it collides with other particles nearby and the vibrations quickly pass from one particle to another. Conduction of heat is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles. This process continues throughout the solid and gradually some of the extra kinetic energy (or heat) is passed all the way through the solid causing a rise in temperature at the other side. 

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Caption: : Song to help you remember

Heat Convection

Convection of Heat - Liquids and Gases Only...Convection occurs when the more energetic particles move from the hotter region to the cooler region - and take their heat energy with them. The immersion heater: Heat energy is transferred from the heater coils to the water by conduction. (particle collisions.)The particles near the coils get more energy so they start moving around faster.This means there's more distance between them, i.e. the water expands and becomes less dense. This reduction in density means that hotter water rises above the denser and cooler water. As the hot water rises it displaces the colder water out of the way causing it to sink towards the heating coils. The cold water is then heated by the coils and rises and so it goes on. You end up with convection currents going up, round and down. (Circulating.) CONVECTION CURRENT! 

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Convection Continued

CONVECTION CURRENTS ARE ALL ABOUT CHANGES IN DENSITY. Heated less dense air rises. Warm air displaces cooler air. Cool and denser air falls. Cool air flows to fill the gap left by the rising heated air. 

Reducing Energy Transfer

Insulating your house reduces heat transfer. LOFT INSULATION: Thick layer of fibre glass wool laid out across the loft floor and ceiling reduces heat loss by conduction and convection.  HOT WATER TANK JACKET: Lagging such as fibre glass wool reduces conduction and radiation.  DRAUGHT-PROOFING: Strips of foam and plastic around doors and windows stop draughts of cold air blowing in, i.e. they reduce heat loss due to convection.  DOUBLE GLAZING: Two layers of glass with a narrow air gap in between to reduce conduction and convection. THICK CURTAINS: Big bits of cloth over windows to reduce heat loss by conduction and radiation.  CAVITY WALL INSULATION: Foam squirted into the gaps between the grips stops the convection currents being set up in the gap and radiation across the gap. The insulating foam and trapped air pockets also reduce conduction. 

Humans have ways of reducing heat transfer too... In the cold hairs on the skin rise, 'stand up' to trap a thicker layer of insulting air around the body. This limits the amount of heat loss by convection. Humans wear layers of clothes to reduce heat transfer. Pockets of air trapped in clothes and between layers mainly reduce heat transfer by conduction and convection.  Clothes also reduce heat loss by radiation from the body as the material absorbs some heat radiated out by bodies.