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Conduction and Convection

The energy that is transferred between two bodies that are in contact is called heat.

Definition Heat is the energy that is transferred when a hot body is in contact with acold body.

The process of heat transfer is classified into three groups: conduction, convection andradiation.

1. Conduction of heat where heat is transferred through contact.2. Convection in liquids and gases where heat is transferred due to changes of

density.. !adiation of heat where heat and other for"s of energy are transferred

due to its own energy.

Activity 1 Investigation into Conduction and Convection

#n this activity, we e$plore both conduction and convection in "ore detail and then go on toe$plain the concepts "ore thoroughly.

1. Conduction of heat

#f you were required to investigate which of a "etal spoon or wooden spoonconducts heat best without da"aging the spoons in the investigation, what "ethodwould you use% &ow answer the following questions:

a' (hy does the "etal spoon heat up faster%b' )sing the particle "odel of "atter e$plain why this is so.

2. Convection in liquids and gases

*ou are given one crystal of potassiu" per"anganate, so"e water, a bea+er anda unsen burner. -lan an investigation in which you show how the purple colourspreads throughout the liquid without stirring the contents. &ow answer thefollowing questions:

a' tate what you observe.b' /$plain why this occurs.

Conduction and Convection Explained

a' Conduction of heat through a substance is the flow of heat fro" hot parts to coldparts. #f a nail is held at one end and heated up at the other, the colder endeventually beco"es too hot to handle because heat is conducted to it fro" the hotend. The sa"e happens if a "etal spoon is placed in hot food eventually thespoon cannot be held because it is too hot. -reventing heat conduction is ofteni"portant thin+ of the ho"e it needs to be insulated properly in order to +eep a"oderate te"perature. Ther"al conductivity does not necessarily involve the"ove"ent of parts of the conductor it is the contact each part "a+es with the

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other that deter"ines whether it will be a conductor or insulator. 0n e$a"ple of ather"al conductor is a ther"o"eter. 0 s"all volu"e of alcohol or "ercury' isplaced into a very narrow tube. 0s the alcohol heats up, it e$pands into the thintube which is "ar+ed off and calibrated to read the correct te"perature.

b' Convection in liquids and gases as aliquid or gas heats up, it e$pands,beco"es less dense and rises. Coolerliquid or gas then ta+es its place and aconvection current is for"ed. Thisprinciple is used in chi"neys and oilheaters in the ho"e. #t also accounts forsea bree es during the day and nightwhere, during the day cool wind will co"ein off the sea because the land heats upquic+er than the sea, and at night the windblows in the opposite direction becausethe sea cools down slower than the land i.

Activity 2 Questions on Conduction and Convection1. )se the table below to write down three e$a"ples of good conductors of heat and

three of bad conductors of heat that are used in your ho"e or school.

Good Conductors ad Conductors

1 1

2 2

2. (hat is it that "a+es a substance a good conductor of heat%

. (hy, when in a coastal area, does the wind blow onto the shore during the day andaway fro" the shore at night%

3. (hy do you thin+ a solar water heater will use blac+ piping and not white piping%4. There are "any people around the world who have a cat or a dog as a pet. (hy do

you thin+ these ani"als "oult at the end of winter% (hat was the function of the furthey are now losing%

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!adiation of "eat

Conduction and convection of heat depends on there being contact between the syste"sthat are transferring the heat. !adiated heat does not have to have a "aterial "ediu"through which to travel.

!adiation of heat . This is often referred to as 5infra6red7 radiation and is only detectedwhen the radiation falls on so"ething that detects it. Heatradiation has the ability to travel through e"pty space.

#he la$ of radiation is% 50 good e"itter of heat radiation is a good absorber and abad e"itter is a bad absorber7.

Activity & Questions on "eat #ransfer 1. #n the following e$a"ples, is the effect of the heating or cooling by conduction,

convection or radiation% 8otivate your answer in each case.

a' (hen standing in front of a fire.b' The heating of a roo".c' The heating of water in a hot water cylinder.d' 0n onshore wind at the coast.e' The heating of a teaspoon while stirring a cup of coffee.

2. (hy should the layer of ice be re"oved fro" a free er at regular intervals%

. 0 piece of paper wrapped round a brass rod "ay be held in a unsen burner fla"ewithout being scorched. #f wrapped round a wooden rod, it starts to burn "orequic+ly. (hy do you thin+ this happens%

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i 9ohn yron, 1 ; , GCSE Steps in Physics ,