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 a
cold body.
The process of heat transfer is classified into three groups: conduction, convection and
radiation.
1.
2.
3.
Conduction of heat – where heat is transferred through contact.
Convection in liquids and gases – where heat is transferred due to changes of
density.
Radiation of heat –
where heat and other forms of energy are transferred
due to its own energy.
Activity 1
Investigation into Conduction and Convection
In this activity, we explore both conduction and convection in more detail and then go on to
explain the concepts more thoroughly.
1.
Conduction of heat
If you were required to investigate which of a metal spoon or wooden spoon
conducts heat best without damaging the spoons in the investigation, what method
would you use? Now answer the following questions:
a)
b)
2.
Why does the metal spoon heat up faster?
Using the particle model of matter explain why this is so.
Convection in liquids and gases
You are given one crystal of potassium permanganate, some water, a beaker and
a Bunsen burner. Plan an investigation in which you show how the purple colour
spreads throughout the liquid without stirring the contents. Now answer the
following questions:
a)
b)
State what you observe.
Explain why this occurs.
Conduction and Convection Explained
a) Conduction of heat through a substance is the flow of heat from hot parts to cold
parts. If a nail is held at one end and heated up at the other, the colder end
eventually becomes too hot to handle because heat is conducted to it from the hot
end. The same happens if a metal spoon is placed in hot food – eventually the
spoon cannot be held because it is too hot. Preventing heat conduction is often
important – think of the home – it needs to be insulated properly in order to keep a
moderate temperature. Thermal conductivity does not necessarily involve the
movement of parts of the conductor – it is the contact each part makes with the
other that determines whether it will be a conductor or insulator. An example of a
thermal conductor is a thermometer. A small volume of alcohol (or mercury) is
placed into a very narrow tube. As the alcohol heats up, it expands into the thin
tube which is marked off and calibrated
to read the correct temperature.
b) Convection in liquids and gases – as a
liquid or gas heats up, it expands,
becomes less dense and rises. Cooler
liquid or gas then takes its place and a
convection current is formed.
This
principle is used in chimneys and oil
heaters in the home. It also accounts for
sea breezes during the day and night
where, during the day cool wind will
come in off the sea because the land
heats up quicker than the sea, and at
night the wind blows in the opposite
direction because the sea cools down
slower than the landi.
Activity 2
1.
Questions on Conduction and Convection
Use the table below to write down three examples of good conductors of heat and
three of bad conductors of heat that are used in your home or school.
Good Conductors
Bad Conductors
1
1
2
2
3
3
2.
What is it that makes a substance a good conductor of heat?
3.
Why, when in a coastal area, does the wind blow onto the shore during the day and
away from the shore at night?
4.
Why do you think a solar water heater will use black piping and not white piping?
5.
There are many people around the world who have a cat or a dog as a pet. Why do
you think these animals moult at the end of winter? What was the function of the fur
they are now losing?
Radiation of Heat
Conduction and convection of heat depends on there being contact between the systems
that are transferring the heat. Radiated heat does not have to have a material medium
through which to travel.
Radiation of heat.
This is often referred to as ‘infra-red’ radiation and is only detected
when the radiation falls on something that detects it. Heat
radiation has the ability to travel through empty space.
The law of radiation is:
Activity 3
1.
‘A good emitter of heat radiation is a good absorber and a
bad emitter is a bad absorber’.
Questions on Heat Transfer
In the following examples, is the effect of the heating or cooling by conduction,
convection or radiation? Motivate your answer in each case.
a)
b)
c)
d)
e)
When standing in front of a fire.
The heating of a room.
The heating of water in a hot water cylinder.
An onshore wind at the coast.
The heating of a teaspoon while stirring a cup of coffee.
2.
Why should the layer of ice be removed from a freezer at regular intervals?
3.
A piece of paper wrapped round a brass rod may be held in a Bunsen burner flame
without being scorched. If wrapped round a wooden rod, it starts to burn more
quickly. Why do you think this happens?
i
John Byron, 1979, GCSE Steps in Physics, Oxford University Press, page 75