Thermal transfer mechanisms - conduction, convection and radiation
Hot things tend to cool down and cold things tend to warm up. Heat energy spontaneously moves from hot places to cold places. There are 3
ways it does this.
Conduction
Heat energy is passed through solids as vibrational energy from atom to
atom. Metals are very good thermal conductors as they have free electrons
which also carry energy.
Heat energy from the cup conducts into the table.
Convection
Remember- hot air rises. All hot liquids and gases rise and cool ones sink.
This is how heat energy is carried through a fluid.
The air above this
coffee gets hot and rises.
Radiation
Infra red radiation carries heat energy through space. All hot objects emit
heat radiation. Black objects are very good emitters and absorbers. White
and shiny objects are poor emitters and good reflectors.
The hot coffee emits heat radiation into the surroundings.
In the winter we spend a lot of money trying to keep our houses warm and
comfortable. There are many reasons why we should make an effort to reduce the
amount of heat which is lost from our houses.
If we use less energy to heat our houses then we save money and less
energy needs to generated. We have seen how electricity generation causes
environmental problems. We are also conserving natural resources such as
fossil fuels.
Industry
takes energy losses very seriously as it can reduce their profit margins
greatly if they are wasting money.
Waste heat from power stations is often used to heat homes or
greenhouses.
Notice in the infra red picture above that most heat will be lost from the
roof of the house. Why do you think this is? How can we reduce this heat loss?
How automatic feedback can control the temperature of a system
Imagine we want to keep the temperature in a greenhouse at around 300C.
We wouldn't have a heater on all the time as this might make the greenhouse too
hot and would be wasteful.
What we could do is monitor the temperature in the greenhouse constantly
using a temperature sensor and whenever it falls below a certain value
turn a heater on. When the temperature rises above a certain value the heater is
then turned off.
There is usually a difference between the upper and lower temperatures
so that the heater is not constantly switching on and off as this might
damage it.
The system could be improved by increasing the power of the
heater when there is a bigger difference between the desired and the actual
temperatures. This is called proportional control.
The temperature of our bodies is controlled in a similar way. When we are
too hot or cold things happen automatically to bring our temperature back to
the correct value.
