Factors Affecting Energy Transfer by Heating (Oxford AQA IGCSE Physics)
Revision Note
Written by: Ann Howell
Reviewed by: Caroline Carroll
Factors Affecting Energy Transfer By Heating
The rate at which an object transfers energy by heating depends on:
Its surface area and volume
The material from which the object is made
The nature of the surface with which the object is in contact
The temperature difference between the object and its surroundings
Surface area and volume
Smaller objects cool (transfer energy) more quickly than larger objects because of their high surface area to volume ratio
The surface area of an object is the area in contact with the surrounding environment
The volume of an object is the total amount of space it occupies
A smaller object has a relatively large surface area compared to its volume
A larger object has a small surface area compared to its volume
So, a smaller object has relatively more surface area available through which it can transfer energy than a larger object with the same material and density
In the example below:
A small cube has a volume of 1 cm3 and a surface area of 6 cm2
A medium cube has a volume of 8 cm3 and a surface area of 8 cm2
A large cube has a volume of 27 cm3 and a surface area of 54 cm2
When the surface area of each shape is divided by the volume then this gives the volume-to-surface area ratio
For the smaller shape, this is 6:1, for the medium shape this is 3:1 and for the larger shape, this is 2:1
Hence the smaller shape has a larger surface area compared to its volume so it will transfer thermal energy away from itself quicker
Surface area to volume ratio
Type of material
The properties of a material determine whether it is a good thermal conductor or a good thermal insulator
Metals are a good thermal conductor
Rubber is a good thermal insulator
More information on this is available on the revision note page for conduction
Thermal conduction is also affected by the thickness of the material and its mass
The rate of energy transfer is reduced by increasing the thickness of the material and therefore increasing the mass
Thermal conductors and insulators
Nature of contact surface
Materials have different surfaces that allow varying amounts of thermal energy transfer
Dull black or matt surfaces are the best emitters and absorbers of radiation
This is why you feel warmer in a black T-shirt than in a white T-shirt on a hot day
White or shiny surfaces are poor absorbers and emitters of radiation
Following a race runners are given foil blankets to keep them warm
These reflect their thermal energy back onto themselves
Temperature difference
The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred by heating
When it is cold outside thermal energy is lost through the walls of a house much more quickly than when it is warm outside
Decreasing a temperature difference can reduce the rate of energy transfer
This can be done through the use of insulation, such as cavity wall insulation in a house
Insulated mugs can be used to keep hot drinks hotter for longer
Cavity wall insulation
Energy transfer by design
Cooling fins
Cooling fins are found at the back of most appliances
They comprise of rows of metal pieces that look like fins sticking up at right angles to an appliance
They are designed to increase the surface area from which thermal energy can be transferred away from the device
Cooling fins are used on motorbikes to help the engine cool
Cooling fins on a motorbike
Animal adaptations
Animals are adapted to survive best in their own environment and maintain a more comfortable temperature
A desert fox for example has much larger ears than an artic fox
This is because desert foxes get much hotter, so they need to transfer thermal energy away from their body more quickly to avoid overheating
Desert and artic foxes
Worked Example
The component below is found at the back of a computer and it gets very hot when the computer is in use.
The component is designed to help transfer energy quickly away from the computer.
a) Explain why the component has a dark matt surface and a large number of fins.
b) Suggest a suitable material for the fins and explain your answer.
Answer:
Part a)
Step 1: Explain why a dark matt surface has been used:
It makes the component a good emitter of infrared radiation
Step 2: Explain why there is a large number of fins:
They create a large surface area for thermal energy to be transferred away from the computer
Part b)
Step 1: State a suitable material that could be used:
Metal
Step 2: Give a reason:
Metals are good thermal conductors as they contain free electrons which speed up the process of conduction
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