Factors Affecting Energy Transfer by Heating (Oxford AQA IGCSE Physics)

Revision Note

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

Surface area to volume ratio of different sized cubes for IGCSE & GCSE Physics Revision Notes
The ratio of a smaller object's surface area to volume is larger than that of a bigger object

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

A person is standing in a bathroom with one foot on the tile floor and one foot on a bath mat. The foot in direct contact with the tile will loose thermal energy more quickly than the foot in contact with the mat
Bathroom tile is a good thermal conductor whilst a bathroom rug is a poor thermal conductor

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

The left hand side diagram shows that less thermal energy is transferred out of the building because of the filled cavity. The right side diagram shows more thermal energy is transferred out of the building because there is no filling in the cavity
Less energy is transferred by conduction and convection if there is cavity wall insulation instead of an air gap

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

A motorbike has metal cooling fins next to the engine, for IGCSE & GCSE Physics revision notes
Cooling fins on the outside of a motorbike engine help to keep it cool

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

Desert foxes have big ears and artic foxes have small ears, for IGCSE & GCSE Physics revision notes
Desert foxes and artic foxes have different sized ears suited to their environments and maintaining a comfortable temperature

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 image shows a large number of fins that have a dark matt surface

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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