Factors Affecting Energy Transfer by Heating (Oxford AQA IGCSE Combined Science Double Award)

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