Heat Transfer
Science: KS3, topic 3.1.8
Heat Transfer
Thermal energy can be transferred by conduction, convection, and radiation.
Thermal energy
- If there is a temperature difference between objects, heat will transfer.
- A common misconception is that heat travels ‘up’. This is wrong.
- Heat transfers from a hotter object to a colder object.
- The greater the temperature difference, the faster the transfer of heat.
- Heat transfer will stop when both objects are the same temperature.
- Other factors that affect the thermal energy transfer rate include:
- The surface area.
- The material.
- There are three ways in which heat is transferred.
Radiation
- All objects emit infrared radiation.
- The hotter the object is, the more radiation it emits.
- Radiation does NOT need particles, so it can travel through a vacuum.
- Space is a vacuum and we can feel the Sun's radiation.
- Infrared radiation is part of the electromagnetic spectrum.
- Dark-matt surfaces are the best emitters and absorbers of radiation.
- Shiny surfaces reflect radiation.
- Thermal imaging cameras use infrared radiation to take photographs.
- You feel the heat of a light bulb by radiation.
Conduction
- Conduction occurs when particles in a solid are heated.
- If a substance is able to conduct heat well, such as a metal, it is a good conductor of heat.
- If a substance is unable to conduct heat well, such as plastic or newspaper, it is a poor conductor or an insulator.
- Plastic insulation is wrapped around electrical wires to prevent electric shocks.
How conduction works
- As particles are heated, they gain energy and vibrate more.
- As they vibrate, they collide and so transfer energy to the next particle.
- Examples of this can be seen if you leave a cold spoon in a hot cup of tea:
- The spoon will become hot.
- Pans become hot when you are cooking due to conduction.
- A snowball will melt in your hand.
Convection
- Convection occurs when particles in a fluid (liquids or gases) are heated.
- As particles are heated, they gain energy.
- As they gain energy, they move around more and so take up more space and become less dense.
- Less dense fluid will rise and be replaced by more dense fluid.
- Examples are:
- A radiator.
- Central heating.
- Convection currents causing wind.
- The movement of the mantle inside the Earth.
Heat transfer
- No heat transfer is ever 100% efficient.
- You will always lose heat from an object.
- We say that this heat is 'dissipated' into the surroundings.
- The heat is transferred to the air around you by convection, conduction, and radiation.
Test yourself on Heat Transfer
Which of these is not a method of heat transfer?
Which are true of radiation?