Consequences of Thermal Energy Transfer (Cambridge O Level Physics)

Conduction

• The main means of thermal energy transfer in solids
• When heated, atoms vibrate more, knocking into each other and transferring energy from atom to atom as a result

Conduction in Solids

Energy from the kinetic store of a vibrating particle is transferred to the kinetic store of a neighbouring particle. In this way, energy is transferred throughout the solid.

• Metals are especially good at conducting heat as the delocalised electrons can collide with the atoms, helping to transfer the vibrations through the material and hence transfer heat better

Delocalised Electrons in a Metal

Delocalised electrons in metals speed up thermal conduction

• If a question mentions metals, the answer will probably have something to do with conduction
• Trapped air is a very good insulator. Air is a gas and so is a poor conductor. Trapping air prevents it from circulating and forming a convection current

 

Hot Coffee Heating a Hand by Conduction

Thermal energy is transferred from the hot coffee to the mug and to the cold hands

• Other common examples of conduction are:
  • Heating a pan on a hob
  • Heating water in a kettle
  • A lizard warming its belly on a hot rock

 

Convection

• The means of thermal energy transfer in liquids and gases
  • Convection cannot occur in solids because the in solids the particles are not free to move
• When a fluid (a liquid or a gas) is heated:
  • The molecules push each other apart, making the fluid expand
  • This makes the hot fluid less dense than the surroundings
  • The hot fluid rises, and the cooler (surrounding) fluid moves in to take its place
  • Eventually, the hot fluid cools, contracts and sinks back down again
  • The resulting motion is called a convection current

Convection Current

A convection current caused by heating from the fire

• Heat sources placed at the bottom of things will generally create convection currents. Likewise, cooling units placed high up will cool any rising air, causing it to sink again

Coffee Heats Air through Convection

Thermal energy is transferred from the hot coffee to the air by convection currents rising from the surface

•  Other common examples of convection are:
  • A radiator heating a room
  • Air conditioning cooling a room
  • Ice cubes cooling a drink
  • A hot air balloon

Thermal Radiation

• The only means of thermal energy transfer that does not require a medium

• The temperature of a body can be regulated by balancing how much incoming radiation is absorbed and emitted (or reflected)
• If an object starts to absorb radiation at a higher rate than it radiates it, then the object will heat up
  • Likewise, if it loses radiation at a greater rate than it absorbs it, then the object will cool down

• This is how an emergency blanket works, to keep a trauma victim warm:
  • Rescue teams use light-coloured, shiny emergency blankets to keep accident survivors warm
  • A light, shiny outer surface emits a lot less radiation than a dark, matt (non-glossy) surface
  • This keeps the patient warm, as less infrared radiation is emitted than if an ordinary blanket had been used

Reflective Blanket Being Used to Keep a Patient Warm

The reflective surface of an emergency blanket reflects the infrared radiation emitted by the body back towards the patient, helping to keep them warm

• Other common examples of thermal radiation are:
  • Heating from sunlight 
  • Using an infrared thermometer to measure temperature
  • Using a thermal imaging camera 
  • Using night vision

 

• In real situations there is very rarely only one form of energy transfer
  • Usually all three happen at once

Thermal Energy Transfers in a Hot Drink

• In the diagram below a more complex - and more 'real' - version of the coffee cup is shown
• Thermal energy is transferred from hotter areas (the tea) to cooler areas (the cup, hands and air) by the processes of:
  • Conduction; by direct contact between the tea and the solid sides of the cup and also by direct contact from the cup to the surface it is sitting on 
  • Convection; from the surface of the coffee to the air directly above it
  • Radiation; from the sides of the hot cup in all directions to the surrounding air

Thermal Energy Transfers Occurring in a Hot Drink

Energy is transferred by conduction, convection and radiation from a hot mug of coffee

• Objects will always lose heat until they are in thermal equilibrium (same temperature) with their surroundings
  • For example, a mug of hot tea will cool down until it reaches room temperature
  • Eventually the room, tea and cup will all be at the same temperature

Insulation in the Home

• Insulating the loft of a house lowers its rate of cooling, meaning less energy is lost to the outside
• The insulation is often made from fibreglass (or glass fibre)
  • This is a reinforced plastic material composed of woven material with glass fibres laid across and held together
  • The air trapped between the fibres makes it a good insulator
  • Trapped pockets of air ensure that convection currents cannot be formed, therefore reducing energy transfer by convection

• It has a much lower thermal conductivity than the roof material
• Several layers of insulation make it very thick and therefore decrease the rate of cooling

Cavity Wall Insulation

 Less heat is lost from a building with the help of insulation (filled cavity in walls)