Thermal energy and its transferIB MYP Sciences: Revision notes

Temperature tells us how hot something is. It is measured in degrees Celsius (°C). Thermal energy is the total energy stored in the moving and vibrating particles of an object. It is measured in joules (J).

A bath of warm water has a lower temperature than a cup of boiling tea, but it stores far more thermal energy because it has much more water. Heating is the transfer of thermal energy from a hotter place to a colder place.

Conduction is the transfer of thermal energy through a solid. The particles at the hot end vibrate more and pass the vibrations to their neighbours.

Metals are the best conductors, because they also have free electrons that carry energy quickly. Plastic, wood and air are poor conductors, so they are insulators.

Convection is the transfer of thermal energy in liquids and gases by the movement of the fluid itself.

1. The fluid is heated, so it expands and becomes less dense.
2. The warm fluid rises.
3. Cooler, denser fluid sinks to take its place.
4. This creates a circular convection current.

Convection cannot happen in solids, because the particles cannot move from place to place.

Radiation is the transfer of energy by infrared waves. It does not need particles, so it can travel through a vacuum. This is how the Sun warms the Earth.

All objects emit infrared radiation, and hotter objects emit more. Surface matters too:

• Matt black surfaces are the best emitters and absorbers
• Shiny, light-coloured surfaces are poor emitters and absorbers, and good reflectors

That is why solar water heater panels are painted black and a shiny blanket keeps a runner warm.

A vacuum flask keeps drinks hot by stopping all three methods:

• The vacuum between the double walls stops conduction and convection
• The shiny silvered walls reflect infrared radiation back
• The plastic stopper is a poor conductor and traps air to stop convection

Home insulation works in a similar way: loft insulation traps air, which is a poor conductor, and reduces convection. Thick insulating material reduces the rate at which a hot water tank loses energy.

The specific heat capacity (c) of a material is the energy needed to raise the temperature of 1 kg of it by 1 °C. For water, c = 4200 J/kg °C.

Q = m × c × ΔT

Q is the energy in joules, m is the mass in kg and ΔT is the temperature change in °C.

Worked example: heating 0.50 kg of water from 20 °C to 60 °C. ΔT = 40 °C, so Q = 0.50 × 4200 × 40 = 84 000 J.

In an investigation you change one factor (the independent variable, such as the colour of a can), measure another (the dependent variable, such as the fall in temperature) and keep everything else the same (the control variables: mass of water, starting temperature, time, room).

To process the data, work out the temperature change and compare. To evaluate, comment on reliability (repeat and take a mean), sources of error (energy lost by evaporation and convection from open tops, so use lids) and whether the results support your hypothesis. Take care with hot water: use heatproof mats and do not carry it near others.