Heat & Temperature

Temperature vs Heat

These two terms describe related but different things:

Temperature is a measure of the average kinetic energy of the particles in a substance. Measured in degrees Celsius (°C) or Kelvin (K). 0°C = 273 K.

Heat is thermal energy transferred from a hotter region to a cooler region. Measured in joules (J).

Key distinction: a large bucket of warm water contains more thermal energy than a small drop of boiling water, even though the drop is at a higher temperature.

Methods of Heat Transfer

There are three ways thermal energy moves from place to place.

Conduction

Transfer of heat through a solid by vibrating particles passing energy to neighbouring particles. In metals, free electrons also carry energy quickly through the material — this is why metals are much better conductors than non-metals.

Good conductors: copper, aluminium, steel Poor conductors (insulators): wood, plastic, glass, air, water

Convection

Transfer of heat through a fluid (liquid or gas) by bulk movement of the fluid. Solids cannot convect because their particles cannot move freely.

How a convection current forms:

1. Fluid near the heat source warms up
2. It expands and becomes less dense
3. Less dense warm fluid rises; denser cool fluid sinks to replace it
4. A circulating convection current is established

Examples: sea breezes (land heats faster than sea during the day), radiators heating a room, hot water in a kettle.

Radiation

Transfer of heat as infrared radiation — electromagnetic waves that require no medium. This is how the Sun heats Earth across the vacuum of space.

Surface properties matter:

• Dark, matt surfaces are good absorbers and good emitters of infrared
• Light, shiny surfaces are poor absorbers and poor emitters (they reflect more infrared)

This is why solar panels are painted black and why emergency blankets are silver.

Worked Example — Vacuum Flask

A vacuum flask keeps drinks hot or cold. Explain how each feature reduces heat transfer:

• Vacuum between double walls — eliminates conduction and convection (no particles to transfer energy)
• Silvered inner walls — reflect infrared radiation back into the flask, reducing radiative loss
• Plastic or cork stopper — poor conductor, reducing conductive heat loss through the top
• Plastic outer casing — poor conductor, reduces conduction to the surroundings

Thermal Equilibrium

When two objects are in thermal contact, heat flows from hotter to cooler until both reach the same temperature. At this point, net heat transfer = 0. This state is called thermal equilibrium.

Example: a cold spoon placed in hot soup gradually warms up and the soup cools slightly until both are at the same temperature.

The Zeroth Law of Thermodynamics states: if object A is in thermal equilibrium with object B, and B is in equilibrium with C, then A and C are also in thermal equilibrium.

Insulation in Buildings

Good insulation reduces unwanted heat transfer and saves energy:

Feature	Heat transfer reduced
Double glazing	Conduction and convection (trapped air gap)
Loft insulation (fibreglass)	Conduction and convection
Cavity wall insulation	Convection in the air gap
Reflective foil behind radiators	Radiation
Draught excluders	Convection

Specific Heat Capacity

Different materials need different amounts of energy to raise their temperature. Specific heat capacity (c) is the energy needed to raise 1 kg of a substance by 1°C.

Formula: Energy (J) = mass (kg) x specific heat capacity (J/kg°C) x temperature change (°C)

Water has a very high specific heat capacity (4200 J/kg°C), which is why it is used in central heating systems and why the sea warms and cools slowly.

Key Terms

• Temperature — measure of average kinetic energy of particles (°C or K)
• Heat — thermal energy transferred from hot to cold (J)
• Conduction — heat transfer through solids via vibrating particles and free electrons
• Convection — heat transfer through fluids via convection currents
• Radiation — heat transfer as infrared electromagnetic waves (no medium needed)
• Thermal equilibrium — state where two objects in contact reach the same temperature
• Insulator — material that slows heat transfer

Common Mistakes

• Saying "heat rises" — it is warm fluid that rises during convection, not heat itself
• Confusing heat (energy in transit, measured in J) with temperature (measured in °C)
• Thinking shiny surfaces are good absorbers — they are good reflectors and poor absorbers
• Forgetting that radiation does not need a medium — conduction and convection do

Tips and Tricks

• Remember the three methods as C-C-R: Conduction (solids), Convection (fluids), Radiation (no medium)
• Dark/matt = good absorber AND good emitter; shiny/light = poor absorber AND poor emitter
• When asked why metals feel cold even at room temperature, the answer is conduction — they conduct heat away from your hand quickly
• Convection currents always follow the same pattern: hot rises, cool sinks