# Heat Energy: Conduction, Convection and Radiation

> The three ways heat moves, why metal feels cold, why hot air rises and how the Sun warms us across empty space.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/heat-transfer/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The camping trip
On their first camping trip, Ruby's family sat around the fire as the night got cold.

Ruby noticed three things. Her face felt hot from the fire even though she was a metre away. The metal spoon she left in the pot of soup got too hot to hold, but the wooden spoon did not. And sparks and smoke always floated **up**, never sideways.

Her mum, a science teacher, smiled. "You just found all three ways heat can travel."

**In short:** Heat is energy that flows from a hotter place to a colder one. It moves in three ways: conduction (through solids, particle to particle), convection (currents in liquids and gases) and radiation (infrared waves, which can even cross empty space).

## Heat and temperature

**Temperature** tells you how hot or cold something is. It is measured in degrees Celsius (°C) or kelvin (K).

**Heat** (thermal energy) is energy that flows **from a hotter object to a colder one**. It is measured in joules (J).

Heat always flows from **hot to cold**, never the other way by itself. It keeps flowing until both objects reach the same temperature. This is called **thermal equilibrium**.

### Key idea: What is happening inside?
All matter is made of tiny particles that are always moving. In a hotter object, the particles move or vibrate **faster**. Temperature is a measure of the average kinetic energy of the particles.

## 1. Conduction

**Conduction** is heat passing through a material **by particles bumping into their neighbours**. The particles themselves stay in place. They vibrate and pass the energy along.

- Happens best in **solids**, because the particles are packed closely together.
- **Metals** are the best conductors. They have free electrons that carry energy quickly.
- Poor conductors are called **insulators**: wood, plastic, wool, air, polystyrene.

That is why Ruby's metal spoon got hot and the wooden one did not.

### Watch out: Metal is not colder
On a cold morning, a metal bench feels colder than a wooden bench. They are actually the **same temperature**. Metal conducts heat away from your hand much faster, so it feels colder.

## 2. Convection

**Convection** is heat carried by the movement of a **liquid or gas** (a fluid). It cannot happen in solids.

1. The fluid near the heat source warms up.
2. Its particles spread apart, so it becomes **less dense**.
3. The warm, less dense fluid **rises**.
4. Cooler, denser fluid **sinks** to take its place.
5. This sets up a loop called a **convection current**.

**Examples**: smoke rising from a campfire, a heater warming a whole room, sea breezes, boiling water in a pot, hot air balloons.

### Exam tip: Where to put heaters and air conditioners
Heaters work best **low down** in a room, because hot air rises and spreads. Air conditioners work best **high up**, because cold air sinks.

## 3. Radiation

**Radiation** is heat travelling as **infrared waves**. It needs **no particles at all**, so it can travel through empty space. This is how the Sun's energy reaches Earth across 150 million km of vacuum.

All objects give out infrared radiation. Hotter objects give out more.

| Surface | Emitting (giving out) | Absorbing (taking in) | Reflecting |
|---|---|---|---|
| **Dull, dark (black)** | Best | Best | Worst |
| **Shiny, light (silver, white)** | Worst | Worst | Best |

So houses in hot places are often painted white, and emergency blankets are shiny silver.

## Reducing heat transfer

A **vacuum flask** (thermos) stops all three:

- **Vacuum** between the two walls: stops conduction and convection (no particles).
- **Silvered walls**: reflect radiation back.
- **Plastic stopper**: an insulator that stops conduction and convection out of the top.

Home insulation works in a similar way: ceiling batts trap air (a poor conductor) so it cannot move to form convection currents.

**Mind map**

```text
Heat transfer
  Conduction
    Particles pass on vibrations
    Best in solids
    Metals conduct well
    Insulators: wood, air
  Convection
    Liquids and gases only
    Hot rises, cold sinks
    Convection currents
  Radiation
    Infrared waves
    Works in a vacuum
    Black absorbs and emits
    Shiny reflects
  Stopping it
    Vacuum flask
    Trapped air
    Shiny surfaces
```

### Year 11: Specific heat capacity and latent heat
Different materials need different amounts of energy to warm up. The **specific heat capacity** $c$ is the energy needed to raise 1 kg by 1 °C:

$$Q = mc\Delta T$$

Water has a very high value ($c = 4200$ J kg⁻¹ °C⁻¹), which is why the sea warms up and cools down slowly.

When a substance **changes state**, it absorbs or releases energy without changing temperature. This is **latent heat**: $Q = mL$.

The rate of **conduction** through a slab depends on the material's thermal conductivity $k$, area $A$, thickness $d$ and temperature difference:

$$\frac{Q}{t} = \frac{kA\Delta T}{d}$$

### Year 12: Black body radiation
A perfect absorber and emitter is called a **black body**. Two laws describe its glow:

- **Wien's law**: the peak wavelength gets shorter as temperature rises. $\lambda_{\max} T = b$, with $b = 2.898 \times 10^{-3}$ m K. A hot star looks blue. A cooler star looks red.
- **Stefan-Boltzmann law**: the power emitted rises with the fourth power of temperature, $P = \sigma A T^4$. Double the temperature and the power goes up 16 times.

Explaining the shape of the black body curve forced Max Planck to suggest energy comes in packets, which started quantum physics.

## Quick summary

- Heat flows from hot to cold until temperatures are equal.
- **Conduction**: particles pass on vibrations. Best in solids, especially metals.
- **Convection**: hot fluid rises, cold fluid sinks, forming currents. Liquids and gases only.
- **Radiation**: infrared waves, which can cross a vacuum.
- Dark dull surfaces absorb and emit radiation best. Shiny surfaces reflect it.
- Insulators like trapped air reduce heat transfer.

## Quiz

Try these questions from the chapter quiz.

1. Heat travels through a metal spoon mainly by:
   - A. Conduction
   - B. Convection
   - C. Radiation
   - D. Evaporation
   - Answer: A. Conduction. Particles in the solid pass vibrations along, helped by free electrons.

2. How does the Sun's heat reach Earth?
   - A. Conduction
   - B. Convection
   - C. Radiation
   - D. All three equally
   - Answer: C. Radiation. Space is a vacuum. Only radiation can cross it.

3. Convection can take place in:
   - A. Solids only
   - B. Liquids and gases
   - C. A vacuum
   - D. Metals only
   - Answer: B. Liquids and gases. Convection needs a fluid that can flow.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/heat-transfer/#quiz