Know that convection is an important method of thermal energy transfer in liquids and gases

2.3.2 Convection

Learning Objective (AO1)

State that convection is an important method of thermal‑energy transfer in liquids and gases.

What is Convection?

Convection is the transfer of heat by the bulk movement of a fluid (liquid or gas). The moving fluid carries thermal energy from a hotter region to a cooler region.

Why Convection Occurs – The Density‑Buoyancy Connection

1. Heating a portion of a fluid raises its temperature.
2. The fluid expands, so its density ρ decreases.
3. The lower‑density, warmer fluid experiences an upward buoyancy force (Archimedes’ principle) and rises under gravity.
4. Cooler, denser fluid moves down to replace the rising warm fluid.
5. The continuous circulation is called a convection current.

Types of Convection

Key Features (AO1)

• Requires a fluid medium – does not occur in solids.
• Driven by density differences caused by temperature gradients (buoyancy).
• Can be natural or forced.

Everyday Examples (AO1)

• Boiling water in a kettle – hot water rises, cool water descends.
• Atmospheric circulation – warm air rises at the equator, cool air sinks at the poles.
• Heating a room with a radiator – natural convection spreads warmth.
• Ocean currents – surface water warmed by the Sun moves toward colder regions.
• Engine‑cooling systems – forced convection removes excess heat.

Factors Affecting the Rate of Convection (AO2)

Quantifying Convective Heat Transfer (AO2)

The rate of heat transfer by convection is given by

\( Q = h\,A\,\Delta T \)

• \(Q\) – heat transferred per unit time (W)
• \(h\) – convective heat‑transfer coefficient (W m⁻² K⁻¹)
• \(A\) – area of the surface in contact with the fluid (m²)
• \(\Delta T = T_{\text{surface}} - T_{\text{fluid}}\) (K)

Worked Example (AO2)

Problem: A vertical radiator has a surface area of 1.2 m² and a temperature of 55 °C. The surrounding air is 20 °C. For natural convection over a vertical plate, take \(h = 10\) W m⁻² K⁻¹. Calculate the heat loss \(Q\).

Solution:

1. Temperature difference: \(\Delta T = 55 - 20 = 35\) K.
2. Insert values: \[ Q = h A \Delta T = 10 \times 1.2 \times 35 = 420\;\text{W} \]
3. The radiator loses 420 W of heat to the room.

Practice Question (AO2)

A small fan blows air across a flat metal plate of area 0.5 m². The plate temperature is 40 °C, the air temperature is 25 °C, and the measured heat‑transfer coefficient is \(h = 30\) W m⁻² K⁻¹.

1. Calculate the rate of heat transfer \(Q\).
2. Re‑arrange the formula to solve for \(h\) and verify the given value using the measured \(Q = 225\) W.

Simple Classroom Experiment – Visualising Convection (AO1 + AO2)

1. Materials: clear 250 mL beaker, hot water (~70 °C), cold water (~20 °C), food‑colouring, thermometer, timer, safety goggles.
2. Procedure:
   1. Fill the beaker half‑full with cold water and place it on a heat‑proof mat.
   2. Heat the water from below using a hot‑plate until the temperature reaches about 60 °C (monitor with the thermometer).
   3. Add a single drop of food‑colouring at the centre of the surface and observe.
   4. Watch the coloured plume rise, spread, then sink as it cools – a visible convection current.
   5. Repeat the observation with a small desk fan positioned to blow across the surface (demonstrates forced convection).
3. Safety note: Handle hot water carefully, wear goggles, and never leave the hot‑plate unattended.
4. Observations to record: time for the plume to reach the top, shape of the current, effect of the fan on speed of motion.

Comparison with the Other Modes of Heat Transfer (AO1)

Why Convection Is Important in Everyday Life (AO1)

• Distributes heat in cooking appliances, ovens, and domestic heating systems.
• Drives weather patterns and oceanic currents, influencing climate.
• Engine and electronic cooling rely on forced convection to prevent overheating.
• Natural ventilation in buildings uses convection currents to improve indoor air quality.