Convection

Convection is the transfer of thermal energy by the movement of a fluid (liquid or gas). It does not happen in solids because their particles cannot flow. Warm regions of a fluid move and carry energy with them.

Why convection happens

When a fluid is heated, its particles move faster and spread out. The fluid expands and becomes less dense. Density is mass per volume: $\rho = \frac{m}{V}$. Less dense, warmer fluid rises. Cooler fluid is denser, so it sinks. This sets up a continuous loop called a convection current.

Convection in liquids and gases

• Heat at the bottom: Fluid near the heat source warms, expands, and rises.
• Cooling at the top: As it rises away from the heat, it cools, becomes denser, and sinks.
• Circulation: This rising and sinking creates a convection current that transfers energy through the fluid.

Simple classroom demonstrations

• Dyed water: Place a small crystal of dye at one side of a beaker of water. Gently heat beneath the dye. You will see colored streaks rise on the hot side and sink on the cool side, showing the current.
• Smoke and candle: Hold a smoking splint or incense near the base of a lit candle. Smoke is drawn in at the bottom and rises in the warm air above the flame.
• Two chimneys: A box with two vertical tubes and a candle inside shows smoke entering the cool tube and leaving the warm tube—an airflow convection loop.

Everyday applications and effects

• Heating a room: A radiator or heater warms nearby air. Warm air rises, cool air moves in to replace it, setting up a convection current that spreads heat.
• Boiling soup or water: You can see bits of food circulate as convection currents form.
• Sea breeze: Daytime land heats air, which rises; cooler sea air moves in to replace it.
• Car radiator: Hot engine coolant transfers heat to air passing through the radiator; moving air carries the heat away by convection.

Common misconceptions

• “Heat rises.” In fact, warm fluid rises because it is less dense. Heat itself does not rise.
• Convection does not occur in solids, and it cannot occur in a vacuum (no particles to move).

Key points

• Convection moves thermal energy by bulk movement of fluids.
• Density changes drive convection currents: heating lowers density; cooling increases density.
• Seen in kettles, rooms, weather, and engines.