Explain convection in liquids and gases in terms of density changes and describe experiments that illustrate convection.
What is Convection?
Convection is the transfer of heat by the bulk movement of a fluid (liquid or gas). The motion is driven by differences in density that arise when part of the fluid is heated or cooled.
How Density Changes Produce Motion
When a fluid is heated, its temperature rises and its density decreases. The warmer, less‑dense portion rises, while the cooler, denser portion sinks. This creates a circulating flow known as a convection current.
Mathematically, the change in density ρ with temperature T (for small temperature ranges) can be expressed as
\$\rho = \rho0 \bigl[1 - \beta (T - T0)\bigr]\$
where \$\rho0\$ is the density at a reference temperature \$T0\$ and \$\beta\$ is the coefficient of volumetric expansion.
Convection in Liquids
Liquids are generally incompressible, so density changes are almost entirely due to thermal expansion.
Typical examples: heating water in a pot, ocean currents, mantle convection inside the Earth.
Convection in Gases
Gases are compressible; both temperature and pressure affect density.
Typical examples: warm air rising from a heater, atmospheric circulation, smoke rising from a fire.
Key Features of Convection Currents
Feature
Explanation
Direction of flow
From hot (low‑density) region upward to cooler (high‑density) region.
Driving force
Buoyancy – the net upward force on a less‑dense parcel of fluid.
Closed loop
Rising warm fluid is replaced by sinking cool fluid, forming a continuous loop.
Experiments to Illustrate Convection
Experiment 1 – Heated Water in a Transparent Beaker
Fill a clear beaker with room‑temperature water and add a few drops of food colouring near the side.
Place a small electric heater at the bottom of the beaker and turn it on.
Observe the coloured water being drawn downwards toward the heater and then rising as it warms.
Explain: The heated water near the heater becomes less dense and rises, pulling cooler water downwards, creating a visible convection current.
Experiment 2 – Candle Flame and Air Currents
Light a candle on a stable surface in a draft‑free room.
Hold a thin piece of paper above the flame and gently move it sideways.
Notice the paper being pulled upward toward the flame.
Explain: The flame heats the surrounding air, reducing its density. The warm air rises, and cooler air from the sides moves in to replace it, producing a convection current that lifts the paper.
Experiment 3 – Colour‑Water Convection Tank
Set up a rectangular transparent tank filled with water.
Place a heating element at one end of the bottom and a cooling element (e.g., ice pack) at the opposite end.
Add a thin line of coloured dye vertically in the centre of the tank.
Watch the dye bend and form a circulating pattern as warm water rises on the heated side and cool water sinks on the cooled side.
Explain: The temperature gradient creates a density gradient, driving a steady convection roll across the tank.
Suggested diagram: Schematic of a convection cell showing warm fluid rising, cool fluid sinking, and the resulting circular flow.
Summary Checklist
Convection requires a fluid medium (liquid or gas).