K323 / 2027
Thermal processes overview

Topic 2 of 4

Convection in fluids

Convection transfers energy with the bulk movement of a liquid or gas. Natural convection develops when density differences set the fluid in motion.

A fluid can flow. Density is mass divided by volume. If a portion of fluid expands while its mass stays the same, its density decreases. Energy still transfers overall from hotter regions to cooler ones.

Follow a complete circulation

Heat a liquid locally near one side of the bottom of a vessel. Assume that the liquid expands when warmed over the temperature range used.

  1. Warming: the portion near the heater gains energy and expands.
  2. Density change: its mass is spread over a larger volume, so it becomes less dense than the surrounding cooler liquid.
  3. Rising: the surrounding denser fluid displaces the warmer, less dense portion upwards.
  4. Return flow: cooler liquid moves into the region left behind. The moving fluid carries energy and a circulation develops.

Heating one lower region can set up a circulation

This liquid expands when warmed over the temperature range shown. Blue arrows show bulk liquid motion; the orange arrow shows energy entering from the heater.

A complete convection loop above a heater at the lower leftA container holds one continuous body of liquid. A local heater transfers energy into the lower-left region. The warmer liquid expands and becomes less dense than the cooler surrounding liquid, which displaces it upwards. Blue arrows run upwards at the left, across the top, downwards at the cooler right and back along the bottom, making one connected circulation. The blue arrows represent fluid motion that transports energy. The orange arrow from the heater represents energy transfer into the liquid. The coloured regions and arrows are qualitative.Warmer liquidLower densityCooler liquidHigher densityMovingliquidLocal heater

The returning cooler liquid completes the circulation. Energy is carried with the moving liquid.

Warmer, less dense liquid rises near the heated side. Cooler, denser liquid returns towards the lower region, completing a circulation. The arrows represent fluid motion, not a separate substance called heat.

The warmed portion does not have to lose mass or become weightless. Its smaller density follows from greater volume for the same mass. A complete explanation connects the temperature change to expansion, density and movement.

Why the heater position matters

Worked comparison

Heating water at the bottom or at the top

Consider water initially at 20°C and warming through a range in which it expands.

  • Heating near the bottom: the newly warmed, less dense water is below denser water, so it rises and promotes circulation through the sample.
  • Heating near the top: the warmed, less dense water is already above the cooler, denser water. Much less natural circulation develops through the whole sample.

Lower regions can still gain energy through other transfers. The top-heating case does not show that energy can never travel downwards.

A room heater placed low can start a similar circulation in air: warmed air expands and rises, while cooler air moves into the lower region. The process redistributes energy with the air rather than sending every warmed particle directly to every part of the room.

Convection needs a fluid that can move

Particles in an ordinary solid remain near fixed positions, so they cannot form this fluid circulation through the material. A liquid or gas can move bodily from one region to another.

Fluid can also be moved by a fan or pump. This forced movement can carry energy even when natural circulation is weak. It should not be explained by pretending that every airflow is caused by warmer fluid rising.

"Heat rises" is not a complete explanation. Under the stated conditions, the warmer fluid rises because it is less dense than its surroundings. Conduction and radiation can transfer energy in other directions, and a fan can direct fluid flow.

Optional check A sample of water initially at 20 degrees C is heated gently only near its top. Assume ordinary expansion over the temperatures used. Why is natural convection through the whole sample much weaker than when it is heated near the bottom?
A sample of water initially at 20 degrees C is heated gently only near its top. Assume ordinary expansion over the temperatures used. Why is natural convection through the whole sample much weaker than when it is heated near the bottom?