Week 6 · Lesson 4 Methods of Heat Transfer (B): convection currents, sea breeze, radiation, thermal imaging · 3 lessons
1
Warm rises, cool sinks: convection
Science Prep 2 Term 1, Unit 1: pp. 35–38
GoalExplain convection as heat carried by a moving fluid, and say why warm fluid rises and cool fluid sinks.
New words
convection · heat carried from place to place by a moving liquid or gasconvection current · the circular flow of a fluid caused by heatingdensity · how much mass there is in a given volume
Explanation
Convection is how heat moves in liquids and gases, never in solids. When part of a fluid is warmed, its particles move faster and spread farther apart. The warmed fluid expands, so the same volume has less mass: it is less dense. Less dense fluid rises. Cooler, denser fluid from the sides sinks to take its place. Then it is warmed and rises too. This circular flow is a convection current, and it carries heat with it. We should say that warm fluid rises, not that “heat rises”. A heater placed low in a room warms the air next to it; the warm air rises, spreads along the ceiling, cools and sinks again, so the whole room warms. Soaring birds such as eagles use convection: they circle in rising warm air without flapping, and the air lifts them. A heater on the ceiling would be much less effective, because the warm air would stay near the top.
Worked examples
A pan of water on a warm sunny windowsill: water near the sunny wall warms, rises and flows along the top, while cooler water sinks on the other side. A drop of food colour would show the loop.
Why is a heater placed near the floor, not near the ceiling? Warm air rises from the floor heater and the cooler air sinks to replace it, so the room mixes and warms. A ceiling heater would leave cool air below.
Is convection possible in a metal bar? No. A solid’s particles stay in place, so no current can form. Heat moves along the bar by conduction.
Class activity
Colour current, with a grown-up. Fill a clear box or big jar with cold tap water and let it stand. Fill a small bottle with warm (not hot) tap water coloured with a few drops of food colour, close it, and lower it gently to the bottom. Open it under water carefully or tip it slowly. Describe what the coloured water does. Do not use hot water or a flame.
Practice
1
In which kinds of matter can convection happen?
Only liquids and gases (fluids).
2
Why does warmed air rise?
It expands, becomes less dense than the cooler air around it, and is pushed up.
3
Put the steps in order: cool fluid sinks, fluid is warmed, warmed fluid rises, fluid cools at the top.
Fluid is warmed, warmed fluid rises, fluid cools at the top, cool fluid sinks.
4
A friend says “heat rises”. Make the statement more exact.
Warm fluid rises, because it is less dense, and carries heat with it.
