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- Recognise convection as the main method of heat transfer in liquids and gases.
Convection occurs when particles with more heat energy move and take the place of particles with less heat energy. Because this process requires the movement of particles, it can only occur in liquids or gases.
It is more efficient than conduction (heat moves through the substance faster) and requires less energy than radiation (radiation is discussed in the next topic – Unit P6.3), so most of the heat is transferred by convection in liquids and gases.
- Relate convection in fluids to density changes.
As a liquid or gas gains heat energy, its particles move faster, causing the space between them to increase. Therefore, the density of the liquid or gas also decreases.
Remember that density=mass/volume?
Well, that means that the less dense parts of the liquid or gas (as in, the parts with more heat energy) will have a lower mass per unit volume, making them lighter, and causing them to rise to the top while the colder parts sink.
If the source of the heat energy is at the bottom of the container, like when you boil water on a stove, then the colder part that sank to the bottom will soon become the hotter part, and that will rise, and so on.
This process is convection.
- Describe experiments to illustrate convection in liquids and gases.
There’s actually a video on YouTube that gives a brilliant explanation of the experiment that demonstrates convection in liquids (here’s the link – https://www.youtube.com/watch?v=xPl3foajmcw) but for the lazy butts here who can’t be bothered to go and watch it, or if you simply don’t have the time, I’ll write it down here.
There are four things you need: Blue food colour (to represent the cold water), red food colour (representing the warmer water), water and a large transparent container.
Add blue food colour to some water, and freeze it in an ice cube tray to create blue ice cubes.
Fill the large transparent container with water.
Add the blue ice cubes to one side, and add a decent amount of red food colour (2 – 3 teaspoons should do the trick) to the other side.
You’ll notice that the blue water almost immediately sinks to the bottom of the tub, and the red water mostly stays at the top. You’ll also notice that the red water moves towards the blue water and the blue water spreads to the red. What you’re seeing is convection.
In order to demonstrate convection in gases, we have the convection chimney experiment:
Here’s a link to a video if you want to see it in action – https://www.youtube.com/watch?v=6LztGrDrtpE
You’ll need a box in with a glass front, two glass chimneys, a candle, a lighter or matches to light the candle, and smoke matches or an incense stick or anything that gives off a good amount of smoke.
Set up the box and chimneys as shown in the picture. Light a candle and place it under one of the glass chimneys.
The air around the candle’s flame will be warmer than the rest of the air.
Light the smoke match/ incense stick and hold it above the other chimney. You’ll notice that instead of the smoke rising, which is what it usually does, it gets sucked down through the chimney and emerges from the other chimney.
This is because of convection. The air heated by the candle becomes less dense, and rises, leaving a vacuum in its wake. The colder air moves to replace it, causing the smoke to be sucked along with it.
Notes submitted by Sarah.
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