Thermal Convection (DP IB Physics)
Revision Note
Thermal Convection
Convection occurs when:
A fluid is heated causing the movement of groups of atoms or molecules due to variations in density
Convection is the main way that heat travels through liquids and gases
Convection cannot occur in solids
This is because the particles in a solid are unable to travel relative to one another
When a fluid (a liquid or a gas) is heated from below:
The heated molecules gain kinetic energy and push each other apart, increasing the space between particles, thus making the fluid expand
This makes the hot part of the fluid less dense than the surrounding fluid
The hot fluid rises because of this, and the cooler (surrounding) fluid moves in to take its place
Eventually, the hot fluid cools, contracts and sinks back down again
The resulting motion is called a convection current
How is thermal energy transferred during convection?
The thermal energy from a fire forms a convection current in the air around it. The hot air rises and the cool air sinks.
Worked Example
Discuss one example, in nature or in the lab, in which convection takes place.
Answer:
Step 1: Draw a diagram to illustrate the convection currents
Step 2: Describe the situation
One method of observing a convection current is by heating a beaker of water containing potassium permanganate crystals
Heat is initially transferred through the glass wall of the beaker by conduction
The water in the region of the Bunsen flame is heated and expands, becomes less dense and rises
This causes the dissolved purple crystal to flow up with the water
Meanwhile, when the water at the top of the beaker cools, it becomes denser again and falls
The process continues which leads to a convection current where heat is transferred through the liquid
The dissolved purple crystal follows this current and can be observed during this experiment
Other examples of convection:
Atmospheric convection / winds / sea breezes
Thunderheads (a cloud that appears before a thunderstorm)
Convection currents in the Earth's mantle (which can lead to continental drift)
Ocean currents
Solar conventions / sunspots / flares
Examiner Tips and Tricks
If a question refers to thermal energy transfers in a fluid (a liquid or gas) that isn’t trapped, then make sure your answer mentions that convection currents will likely form!
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