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 Tip
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!
