Stellar convection

In this demonstration students see that if there is temperature difference between the bottom and top of a coloured liquid, the top surface moves. You can use it to introduce solar convection.

Equipment

• Electrical hotplate
• Flat bottomed aluminium food tray or pie tin
• 5 coins all of the same denomination
• 50 ml liquid soap (eg moisturising face wash)
• A few drops of red food colouring
• 500 ml of water
• Torch (or other white light source)

Preparation & safety

The liquid soap/shampoo will need to contain glycol stearate, glycol distearate, or glycerol stearate in order to make the convection cells visible. Moisturising products with a pearlescent appearance often contain one of these. Check ingredients on the bottle.

Be careful not to touch the hotplate when it is on. The liquid temperature should not exceed 50°C (check with a thermometer).

Procedure

1. With the hotplate off, place the five coins in a cross pattern on top of the plate.
2. Place the food tray on top of the coins.
3. Pour in cold water until the tray is half full.
4. Add 50 ml of liquid soap and a few drops of food colouring; mix well using a finger.
5. Switch the hotplate on to a low setting. Leave for a few minutes.
6. Shine a torch at an angle onto the tray to make it easier for the students to see the water rise and sink.

Discussion prompts

• Why does the surface of the water move?
• The Sun’s surface also moves. Why do you think that is?

Teaching notes

Students may talk about heat or energy rising. Emphasise that neither energy nor heat are substances. Convection is mechanical process that it is best described in terms fluids at a higher temperature expanding and floating, and then cooling and sinking. In this experiment it is driven by the hotspots created by the coins at the bottom of the tray.

The columns of rising and falling fluid are called convection cells. When we look down on the tray we see the top of the cells: the liquid appear as it rises to the surface, moves across the surface and then disappears at is sinks back below. The process is a repeating one so the water gets circulated continuously as long as there is a temperature difference between the bottom and top.

Link the demonstration to stellar convection by providing an image of the sun’s surface. The giant granules they can see are the top of very large convection cells formed by plasma rising upwards from the hot interior to the cooler surface.

Teaching notes

Students describe how convection cells are formed and why they are responsible for the grainy appearance of the Sun.

This experiment was safety-checked in March 2020.