The diagram shows a way of demonstrating a convection current in air. 

Explain why colder air flows downwards through the other 'chimney'.

Liquid nitrogen, at a temperature of $-196°c$, is stored in a wide-necked vacuum flask, as shown.

Explain the features of the design of this flask that help to keep the liquid nitrogen cold.

Liquid nitrogen, at a temperature of $-196°\mathrm{C}$, is stored in a wide-necked vacuum flask, as shown.

When hot drinks are stored in a vacuum flask, it is important to keep the stopper in the flask. Why is it less important to have a stopper in a flask that is being used to keep things cold$?$

Some water in a glass beaker is heated from below, as shown in the diagram.

Name the process by which thermal energy is transferred$:$

Through the glass.

Some water in a glass beaker is heated from below, as shown in the diagram.

Name the process by which thermal energy is transferred$:$

Throughout the water.

Some water in a glass beaker is heated from below, as shown in the diagram.

As thermal energy is supplied, the temperature of the water begins to rise. Although the supply of energy remains constant, eventually the temperature becomes steady at $80°C$. Suggest why this happens.