Convection

Describe and explain an experiment to demonstrate convection currents in gas.

1) A box with two chimneys is used. 2) A smouldering paper is placed above one of the chimneys. The smoke from the smouldering paper is used to demonstrate the movement of the convection current. 3) A burning candle is placed below the other chimney. The burning candle acts as a source of heat to create convection current. 4) When the air in the box is heated, its density decreases and rises up to the chimney together with the smoke above the candle. 5) The cooler surrounding air is denser at the other chimney, so it sinks into the box to replace the less dense air. This causes the smoke produced from the smouldering paper to flow downwards together.

Decribe how a convection is created.

1) In region X, fluid is heated. The particles in this region move further apart when they gain heat energy, so the volume increases. As a result, the density of the fluid at this particular region becomes lower and hence, it rises up. When the fluid rises up, they carry the heat energy with it. 2) When the hotter fluid reaches region Y, the partivles lose heat energy and becomes closer together, so the volume decreases. The density of the cold fluid becomes higher again, so it sinks. 3) The partcles in the fluid are then heated again when they reach region X and the cycle repeats. This creates the convection current in the fluid. Remark: Density is the ratio of mass/volume.

What causes a convection current in a fluid?

It is created due to change in density in different parts of the fluid.

Define convection.

It is the heat transfer in a fluid by the movement of particles in it. Remark A fluid refers to a substane which is able to flow easily. Hence, the substance is generally a liquid or gas.

Describe how convection affects sea breeze and land breeze.

Sea breeze: During the day, land heats up faster than the sea. Air above the land is warmer, so it rises up. Air above the sea is cooler, so it flows towards the lamnd to replace air that has been risen up. The air that flows towards the land from the sea is called the sea breeze. Land breeze: At night, the sea loses heat slower than the land. The air above the sea is warmer, so it rises up. The cooler air abovet the land flows towards the sea to replace air that has risen up. The air that flows towards the sea from the land is called the land breeze.

Explain why convection can only take place in fluid but not in solids.

The particles in a fuid (liquid or gas) are able to move about in the medium to transfer heat energy from one region to another through convection. However, the particles in solids are held at fixed positions. They cannot move freely in the medium to transfer heat energy from ne region to another. Remark: Recall the knetic model of matte in describing solids, liquids and gases.

How does heat transfer by convection in a fluid?

Through a convection current caused by the 'hot fluid rises, cold fluid sinks' phenomenon.

How do we demonsterate convection currents in water?

We can use a dye to demonstrate the convection currents in water. When water is heated at one point, the convection currents can be seen by the circulation of the dye in the water.