CONVECTION

Activity H

Keep ice wrapped in wire mesh at the bottom of water in a test tube. Heat the water at the surface as shown in the diagram.

Observation

Water at the surface becomes hot but ice dose not melt.

Can you explain, why?

 

Activity I

Wrap thin piece of paper around a rod made half of wood and half of copper. Hold it over a flame.

Observation

Paper burns only where it is in contact with wood. Justify

 

Convection is a process of heat transfer in which each part of the material comes to the source of heat, collects it share and moves away from the source of heat.

Convection occurs only in Fluids (i.e. liquids and gases).

 

Activity J

Take a wide beaker and fill it with water. Place it on a tripod stand. Carefully drop a large crystal of potassium permanganate in it so that it settles down at the bottom in the center. Heat the bottom of the beaker with a very small gas flame.

Observation

Colored water rises from the place where the heat is applied. When it reaches the top it spreads out and comes down along the sides of the beaker. Thus, as water is warmed up, it rises and its place is taken by cold water from the sides. This forms currents in water known as convection current. This will happen till the whole water gets heated.

Explanation

Water expands on heating. Its density decreases, becomes lighter and thus goes up. Cold water takes it place.

In this experiment we can replace potassium permanganate by saw-dust

 

Activity K

Observe convection current in the Air. You will see dry leaves on the ground lifting in the air and moving along the current. You will also be able to see lose soil also getting into the air currents. Movement of the clouds are along the currents.

When glass tube is placed over a lighted candle it soon extinguishes. This does not happen if the tube is provided with a central partition to facilitate convection current.