NCERT 7th Class (CBSE) Science: Heat and Temperature

Question: Why are air conditioners fitted at higher level on the wall of the room?

Answer: Cold air is heavier than warm air, the cold air from the cooler or air conditioner being heavier moves downwards, whereas the hot air at the lower level moves upwards and gets cooled and comes down once again. thus the convection currents are set in the air and room gets cooler faster.

Question: Why is the freezer mostly located at the top in a refrigerator?

Answer: The air inside and around the freezer compartment gets cool and moves downwards and lesser cool air moves up to get cooled. Thus convection currents are set in the air and the lower section of the refrigerator also gets cooled faster.

Question: Explain briefly how oceanic currents take place.

Answer: The ocean water near the equator is heated by the sun to a much higher temperature than the water near the poles of the earth. This is because the sun rays fall almost normally in the equatorial region but slantingly in the polar region. The ocean water in the equatorial region expands and become lighter, but the water in the polar region remains cold and heavy. Therefore, convection currents of warm water flows on the surface of ocean from equator towards the poles. Below the surface of the ocean, currents of cold water flows from the poles towards the equator. These currents control the temperature of the ocean and are called oceanic currents.

Question: Give an experiment to show that wood is a bad conductor of heat.

Answer: Take a rod of uniform diameter half of which is made of brass and half of wood. Wrap a white paper round the junction. Now heat it uniformly with a flame at the junction of the two rods. The paper on the wooden portion will be found to be smeared while the paper on the brass remains unaffected.

This shows that wood is bad conductor of heat. It cannot take away heat from the paper which is therefore smeared. On the other hand, brass conducts away heat from the paper so quickly that the paper does not reach the burning point.

Question: Write an experiment to prove that heat radiations can travel through vacuum.

Answer: Take a flat bottomed flask. In the mouth of the flask fix rubber stopper having two holes. Pass a thermometer through one hole and a glass tube with stopper through the other hole.

Remove air from the flask, by connecting to a vacuum pump. Record the temperature from the thermometer. Place the flask in bright sunlight or in front of an electric heater. It is observed that the thermometer records a rise in temperature. Thus, the experiment proves that transference of heat by radiation requires no medium.

Question: Give an activity to show the process of convection of heat in a gas.

Answer: Material required: A glass box with two holes in it, a candle , some papers in order to burn.

Procedure:

1. Take a glass box with a hole on side and another hole at the top.
2. Place a burning candle in the box. Burning candle is used to create convection of heat in the air present in the box.
3. The heated air expands and becomes lighter than the surrounding air. If you place your hand above the hole, you can feel hot air rising upwards.
4. Air is heated by convection. In order to make convection of air visible to eye, produce some smoke by burning paper near the side hole of the box. You will smoke entering the box and moving towards candle and then rising upwards, finally going out of the hole at the top.

Conclusion:

Gases are heated up by convection. They are bad conductors of heat. Heat travels in them by convection.