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

Question: The quantity of heat absorbed or given out by a substance during a thermal change depends on what factors?

Answer: The quantity of heat absorbed or given out by a substance during a thermal change depends on:

1. Mass: A larger mass of a substance absorbs or gives out more heat than a smaller mass of it for a certain change of temperature. So, if ‘H’ be the quantity of heat absorbed or given out by a substance of mass ‘m’ for a given change of temperature, H = m.
2. Temperature: The heat absorbed or released by a certain quantity of a given substance increases or decreases accordingly as the difference between the initial and final temperatures is large and small. Thus, H= (T-t) where, (T-t) is the difference between the initial and the final temperatures of the body.
3. Specific heat: This is a fundamental property of matter. An equal mass of different materials absorb or give out different quantities of heat, even if they are heated or cooled through the same range of temperature.This is also called specific heat capacity (abbreviated as SHC) in S.I. system.

Heat (H) absorbed or given out by body of mass (m) for rise or fall of temperature through t is given by H = mst.

Question: State similarities between the laboratory thermometer and the clinical thermometer

Answer: Similarities between the laboratory thermometer and the clinical thermometer are:

1. Laboratory thermometer and the clinical thermometer consist of a long, narrow, uniform glass tube.
2. Laboratory thermometer and the clinical thermometer have a bulb containing mercury at the end of the tube.
3. Laboratory thermometer and the clinical thermometer are marked with Celsius scale on the glass tube.

Question: Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

Answer: In between the layers of cloths there is trapped air. As air is bad conductor of heat so the out side low temperature do not get transferred to body as well as prevent our body heat to escape out side. Hence more layers of cloths keep us warmer during cold winter.

Question: In places of  hot climate it is advised that the outer walls of houses be painted white. Explain.

Answer: In places of  hot climate it is advised that the outer walls of houses be painted white because a light colour absorb very less radiant heat  and we feel comfortable inside such houses due to lower temperature inside house.

Question: Which two conditions must be satisfied for heat transfer by conduction?

Answer: Two conditions are:

1. The two objects should be in direct contact.
2. Temperature of the two objects should be different.

Question: What is ventilation?

Answer: The process by which impure and warm air inside a room is continuously replaced by fresh air from outside is called ventilation.

During ventilation, what is the direction of the convection current?

Answer: Warm air being lighter rises up and leaves the room through the ventilators. Fresh air from outside enters the room through the doors and windows.