Heat

Q38. When we come out in the sun, we feel warm. How does the heat from the sun reach us?

Ans. It cannot reach us by conduction or convection as there is no medium such as air in most part of the space between the earth and the sun. From the sun the heat comes to us by another process known as radiation. The transfer of heat by radiation does not require any medium.

Q39. In summer we prefer light-coloured clothes and in winter we usually wear dark-coloured clothes. Why is it so?

Ans. Dark surfaces absorb more heat and, therefore, we feel comfortable with dark coloured clothes in the winter. Light coloured clothes reflect most of the heat that falls on them and, therefore, we feel more comfortable wearing them in the summer.

Q40. What is sea breeze?

Ans. During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze.

Q41. Two thin blankets joined together are usually warmer than one thick blanket. Give reason.

Ans. Two thin blankets joined together are usually warmer than one thick blanket because air gets trapped between the layers of blankets and being a bad conductor of heat, prevents the flow of heat from our body to the cold surroundings.

Q42. How water gets heated when kept on flame?

Ans. When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

Q43. An iron ball at 40°C is dropped in a mug containing water at 40°C.

The heat will

(a) flow from iron ball to water.

(b) not flow from iron ball to water or from water to iron ball.

(c) flow from water to iron ball.

(d) increase the temperature of both.

Ans. (b) not flow from iron ball to water or from water to iron ball.

Q44. A wooden spoon is dipped in a cup of ice cream. Its other end

(a) becomes cold by the process of conduction.

(b) becomes cold by the process of convection.

(c) becomes cold by the process of radiation.

(d) does not become cold.

Ans. (d) does not become cold.

Q45. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that

(a) copper bottom makes the pan more durable.

(b) such pans appear colourful.

(c) copper is a better conductor of heat than the stainless steel.

(d) copper is easier to clean than the stainless steel.

Ans. (c) copper is a better conductor of heat than the stainless steel.

Q46. State the similarities and differences between the laboratory thermometer and the clinical thermometer.

Ans. Similarities

                          i.        Both thermometers consist of a long, narrow, uniform glass tube.

                         ii.        Both have a bulb at one end. This bulb contains mercury.

                        iii.        Both have celsius scale.

Differences

                          i.        A clinical thermometer reads temperature from 35°C to 42°C whereas the range of a laboratory thermometer is generally from –10°C to 110°C.

                         ii.        A clinical thermometer has a kink in it whereas there is no kink in laboratory thermometer.

Q47. What are the precautions that need to be observed while reading a clinical thermometer?

Ans. Precautions to be observed while reading a clinical thermometer

                          i.        Thermometer should be washed before and after use, preferably with an antiseptic solution.

                         ii.        Ensure that before use the mercury level is below 35°C.

                        iii.        Read the thermometer keeping the level of mercury along the line of sight.

                        iv.        Handle the thermometer with care. If it hits against some hard object, it can break.

                         v.        Don’t hold the thermometer by the bulb while reading it.

Q48. State the precaution to be observed while using a laboratory thermometer.

Ans. Precaution to be observed while using a laboratory thermometer

                           i.        Handle the thermometer with care. If it hits against some hard object, it can break.

                          ii.        Thermometer should be kept upright not tilted.

                         iii.        Bulb should be surrounded from all sides by the substance of which the temperature is to be measured. The bulb should not touch the surface of the container.

                         iv.        Do not move the thermometer while measuring the temperature of the substance.

                          v.        Read the temperature of the object when the thermometer is in the substance.

Q49. Why can't we use a laboratory thermometer to measure human body temperature?

Ans. We can’t use a laboratory thermometer to measure human body temperature because the range of a laboratory thermometer is high generally from –10°C to 110°C while the normal body temperature of human body is 37°C. Moreover, a laboratory thermometer does not have a kink, so the mercury falls on its own upon removing it from the body orifice. Thus, it does not give accurate temperature of the human body.