Science Class 7 : Heat and its Transfer

             
                                 Heat and Its Transfer



 Q1 You may have noticed that a few sharp jerks are given to clinical thermometer before using it. Why is it done so?

Ans. The jerk to the thermometer will allow the mercury in or above the kink to flow into the bulb so that the mercury level falls below 35 degree Celsius

Q2 Why is it advised not to hold the thermometer by its bulb while reading it?

Ans. If we hold a thermometer by its bulb, the mercury level will change to show the temperature of our hand and the desired reading will not be observed.

Q3 At a camp site, there are tents of two shades one made with black fabric and the other with white fabric. Which one will you prefer for resting on a hot summer afternoon? Give reason for your choice. Would you like to prefer the same tent during winter?

Ans. On a hot summer afternoon, the tent made up of white fabric, will be preferred as white colour is a bad absorber and good reflector of heat which will keep the tent cool.

No, the black fabric tent will be preferred during winter because black colour is a good Absorber of heat and will keep the tent warm.

Q4. To keep her soup warm Tina wrapped the container in which it was kept with a woollen cloth. Can she apply the same method to keep a glass of cold drink cool? Give reason for your answer.

Ans. Yes, she can, because the air trapped in the layers of wool is a poor conductor of heat which prevents the heat from surroundings to get absorbed by the glass of cold drink. This keeps the glass of cold drink cool.

Q5 In a mercury thermometer, the level of mercury rises when its bulb comes in contact with a hot object. What is the reason for this rise in the level of mercury?

Ans.  Mercury expands when heated. Hence, it rises in the capillary tube.

Q6 State similarities and differences between the laboratory thermometer and the clinical temperature.

Ans.  Similarities :

  a. both made up of uniform glass tube.

  b. both contain mercury

  c. both have bulb at one end.

  d. both generally have Celsius scale.

  Class 7 Heat and it Transfer

Differences :

A. Range of laboratory thermometer is from 10 degree Celsius to 110 deg Celsius while range of clinical thermometer is from 35 degree Celsius to 42 degree Celsius

B. Laboratory thermometer is used to take the reading of temperature in laboratory while clinical thermometer is used to measure the temperature of human body.

C. Mercury level falls when removed from the source in case of laboratory thermometer while in case of clinical thermometer.

Q7 In place of hot climate, it is advised that the outer walls of houses f painted white. Explain.

Ans. In places of hot climate it is advised that the outer walls of houses be painted white because white color is absorb least heat. It reflects most of the heat and hence keeps the house cooler.

Q8 An iron ball at 40 deg Celsius is dropped in a mug containing water at 40 deg Celsius 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.