Saturday, May 19, 2012

Class 7 - Science - CH4 - Heat



NCERT Solution and Q & A


Q1: Define Heat.
Answer: Heat is a form of energy which produces a sensation of hotness or coldness. It is measured in Joules (J) or in Calories. Heat energy flows from one body to another due to temperature difference between the two. Heat is an internal energy of the substance due to molecular motion of the substance.



Q2: State similarities and differences between the laboratory thermometer and the clinical thermometer.
Answer:
Similarities:
  1. Construction and working principle are same.
  2. Both are used to measure temperature.
  3. Both have a thick walled glass tube enclosing a fine uniform bore capillary tube.
  4. Mercury is used in both thermometers to measure temperature.
  5. Both have Celsius and Fahrenheit markings.
Differences


Laboratory ThermometerClinical Thermometer
It is used to measure temperature of different objects in factories and laboratory.It is used to measure the temperature of human beings only.
Generally, it has temperature range from -10°C to 110°CIt has temperature range from 35° C to 42°C.
Usually it does not have any constriction.It has the small constriction near the mercury bulb.
Has to be kept upright while taking the readingCan be tilted while taking reading.

Q3:Give two examples each of conductors and insulators of heat. 

Answer
   Conductors: Aluminum, Iron 
    Insulators: Plastic, Wood. 

Q4: Fill in the blanks: 

Answer
(a) The hotness of an object is determined by its _temperature_. 

(b) Temperature of boiling water cannot be measured by a _Clinical_ thermometer. 

(c) Temperature is measured in degree _Celcius__. 

(d) No medium is required for transfer of heat by the process of _radiation_. 

(e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of _conduction_. 

(f ) Clothes of _dark_ colours absorb heat better than clothes of light colours.


Q5: Define temperature. What are the units used to measure temperature?

Answer: The degree of hotness or coldness is called temperature. It is measured in degree Celcius (°C), Fahrenheit and Kelvin (SI unit).


Q6: How are  heat and temperature of a substance related to each other?

Answer: Heat is the total energy of the molecular motion of the substance. Temperature is a measure of the average energy of the molecular motion in the substance. When you heat a substance, it increases the motion of the vibrating molecules of the substance thereby increasing the temperature of the substance.

Q7: Match the following:

Answer:
(i) Land breeze blows during  - (d) night
(ii) Sea breeze blows during  - (c) day
(iii) Dark coloured clothes are preferred during - (b) winter
(iv) Light coloured clothes are preferred during - (a) summer

Q8: Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

Answer: The air gets trapped between two layers of warm clothes.Air acts as insulator of heat. This layer prevents our body heat to escape in the surroundings. More layers of thin clothes will allow more air to get trapped and as a result we will not feel cold. So wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

Q9: Look at Figure. Mark where the heat is being transferred by conduction, by convection and by radiation

Answer:
  • Water in the pan gets heat through  Convection. Convection occurs in liquids and gases.
  • Heat is transferred by hot plate to pan by conduction. Conduction occurs in solids.
  • Surroundings is also receiving heat through Radiation by warm bodies (burner, pan, hot plate etc.). Radiation does not require any medium.

Q10: 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 light colour reflects most of the heat that falls on it.Or we can say it absorbs less amount of heat. Therefore, the temperature inside the house does not increase very much.



Q11: What is meant by 'Thermal Equilibrium'?

Answer: Heat flows from body of higher temperature to body of lower temperature until the temperature of both bodies are equal. Then the two bodies are said to be in thermal equilibrium.

Q12: One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be
(a) 80°C
(b) more than 50°C but less than 80°C
(c) 20°C
(d) between 30°C and 50°C

Answer: (d) between 30°C and 50°C


Q13: 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.

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

Q14: 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.

Answer: (d) does not become cold.

Q15: 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.

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



Q16: Which property of liquids is used in making thermometers?

Answer: The property of thermal expansion of liquids is used in making thermometers. It states liquid expands on heating and contracts on cooling.

Q17: What are the different temperature scales used? What are the relations among them?

Answer: Different temperature scales used are:
  1. Kelvin Scale (K) - SI Unit
  2. Degree Celsius (°C)
  3. Fahrenheit (F)
          K = 273 + °C  e.g. 7°C on Kelvin scale = 273 + 7 = 280K
          T =  (F-32) x 5 / 9  e.g. 98°F on Celsius scale = (98-32)x5/9 =36.67 °C
 
Q18: Name the temperature reading which is has same value on both  Celsius and Fahrenheit scales.

Answer: -40 °C


Q19: Name the temperature scale which does not have -ve temperature readings.

Answer: Kelvin scale.


Q20: Why mercury is chosen as thermometric liquid? State the reasons.

Answer:
  1. Mercury being liquid metal, is a good conductor of heat.
  2. It has uniform rate of thermal expansion.
  3. It does not wet the glass.
  4. Its appearance is like shining silver, hence it can be seen easily through glass tube.
  5. It quickly measures the temperature of the object absorbing very less amount of heat.
  6. It can measure good range of temperature because it has high boiling point (357 °C) and low melting point (-39°C).

Q21: Define 1 Calorie. How it is related to Joules?

Answer:  The amount of heat energy required to raise temperature of 1 gram of water by 1°C is called 1 calorie.
        1 Calorie = 4.2 Joules.


Q22: Why do you feel warm when you rub your palms and touch your cheeks with them?

Answer: While rubbing the hands, due to friction between them heat energy is produced. In this case mechanical energy is converted to heat energy. 

Q23: What is the human body temperature in Fahrenheit scale?

Answer: 98.6 °F

See also Chapter Heat MCQs

26 comments:

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