Class 7 Science Ch 04: Heat

    I. Multiple Choice Questions (MCQs).
    Q.1. The reliable measure for knowing the hotness or coldness of a body is known as
    a) thermometer
    b) heat
    c) temperature
    d) all of these
    Answer: (c) temperature
    Q.2. The temperature scale used for scientific purpose is
    a) Celsius scale
    b) Fahrenheit scale
    c) Kelvin scale
    d) all of these
    Answer: (c) Kelvin scale
    Q.3. The lower fixed point on Fahrenheit scale is
    a) 32 °F
    b) 0 °F
    c) 25 °F
    d) none of these

    Answer: (a) 32°F
    Q.4. The thermometer used to measure our body temperature is known as
    a) Clinical thermometer
    b) Doctor’s thermometer
    c) both (a) and (b)
    d) none of these
    Answer: c) both (a) and (b)
    Q.5. The fastest mode of transfer of heat is
    a) conduction
    b) convection
    c) radiation
    d) none of these
    Answer: (c) radiation
    II. Fill in the blanks by selecting appropriate words from the given list :
    List : Convection, Thermometer, Heat, kink, poor
    Q.1. ______________ flows from a hotter body to a cooler body.
    Q.2. ______________ is used to measure a temperature and it follows the principle
    of expansion.
    Q.3. The clinical thermometer has a ____________ in the stem just above the bulb.
    Q.4. Water and air are ______________ conductors of heat.
    Q.5. ______________ can take place only in liquids and gases, but not in a solid.

    Answer: 1-heat 2-thermometer 3- kink 4-poor 5-convection
    III. Match the items of Column A with those of Column B.

    Column A

               Column

    1. thermometer reads temperature from 35°C to
    42°C           

    (a) Laboratory thermometer 

    2. The range of thermometer lies
    between –10°C and 110°C

    (b)
    Insulator

    3. The material through which heat
    energy flows.

    (c) Clinical thermometer 

     

    4. The material through which heat
    energy does not flow.

    (d)
    Radiation

    5. A process of transfer of heat,
    which does not require any material medium.

    (e) Conductor

    Answer: 1-c, 2-a, 3-e, 4-b, 5-d
    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. Most thermometers use mercury as a thermometric liquid.

    Q.2. One should measure the temperature of boiling water with clinical thermometer.

    Q.3. The rate of conduction of heat differs from metal to metal.

    Q.4. In cold countries, double glass window panes are used leaving a gap filled with air.

    Q.5. Light-coloured objects absorb and radiate more heat than dark-coloured objects.
    Answer: 1-T, 2-F, 3-T, 4-T, 5-F
    V. Very Short Answer Questions.
    Q.1. Name the different kinds of temperature measuring scales.

    Answer: Celsius, Fahrenheit, or Kelvin temperature scale to measure the temperature.

    Q.2. What is the cause of heating or cooling of a body?

    Answer: Heating or cooling of a body is due to gain or Loss of heat to or from the body.

    Q.3. Give the relation between Celsius and Fahrenheit scales.

    Answer:

    Where, If C and F are the temperatures of a body, on the Celsius scale and Fahrenheit scale respectively.

    Q.4. What is use of kink in a Clinical thermometer?
    Answer: Kink in the Mercury thermometer helps to prevent the falling of mercury present in capillary tube into bulb. It ensures that the user takes the correct reading of temperature.
    Q.5. Why the metal surface of a teapot is made shiny?
    Answer: The metal surface of a teapot is shiny, so that tea is kept hot for a longer period.

    VI. Short Answer Questions.
    Q.1. Why is it must to jerk clinical thermometer before to be used?
    Answer: The clinical thermometer has a kink in the stem just above the bulb. When the thermometer is taken out of patient’s mouth, the liquid in the bulb contracts and the mercury column breaks at the kink. Therefore, the level of mercury of the stem remains the same. If the thermometer is to be used again, it is first given a jerk to bring the mercury in the stem back into the bulb.

    Q.2. Why is the base of cooking vessels painted black?
    Answer: The base of cooking vessels is painted black so that it absorbs more heat, and hence, cooking is fast.

    Q.3. Why does convection take place only in liquids and gases?
    Answer: Convection can take place only in liquids and gases, but not in a solid. This process takes place because the particles of a liquid and gas are free to move. The molecules of solids are held strongly due to strong molecular forces. These molecules cannot travel to the source of heat energy and therefore convection is not possible in solids.

    Q.4. What do you mean by heat and temperature? Explain them briefly.
    Answer: Heat is a form of energy which makes us to feel the sensation of coldness and hotness.
    The quantity by which we can compare the hotness or coldness of bodies is called temperature.

    Q.5. What are the different kinds of thermometers? How can you measure the temperature of tap water in a laboratory?
    Answer: Different kinds of thermometer are clinical thermometer and laboratory thermometer.

    To measure the temperature of tap water with a laboratory thermometer we need a beaker with tap water; a laboratory thermometer.

    Fill of the beaker with tap water. Dip the laboratory thermometer along with the bulb in the water. The bulb should not touch the bottom or side of the beaker: You will observe that the mercury in the thermometer tube starts rising. At a certain point it remains fixed, which gives the temperature of water of the tap.

    Q.6. What are the different modes of transfer of heat? Explain them briefly. Answer: There are three methods of heat transfer:
    (a) Conduction,
    (b) Convection, and
    (c) Radiation

    Conduction: The process of transmission of heat energy in solids without the actual movement of particles from their position is known as conduction.

    Convection: The process by which heat is transferred in liquids and gases by actual movement of molecules is called convection.

    Radiation: The transfer of heat energy from a hot body to a cold body directly, without heating space in between the two bodies is called radiation.
    VII. Long Answer Questions.
    Q.1. (a) What do you mean by conductor and insulator? Explain with suitable examples.
    (b) How will you show that the rate of conduction of heat in different materials is different?
    Answer: a) The materials which allow the heat energy to pass through them are called conductors of heat. For example, silver, copper, aluminium, iron, etc.

    The materials which do not allow heat to pass through them are called bad conductors of heat or insulators of heat. For example, plastic, wood, rubber, thermocol, leather, air, water, etc.

    b) To show that the rate of conduction of heat in different materials is different we need rods of equal Length and diameter of aluminium, copper, and iron, two wooden slabs, small metal balls, wax and Bunsen burners.

    Procedure :Take the rods of copper and aluminium, stick the metal balls to the rods at equal distances using wax. Arrange the rods as shown in the figure.

    Light the burner and make your observations. Repeat the experiment with other rods also. You will observe that the balls drop fastest for copper, then aluminium and slowest for iron.
    Thus we can say that the rate of conduction of heat differs from metal to metal.

    Q.2. (a) Write some practical applications of conductors and insulators.
    (b) How will you show the transfer of heat by convection in water?
    Answer: Practical Applications of Conductors

    a) Mercury is used as thermometric liquid as it is a good conductor of heat.

    b) Cooking vessels are made up of metals so that they can readily absorb heat energy and transfer it to food.

    c) Tip of soldering rod is made of copper as it conducts away heat readily to the solder.

    d) Cooling coil of air-conditioners and refrigerators are made of copper as they readily conduct away heat.

    Practical Applications of Insulators

    a) In cold countries, double glass window panes are used leaving a gap filled with air. The trapped air acts as an insulator and does not allow the heat from the room to escape. Thus, rooms keep warm.

    b) A newly made quilt is warmer than the old one because a large amount of air is trapped in the newly ginned cotton. As air is the bad conductor of heat, it does not allow the heat from our body to flow out, and hence we feel warmer.

    c) Slabs of ice are covered with sawdust or gunny bags to prevent them from melting. The sawdust or gunny bags contain a large amount of trapped air, which acts as an insulator and prevents the heat from outside to reach the ice.

    Q.3. What do you mean by land breeze and sea breeze? Explain them with the help of suitable diagrams.

    Answer: During the day, the land warms up more than the sea. The hot air rises upwards.
    To take its place, the cooler air from the sea moves towards the land. This convection current from the sea to the land is called sea breeze.

    After sunset, land loses heat faster than the sea water. So, the air over the sea is warmer at night. The warm air over the sea rises up, and the cooler air from the land starts moving towards the sea to take its place. The convection current from the land to the sea is called land breeze.

    Q.4. How do we get heat from the sun? Write some uses of absorbers and emitters of radiations.
    Answer: We get heat from the sun by radiation.
    Uses of absorbers of radiations
    (a) In the tropical countries, houses are painted in white. This is because white colour is a bad absorber of radiation and therefore, the temperature inside will be bearable. (b) The base of cooking vessels is painted black so that it absorbs more heat, and hence, cooking is fast.

    Uses of emitters of radiations
    (a) The metal surface of a teapot is shiny, so that tea is kept hot for a longer period. (b) A car radiator is painted in black so that heat is radiated away quickly in order to cool down the circulating water. The engine is, therefore, prevented from overheating.

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