Science STD 7 Chapter 3: Heat - Exercises
1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
| Similarities | Differences |
|---|---|
| Both are made of glass and use mercury in a bulb. | A clinical thermometer has a temperature range of 35°C to 42°C, while a laboratory thermometer has a range of -10°C to 110°C. |
| Both have Celsius scales to measure temperature. | A clinical thermometer has a kink near the bulb to prevent the mercury level from falling on its own. A laboratory thermometer does not have this kink. |
| Both are used for measuring temperature. | A clinical thermometer is used to measure human body temperature, whereas a laboratory thermometer is used to measure the temperature of other objects in a lab. |
2. Give two examples each of conductors and insulators of heat.
Conductors of heat: Iron, Aluminium, Copper, Silver.
Insulators of heat: Plastic, Wood, Air, Water.
3. Fill in the blanks:
- (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 Celsius.
- (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. Heat is transferred to its other end by the process of conduction.
- (f) Clothes of dark colours absorb more heat better than clothes of light colours.
4. Match the following:
| Column I | Column II |
|---|---|
| Land breeze blows during | (d) night |
| Sea breeze blows during | (c) day |
| Dark coloured clothes are preferred during | (b) winter |
| Light coloured clothes are preferred during | (a) summer |
5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Wearing more layers of thin clothing keeps us warmer than wearing one thick piece of clothing because a layer of air gets trapped between each layer of clothing. Air is a poor conductor of heat (an insulator). This trapped air prevents the heat from our body from flowing out into the cold surroundings, thus keeping us warm. A single thick piece of clothing traps less air compared to multiple layers.
6. Look at Fig. 3.13. Mark where the heat is being transferred by conduction, by convection and by radiation.
In the diagram showing a pan of water being heated on a stove:
Conduction: The heat is transferred from the flame to the metallic pan and from the bottom of the pan to the water. The handle also gets hot through conduction.
Convection: The heat is transferred through the water itself. The water at the bottom of the pan gets hot, rises up, and is replaced by cooler water from the top, creating a cycle.
Radiation: The heat from the flame is transferred to the surroundings, including the person standing nearby, and to the sides of the pan. This transfer does not require any medium.
7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
In places with a hot climate, it is advised to paint the outer walls of houses white because white or light-coloured surfaces reflect most of the heat that falls on them. In contrast, dark-coloured surfaces absorb more heat. By painting the walls white, less heat is absorbed by the house, keeping the inside cooler and more comfortable.
8. One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be
- 80°C
- more than 50°C but less than 80°C
- 20°C
- between 30°C and 50°C
9. An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will
- flow from iron ball to water.
- not flow from iron ball to water or from water to iron ball.
- flow from water to iron ball.
- increase the temperature of both.
10. A wooden spoon is dipped in a cup of ice cream. Its other end
- becomes cold by the process of conduction.
- becomes cold by the process of convection.
- becomes cold by the process of radiation.
- does not become cold.
11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that
- copper bottom makes the pan more durable.
- such pans appear colourful.
- copper is a better conductor of heat than the stainless steel.
- copper is easier to clean than the stainless steel.
Suggested Activities and Projects
1. Go to a doctor or your nearest health centre. Observe the doctor taking temperature of patients. Enquire: (a) why she dips the thermometer in a liquid before use. (b) why the thermometer is kept under the tongue. (c) whether the body temperature can be measured by keeping the thermometer at some place other than the mouth. (d) whether the temperature of different parts of the body is the same or different. You can add more questions which come to your mind.
Outline: This activity is designed for students to learn about the practical use of a clinical thermometer. They would observe a doctor's procedure and ask questions to understand the safety and accuracy of temperature measurement. For example, the thermometer is dipped in an antiseptic liquid to sterilise it. It is kept under the tongue for an accurate reading of core body temperature. Students would also learn that a person’s body temperature can vary slightly in different parts of the body.
2. Go to a veterinary doctor (a doctor who treats animals). Discuss and find out the normal temperature of domestic animals and birds.
Outline: This project involves visiting a veterinary doctor to learn about animal body temperatures. Students would find that the normal body temperature of different animals varies from humans. For example, a dog's normal temperature is typically higher than a human's. They would also learn about how a vet measures an animal's temperature, as a clinical thermometer is not always suitable.
3. Wrap a thin paper strip tightly around an iron rod. Try to burn the paper with candle while rotating the iron rod continuously. Does it burn? Explain your observation.
Outline: In this activity, the paper will not burn easily. This is because the iron rod, being a good conductor of heat, absorbs the heat from the candle flame and transfers it away from the paper. The paper does not reach its ignition temperature because the heat is continuously being dissipated into the metal rod, preventing it from catching fire.
4. Take a sheet of paper. Draw a spiral on it as shown in the Fig. 3.14. Cut out the paper along the line. Suspend the paper as shown in Fig. 3.14 above a lighted candle. Observe what happens. Think of an explanation.
Outline: When the spiral paper is suspended over a lighted candle, it will start to spin. This happens because the air directly above the candle flame gets heated, becomes less dense, and rises. This creates a continuous upward current of hot air, which pushes the paper spiral and makes it rotate. This demonstrates the transfer of heat by convection in the air.
5. Take two similar transparent glass bottles having wide mouths. Put a few crystals of potassium permanganate or pour a few drops of ink in one bottle. Fill this bottle with hot water. Fill the other bottle with cold water. Cover the cold water bottle with a thick piece of paper such as a postcard. Press the postcard firmly with one hand and hold the bottle with the other hand. Invert the bottle and place it on top of the hot water bottle. Hold both the bottles firmly. Ask some other person to pull the postcard. Observe what happens. Explain.
Outline: When the postcard is removed, the hot water with the coloured liquid will rise into the bottle containing the cold water, and the cold water will sink into the bottom bottle. The liquids mix because the hot water is less dense than the cold water, causing it to float upwards. This experiment visually demonstrates the concept of convection in liquids.