Heat is a form of energy that flows from hotter objects to colder ones. Our sense of touch can detect hot and cold, but it is not always reliable for measuring temperature accurately.
HOT AND COLD
- Some objects are hot, while others are cold. Touching them helps us discern the difference, but our sense of touch isn’t always reliable.
- In winter, it feels cold indoors but warm in the sun. In summer, it feels hot even inside.
Testing the Reliability of Touch
- Label three containers A, B, and C.
- Put cold water in A, hot water in B, and mixed water in C.
- Dip the left hand in A and the right hand in B for 2-3 minutes.
- Then, put both hands into container C. Both hands will feel differently, showing touch can be misleading.
MEASURING TEMPERATURE
- Temperature is a reliable measure of how hot an object is, measured using a thermometer.
- Clinical Thermometer: Measures body temperature (35°C to 42°C).
- Laboratory Thermometer: Measures other objects’ temperature (-10°C to 110°C).
Precautions for Using Thermometers
- Wash before and after use, preferably with antiseptic solution.
- Ensure mercury level is below 35°C before use.
- Read at eye level and handle with care to prevent breakage.
- Do not hold the thermometer by the bulb while reading.
Reading a Thermometer
- Note the temperature difference between two bigger marks and the number of divisions between them.
- If there are five divisions between marks reading one degree, each small division equals 0.2°C.
Human Body Temperature
- Normal human body temperature is 37°C, though individual temperatures may vary slightly.
- Maximum and minimum thermometers are used to measure daily temperature ranges in weather reports.
TRANSFER OF HEAT
- Heat flows from a hotter object to a colder one through conduction, convection, or radiation.
Conduction
- Heat transfer from the hotter end to the colder end of an object, usually in solids.
- Conductors (e.g., aluminum, iron) allow heat to pass easily, while insulators (e.g., plastic, wood) do not.
Convection
- Heat transfer in liquids and gases.
- Hotter parts rise, and colder parts move in to replace them, creating a cycle.
Radiation
- Heat transfer without a medium.
- Hot bodies radiate heat, which is absorbed, reflected, or transmitted by objects.
Examples
- Sea breeze: Hot air rises from land, and cooler air moves in from the sea during the day.
- Land breeze: Cool air moves from land to sea at night.
- Dark-colored clothes absorb more heat, while light-colored clothes reflect heat.
KINDS OF CLOTHES WE WEAR IN SUMMER AND WINTER
Summer
- Light-colored clothes are preferred as they reflect heat, keeping the wearer cool.
Winter
- Dark-colored clothes absorb more heat, keeping the wearer warm.
- Woolen clothes trap air between fibers, preventing body heat from escaping.
FAQ
- Question: Suppose you are given the choice in winter of using either one thick blanket or two thin blankets joined together. What would you choose and why?
- Answer: Remember that there would be a layer of air in between the blankets. Woollen clothes keep us warm in winter because wool is a poor conductor of heat, and air is trapped in between the wool fibres, preventing heat flow from our body to the cold surroundings.
- Question: Why are you advised to use an umbrella when you go out in the sun?
- Answer: All hot bodies radiate heat. When this heat falls on some object, a part of it is reflected, a part is absorbed and a part may be transmitted. The temperature of the object increases due to the absorbed part of the heat.
- Question: Why is it more comfortable to wear white or light-colored clothes in the summer and dark-colored clothes in the winter?
- Answer: Dark surfaces absorb more heat and, therefore, we feel comfortable with dark colored 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.
- Question: In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
- Answer: Light colored clothes reflect most of the heat that falls on them and, therefore, we feel more comfortable wearing them in the summer.
- Question: Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
- Answer: Woollen clothes keep us warm in winter In the winter, we use woollen clothes. Wool is a poor conductor of heat. Moreover, there is air trapped in between the wool fibres. This air prevents the flow of heat from our body to the cold surroundings. So, we feel warm.
- Question: One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be…?
- (d) between 30°C and 50°C
- Question: An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will…?
- (b) not flow from iron ball to water or from water to iron ball
- Question: A wooden spoon is dipped in a cup of ice cream. Its other end…?
- (d) does not become cold
- Question: Stainless steel pans are usually provided with copper bottoms. The reason for this could be that…?
- (c) copper is a better conductor of heat than the stainless steel
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