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Hot and Cold
Why Do We Wear Different Clothes in Different Seasons?
- Winter: Woollen clothes keep us warm because they are made from animal fibers.
- Summer: Light-colored cotton clothes keep us cool because they are made from plant fibers.
Hot and Cold
- Observations: Some objects are hot (like tea), and some are cold (like ice).
- Touch Test: We often use touch to feel if something is hot or cold, but this can be unreliable.
Activity 3.1: Touch Test (click here)
- Take three containers labeled A, B, and C.
- Fill A with cold water, B with hot water, and C with a mix of both.
- Dip your left hand in A and right hand in B for 2-3 minutes.
- Then, dip both hands in C. Do they feel the same?
Conclusion: Our sense of touch is not always reliable for measuring temperature.
Measuring Temperature
- Thermometer: A tool to measure how hot or cold something is.
- Clinical Thermometer: Measures body temperature, usually used when someone has a fever.
Parts of a Clinical Thermometer
- Glass Tube: Long, narrow, and uniform.
- Bulb: Contains mercury, which rises when temperature increases.
- Scale: Shows temperature in degrees Celsius (°C), usually from 35°C to 42°C.
Activity 3.2: Reading a Thermometer
- Note the temperature difference between two bigger marks.
- Count the smaller divisions between these marks. Each small division equals 0.2°C if there are five divisions between the bigger marks.
- Wash the thermometer with an antiseptic solution.
- Shake it to bring mercury level below 35°C.
- Place the bulb under your tongue for one minute, then read the temperature.
- Normal body temperature is 37°C.
Precautions for Using a Clinical Thermometer
- Wash before and after use.
- Ensure mercury is below 35°C before use.
- Read with the mercury level at eye level.
- Handle carefully to avoid breaking.
- Do not hold by the bulb.
Activity 3.3: Measuring Friends’ Temperatures
- Measure and record the body temperature of at least 10 friends.
- Observe if everyone has a body temperature of 37°C.
- Note that body temperatures can slightly vary; 37°C is an average.
Important Points
- Range: Clinical thermometers are designed to measure between 35°C and 42°C.
- Caution: Do not use a clinical thermometer for objects other than the human body. Avoid exposure to sun or flames as it may break.
Laboratory Thermometer
Purpose and Range
- Used to measure the temperature of objects (not the human body).
- Range: -10°C to 110°C.
How to Use a Laboratory Thermometer
- Preparation:
- Look at the thermometer carefully to note its highest and lowest temperatures.
- Check how much a small division reads to ensure accurate reading.
- Measuring Temperature:
- Take some tap water in a beaker or mug.
- Dip the thermometer so the bulb is immersed in water but doesn’t touch the bottom or sides.
- Hold it vertically.
- Wait until the mercury thread stops moving.
- Note the reading when the mercury is steady.
Precautions
- Keep the thermometer upright.
- Make sure the bulb is surrounded by the substance being measured.
- Do not let the bulb touch the container’s surface.
Activity 3.4: Measuring Water Temperature
- Dip the thermometer in tap water.
- Compare readings with classmates to discuss any variations and possible reasons.
Activity 3.5: Observing Mercury Fall
- Dip the thermometer in hot water.
- Note the temperature when mercury is steady.
- Take the thermometer out and observe that mercury falls, meaning the temperature should be read while it’s in water.
Differences from Clinical Thermometer
- Clinical Thermometer: Has a kink to prevent mercury from falling when taken out of the mouth.
- Laboratory Thermometer: Mercury level falls immediately when removed from the substance.
Additional Information
Maximum-Minimum Thermometer
- Used for measuring daily maximum and minimum temperatures in weather reports.
Digital Thermometers
- Modern alternative to mercury thermometers.
- Safer as they do not contain toxic mercury.
Key Points to Remember
- Laboratory thermometers are not suitable for measuring body temperature.
- Always handle thermometers carefully and read them correctly to get accurate results.
Transfer of Heat
Heat Flow Basics
- Heat moves from a hotter object to a colder object.
- Example: A frying pan on a flame gets hot because heat transfers from the flame to the pan.
Activity 3.6: Conduction in Solids
- Materials Needed: Metal rod (aluminum or iron), wax pieces, stand or bricks.
- Steps:
- Fix wax pieces on the rod at equal distances.
- Clamp the rod and heat one end.
- Observe which wax pieces fall first.
- Conclusion: Heat travels from the hotter end to the colder end by conduction.
Conductors and Insulators
- Conductors: Materials that allow heat to pass through easily (e.g., aluminum, iron, copper).
- Insulators: Materials that do not allow heat to pass through easily (e.g., plastic, wood).
Activity 3.7: Testing Conductors and Insulators
- Materials Needed: Steel spoon, plastic scale, pencil, divider, hot water.
- Steps:
- Dip one end of each item in hot water.
- Wait and then touch the other end.
- Observation: Conductors will feel hot; insulators will not.
Convection in Liquids and Gases
Activity 3.8: Convection in Water
- Materials Needed: Round bottom flask or beaker, water, tripod, candle, potassium permanganate crystal.
- Steps:
- Fill the flask with water and heat it.
- Observe the movement of the colored water.
- Conclusion: Hot water rises and cold water sinks, creating a convection current.
Activity 3.9: Convection in Air
- Materials Needed: Candle.
- Steps:
- Light a candle and place one hand above it and one hand to the side.
- Observation: The hand above the flame feels hotter due to convection.
Sea Breeze and Land Breeze
- Day: Land heats up faster than water, causing cooler sea air to move towards the land (sea breeze).
- Night: Land cools down faster than water, causing cooler land air to move towards the sea (land breeze).
Radiation
- Heat transfer that does not require a medium.
- Examples:
- Sun’s heat reaching Earth.
- Feeling warmth from a room heater.
Why Use an Umbrella?
- To protect from direct heat radiation from the sun.
Key Points
- Conduction: Heat transfer in solids.
- Convection: Heat transfer in liquids and gases.
- Radiation: Heat transfer without any medium.
Kinds of Clothes We Wear in Summer and Winter
Summer vs. Winter Clothing
- Summer: We prefer light-coloured clothes.
- Winter: We usually wear dark-coloured clothes.
- Reason: Light and dark colours absorb and reflect heat differently.
Activity 3.10: Heat Absorption by Colours
- Materials Needed: Two identical tin cans, black and white paint, water.
- Steps:
- Paint one can black and the other white.
- Pour equal amounts of water into both cans.
- Leave them in the midday sun for an hour.
- Measure the temperature of the water in both cans.
- Observation: The water in the black can will be warmer because black absorbs more heat.
Activity 3.11: Heat Loss by Colours
- Materials Needed: The same two cans from Activity 3.10, hot water.
- Steps:
- Fill both cans with hot water at the same temperature.
- Leave them in a room or shade for 10–15 minutes.
- Measure the temperature of the water again.
- Observation: The water in the black can cools down slower because it retains heat better.
Conclusion from Activities
- Dark-Coloured Clothes: Absorb more heat, keeping us warm in winter.
- Light-Coloured Clothes: Reflect most heat, keeping us cool in summer.
Why Woollen Clothes Keep Us Warm
- Poor Conductor: Wool is a poor conductor of heat.
- Trapped Air: Air trapped in wool fibers prevents heat from escaping our bodies.
- Layering with Blankets: Two thin blankets are warmer than one thick blanket due to the air layer between them.
Do You Know?
- Buildings can be constructed to be less affected by outside temperatures.
- Hollow Bricks: Using hollow bricks in walls can trap air and insulate buildings effectively.
Chapter Summary:
- Our sense of touch is not always reliable for judging how hot an object is.
- Temperature measures how hot an object is.
- A thermometer measures temperature.
- A clinical thermometer measures body temperature, ranging from 35°C to 42°C.
- Laboratory thermometers are used for other purposes, ranging from -10°C to 110°C.
- The normal body temperature is 37°C.
- Heat flows from a hotter body to a cooler body.
- Heat flows in three ways: conduction, convection, and radiation.
- In solids, heat is transferred by conduction.
- In liquids and gases, heat is transferred by convection.
- Radiation transfers heat without any medium.
- Materials that allow heat to pass through easily are conductors.
- Materials that do not allow heat to pass through easily are insulators.
- Dark-coloured objects absorb more heat than light-coloured ones.
- Light-coloured clothes are more comfortable in summer.
- Woollen clothes keep us warm in winter because wool is a poor conductor of heat and traps air.
Keywords
| No. | Keyword | No. | Keyword |
|---|---|---|---|
| 1 | Celsius scale | 6 | Land breeze |
| 2 | Conduction | 7 | Radiation |
| 3 | Conductor | 8 | Sea breeze |
| 4 | Convection | 9 | Temperature |
| 5 | Insulator | 10 | Thermometer |
