Hot And Cold

We experience differences in temperature in different seasons. Woollen clothes keep us warm in winter because they are made from animal fibres that trap air and prevent heat loss. Light-coloured cotton clothes are preferred in summer because they are made from plant fibres and help us feel cool by reflecting heat.

We often rely on our sense of touch to determine if an object is hot or cold. Some objects are clearly hot (like tea), while others are cold (like ice cream). Some objects can be hotter or colder than others.

Table 4.1: Examples of hot and cold objects:

Object	Cold/Cool	Warm/Hot
Ice cream	✓
Spoon in a tea cup		✓
Fruit juice	✓
Handle of a frying pan (when cooking)		✓
Handle of a frying pan (at room temp)	✓

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Activity 4.1

This activity demonstrates that our sense of touch is **not a reliable** way to determine the exact hotness or coldness of an object. By placing one hand in cold water and the other in hot water, and then placing both hands into a container of lukewarm water, the hand previously in cold water will perceive the lukewarm water as hot, while the hand previously in hot water will perceive it as cold. This shows that touch can be deceiving and only gives a relative sense of temperature based on our recent experience.

To accurately measure the hotness or coldness of an object, we need a reliable measure. This reliable measure is called **temperature**. Temperature quantifies the degree of hotness or coldness.

Temperature is measured by a device called a **thermometer**.

Measuring Temperature

Thermometers are instruments used to measure temperature. Different types of thermometers are used for different purposes.

A **clinical thermometer** is specifically designed to measure human body temperature. It is a long, narrow glass tube with a bulb at one end containing mercury. Outside the bulb, a thin, shining thread of mercury is visible. The thermometer has a scale marked on it.

The scale commonly used in India is the **Celsius scale**, denoted by **°C**. A clinical thermometer typically has a limited range, from 35°C to 42°C, because the normal human body temperature falls within this range.

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Activity 4.2

Reading A Thermometer

Learning to read a clinical thermometer involves understanding its scale. First, note the value of the larger marks and the number of smaller divisions between them to determine the value of each small division. Before use, the thermometer should be washed (preferably with an antiseptic solution) and given jerks to bring the mercury level below 35°C. To measure body temperature, the bulb is placed under the tongue for about a minute. The reading is taken while holding the thermometer horizontally at eye level to avoid parallax error.

The normal temperature of a healthy human body is considered to be **37°C**. The temperature should always be stated along with the unit (°C).

Precautions for using a clinical thermometer:

• Wash before and after use (preferably with antiseptic solution).
• Ensure mercury level is below 35°C before use.
• Read at eye level.
• Handle carefully; glass can break.
• Do not hold by the bulb while reading.

Caution: Clinical thermometers should only be used for measuring human body temperature. They should not be used for other objects or placed near flames or in the sun, as their limited range and structure are not suitable, and they can break.

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Activity 4.3

Measuring the body temperature of several individuals using a clinical thermometer reveals that not everyone has a temperature of exactly 37°C. Individual temperatures may vary slightly, being a bit higher or lower than 37°C. The 'normal temperature' is an average value based on a large healthy population.

Table 4.2: Recording body temperatures (Example Structure):

Name	Temperature (°C)
Paheli	...
Boojho	...
Friend 1	...
...	...

Laboratory Thermometer

To measure the temperature of objects other than the human body, a different type of thermometer called a **laboratory thermometer** is used. Laboratory thermometers have a much wider range compared to clinical thermometers, typically from **–10°C to 110°C**.

Like a clinical thermometer, a laboratory thermometer also has a bulb with mercury and a scale. It's important to understand the value represented by each small division on the scale to read it correctly.

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Activity 4.4

Measuring the temperature of water using a laboratory thermometer involves immersing the bulb fully in the water. It is important that the bulb does not touch the bottom or sides of the container. The thermometer should be held vertically, and the reading is taken once the mercury thread becomes steady. Comparing readings among different students helps identify potential inconsistencies.

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Activity 4.5

Measuring the temperature of hot water with a laboratory thermometer and then taking the thermometer out demonstrates a key difference from a clinical thermometer. As soon as the laboratory thermometer is removed from the hot water, the mercury level starts to fall, indicating that the reading changes as the thermometer's environment changes. This means the temperature must be read **while the laboratory thermometer's bulb is still immersed** in the substance being measured.

This behaviour makes the laboratory thermometer inconvenient for measuring body temperature, as you would have to keep it in your mouth to read it. Clinical thermometers are designed with a **kink** (a slight bend or constriction) near the bulb. This kink prevents the mercury level from falling back down on its own when the thermometer is removed from the mouth, allowing the reading to be taken conveniently.

Note: Due to mercury's toxicity, digital thermometers are increasingly used as a safer alternative, as they do not contain mercury.

Transfer Of Heat

Heat is a form of energy that flows from a region of higher temperature to a region of lower temperature. When a frying pan is placed on a flame, heat flows from the hot flame to the colder pan. When the pan is removed, heat flows from the hot pan to the colder surroundings, causing the pan to cool down.

Heat transfer always occurs from a **hotter object to a colder object**. If two objects are at the same temperature, there is no net transfer of heat between them.

There are three main ways in which heat can be transferred:

1. Conduction: The process of heat transfer in **solids** where heat is transferred from the hotter end to the colder end of an object **without actual movement of the particles** of the material. Particles vibrate and pass energy to neighbouring particles.

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Activity 4.6

Heating one end of a metal rod with wax pieces fixed at equal distances along its length demonstrates conduction. The wax pieces near the heated end melt and fall first, followed by the next pieces, and so on. This shows that heat is conducted from the hotter end towards the colder end along the metal rod.

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Activity 4.7

Testing various materials (steel spoon, plastic scale, pencil, divider) by dipping one end in hot water shows that some materials (like steel, metal) allow heat to transfer easily to the other end (the end gets hot), while others (like plastic, wood) do not. Based on this, materials are classified:

• **Conductors of heat:** Materials that allow heat to pass through them easily (e.g., metals like aluminium, iron, copper).
• **Insulators of heat (Poor conductors):** Materials that do not allow heat to pass through them easily (e.g., plastic, wood). Insulators are used for handles of cooking utensils to prevent burns.

Table 4.3: Conduction of heat by different materials (Example):

Article	Material with which the article is made of	Does the other end get hot (Yes/No)
Steel spoon	Metal (Steel)	Yes
Plastic scale	Plastic	No
Pencil	Wood/Graphite	No (Wood is insulator, graphite is conductor, but less significant effect on this scale)
Divider	Metal (Steel)	Yes

2. Convection: The process of heat transfer in **liquids and gases** where heat is transferred by the **actual movement of the particles** of the medium. Hotter, less dense parts of the fluid rise, while colder, denser parts sink, creating convection currents.

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Activity 4.8

Heating water in a flask with a crystal of potassium permanganate at the bottom demonstrates convection in liquids. As the water near the heat source (candle) gets hot, it becomes lighter and rises, carrying the coloured water upwards. Colder, denser water from the sides sinks down to take its place, gets heated, and also rises. This creates a circulating current within the water, transferring heat throughout the liquid. The process continues until the entire water is heated.

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Activity 4.9

Holding one hand above a candle flame and another hand on the side demonstrates convection in air. The air directly above the flame gets heated, becomes lighter, and rises, creating a strong upward convection current. This causes the hand above the flame to feel much hotter. The air on the sides is not directly heated by convection from below and thus feels less hot, illustrating that heat transfer in air by convection results in upward movement of hot air.

Convection currents in air also explain phenomena like sea breeze and land breeze in coastal areas. During the day, land heats up faster than the sea, causing hot air over land to rise, and cooler air from the sea (sea breeze) to flow towards the land. At night, the land cools faster than the sea, causing cooler air from the land (land breeze) to flow towards the sea.

3. Radiation: The process of heat transfer that **does not require any medium**. Heat is transferred through electromagnetic waves. This process can occur even in a vacuum.

Example: Heat from the Sun reaches the Earth through radiation, as there is no air or other medium in most of the space between the Sun and Earth.

We receive heat from a room heater by radiation. Hot objects lose heat to their surroundings by radiation, and cooler objects receive heat by radiation from hotter objects. When radiant heat falls on an object, some is reflected, some absorbed, and some transmitted. The absorbed heat increases the object's temperature.

Kinds Of Clothes We Wear In Summer And Winter

The colour of clothing affects how much heat is absorbed from the surroundings, influencing comfort levels in different seasons.

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Activity 4.10

Comparing the temperature rise in water in two identical tin cans, one painted black and the other white, when placed in sunlight for an hour. The water in the **black can** will be significantly **warmer** than the water in the white can. This shows that **dark-coloured surfaces absorb more heat** from radiation (like sunlight) than light-coloured surfaces.

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Activity 4.11

Comparing the rate of cooling of hot water in the same black and white painted cans placed in a room or shade. The water in the **black can** will cool down **faster** than the water in the white can. This shows that dark surfaces also radiate heat more effectively than light surfaces, meaning they lose heat faster to cooler surroundings.

These activities explain why we prefer:

• **Dark-coloured clothes in winter:** Dark surfaces absorb more heat from the sun (radiation), and also from our body (though less significant than absorption from external sources in cold), helping us feel warmer.
• **Light-coloured clothes in summer:** Light surfaces reflect most of the heat from the sun (radiation), absorbing very little, thus helping us feel cooler.

Woollen Clothes and Warmth:

**Wool** is an **insulator** (poor conductor) of heat. Woollen fibres trap a large amount of **air** in between them. Air is also a poor conductor of heat. This trapped air pocket acts as an insulating layer, preventing the flow of heat from our body to the cold surroundings in winter. This reduces heat loss from our body, making us feel warm.

Wearing multiple thin layers of clothing in winter often keeps us warmer than one thick layer because the air trapped between the layers provides additional insulation, further reducing heat loss by convection and conduction.

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