Samacheer Class 7 Science - Heat and Temperature

Clinical thermometer is a device used to measure body temperature and has a small temperature range of 35°C to 42°C (or 94°F to 108°F). Heat is a form of energy that flows from a hotter object to a cooler object. The normal temperature of the human body is 37°C (or 98.6°F), which is maintained by various physiological processes. The boiling point of water is 100°C (or 212°F), which is the temperature at which water changes from liquid to gaseous state at standard atmospheric pressure. The melting point of water is 0°C (or 32°F), which is the temperature at which ice changes from solid to liquid state at standard atmospheric pressure. A kink is a special feature found in clinical thermometers that prevents the mercury from flowing back into the bulb when the thermometer is removed from the body, allowing for easy reading of the temperature.

Kodaikanal has greater temperature.
Temperature of srinagar (J &K) = -4°C
Temperature of = 3°C
Difference = -4°C + 3°C = 7°C
Srinagar is colder than that of kodaikanal.

It is wrong to use a clinical thermometer to measure the temperature of hot water. A clinical thermometer has a small temperature range of only 35°C to 42°C (or 94°F to 108°F), which is designed specifically to measure human body temperature. Hot water typically has a temperature much higher than 42°C, often ranging from 50°C to 100°C or more. If a clinical thermometer is used to measure the temperature of hot water, the glass bulb will crack or burst due to the excessive pressure created by the rapid expansion of mercury inside the thermometer. The mercury expands significantly when exposed to temperatures beyond its designed range, and the glass cannot withstand this pressure, resulting in breakage. Additionally, using a clinical thermometer for purposes other than measuring body temperature can damage the instrument and make it unusable. For measuring the temperature of hot water or other high-temperature substances, a laboratory thermometer with a wider temperature range (typically 0°C to 110°C or higher) should be used instead.

The kink in a clinical thermometer is a constriction or narrowing in the tube just above the bulb. Its primary use is to prevent the mercury from flowing back into the bulb when the thermometer is removed from the patient's mouth or body. When the thermometer is in contact with the body, the mercury expands and rises up the tube to indicate the temperature. Once the thermometer is taken out, the mercury would normally contract and flow back down into the bulb due to gravity and the decrease in temperature. However, the kink acts as a barrier that traps the mercury column in the tube, keeping it at the highest level it reached. This allows the person reading the thermometer to note the temperature conveniently without the mercury level dropping immediately, giving sufficient time to record the accurate body temperature reading.

We jerk a clinical thermometer before measuring body temperature to reset the mercury level below the normal body temperature range. When a clinical thermometer is used repeatedly, the mercury remains at the level of the previous reading because of the kink, which prevents it from flowing back into the bulb automatically. By jerking or shaking the thermometer with a quick downward motion, we apply force that pushes the mercury back down into the bulb, lowering the mercury level below 35°C or 95°F. This ensures that the thermometer is ready to measure a new temperature reading. If we do not jerk the thermometer and the previous reading was higher than the current body temperature, the mercury level would not rise further, and we would get an incorrect reading. Therefore, jerking the thermometer is an essential step to prepare it for an accurate measurement.

Mercury is used in thermometers because it has several ideal properties as a thermometric liquid. Mercury remains in liquid form over a wide range of temperatures, from approximately minus 39°C to 357°C, making it suitable for measuring a broad range of temperatures. A small change in temperature causes a noticeable change in the volume of mercury, allowing for accurate and sensitive temperature readings. Water cannot be used as a thermometric liquid for several important reasons. First, water freezes at 0°C and boils at 100°C, so it cannot be used to measure temperatures below 0°C or above 100°C, which severely limits its range. Second, water is transparent and colorless, making it difficult to see the level of water in the glass tube and read the temperature scale accurately. Third, water wets glass, meaning it adheres to the inner walls of the glass tube and does not form a clear, distinct meniscus. This causes the water to stick to the glass and move unevenly, making consistent and reliable readings impossible. Due to these significant limitations, water is unsuitable as a thermometric liquid, and mercury remains the preferred choice for most thermometers.

Yes, I agree with Ramani that Swathi's method was incorrect. A laboratory thermometer does not have a kink in its tube, unlike a clinical thermometer. The kink is essential for trapping the mercury at the highest level reached. Without a kink, when Swathi removes the laboratory thermometer from the hot water, the mercury will immediately begin to contract and flow back down into the bulb as the thermometer cools. This happens because the mercury is no longer in contact with the heat source and the temperature drops rapidly. As a result, the reading will decrease and become inaccurate by the time Swathi looks at it. To measure temperature correctly using a laboratory thermometer, the reading must be taken while the thermometer bulb is still immersed in the hot water and surrounded by the substance whose temperature is being measured. This ensures that the mercury column remains at the correct level corresponding to the actual temperature of the liquid or substance being tested.

Similarities between laboratory thermometer and the clinical thermometer:
Both clinical and laboratory thermometers have long, narrow and uniform glass tubes.
Bulbs contain mercury.
Both have Celsius scale.
Differences:
Laboratory thermometer:
Laboratory thermometer is generally scaled from -10°C to 11 CPC.
Mercury level falls on its own as no kink is present.
Temperature is read while keeping the thermometer in the source of temperature, e.g. a liquid or any other thing.
No need to give jerk to lower the mercury level.
It is used to take temperature in laboratory.
Clinical thermometer:
Clinical thermometer is scaled from 35°C to 42°C or from 94°F to 108°F.
Mercury level does not fall on its own, as there is a kink near the bulb to prevent the fall of mercury level.
Temperature can be read after removing the thermometer from armpit or mouth.
To lower the mercury level jerks are given.
It is used for taking the body temperature.

Let C represent the temperature in Celsius and F represent the temperature in Fahrenheit. The problem states that the temperature in Fahrenheit must be twice its value in Celsius, so we can write this relationship as F = 2C. We also know the standard formula for converting Celsius to Fahrenheit: F = (9/5)C + 32. Now we can substitute the first equation into the second equation: 2C = (9/5)C + 32. To solve for C, we first move the term with C to one side: 2C - (9/5)C = 32. Finding a common denominator, we get (10/5)C - (9/5)C = 32, which simplifies to (1/5)C = 32. Multiplying both sides by 5, we find that C = 160°C. Therefore, the temperature must be 160°C so that it is twice its value in Fahrenheit.

When the bottle gets cooled down, the balloon contracts and returns to its original deflated state. As the temperature within the bottle decreases, the air inside cools down and loses thermal energy. The air molecules slow down and move closer together, occupying less space and exerting less pressure on the balloon. This causes the balloon to shrink and contract inward. This experiment demonstrates an important principle about gases: when a gas is heated, it expands and increases in volume and pressure, and when it is cooled, it contracts and decreases in volume and pressure. This behavior of gases is fundamental to understanding thermal expansion and contraction in various applications.

A tyre gets burst in summer because of thermal expansion of the air inside it. During summer, the temperature of the air surrounding the vehicle increases significantly. The air inside the tyre absorbs this heat and gets heated up. As the air temperature rises, the air molecules move faster and spread out, causing the air to expand. This expansion increases the pressure inside the tyre. Since the tyre is a sealed container with a fixed volume, there is no space for the air to escape. The continuously increasing pressure inside the tyre builds up to dangerous levels that exceed the tyre's structural capacity. Eventually, the internal pressure becomes so great that it exceeds the strength of the rubber material, causing the tyre to burst or explode. This is why vehicle owners are advised to check and adjust tyre pressure in summer and to avoid overinflating tyres during hot weather.