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Hi!!,
If I have to measure the temperature of a hot cup of tea, I insert a thermometer in it. Initial temperature of the thermometer bulb is 00C. When it is in contact with the tea, essentially the temperature of the bulb will increase and of the tea will decrease to attain thermal equilibrium. This means that thermometer heats up a little and tea cools down a little. When they attain a thermal equilibrium, thermometer measures its own temperature(which is equal to the tea's temperature). The problem here is- to attain equilibrium, the tea has cooled down a little, so what we measure is not the actual temperature of the tea but the temperature when the tea is a little cooled off.
If my line of reasoning is correct, then it means we all measure wrong temperature.
Any views over this? (Consider an isolated system)

Jitender Pal , 11 Years ago
Grade 9
anser 1 Answers
Askiitians Tutor Team

Last Activity: 15 Days ago

You've raised an interesting point about measuring temperature and the concept of thermal equilibrium. Let's break this down to clarify how thermometers work and address your concerns about accuracy in temperature measurement.

Understanding Thermal Equilibrium

When you insert a thermometer into a hot cup of tea, the thermometer and the tea are initially at different temperatures. The thermometer bulb, starting at 0 °C, will absorb heat from the tea, while the tea will lose some heat to the thermometer. This process continues until both the thermometer and the tea reach the same temperature, known as thermal equilibrium.

Heat Transfer Dynamics

In an isolated system, heat transfer occurs from the hotter object (the tea) to the cooler object (the thermometer). Here’s how it works:

  • Heat Flow: Heat flows from the tea to the thermometer until they reach the same temperature.
  • Temperature Change: The thermometer's temperature increases, while the tea's temperature decreases slightly.
  • Equilibrium Point: At thermal equilibrium, both the thermometer and the tea will read the same temperature.

Addressing the Measurement Concern

Your concern is valid: the tea does cool down a bit while the thermometer heats up. However, the key point is that the temperature reading on the thermometer reflects the temperature of the tea at the moment they reach equilibrium. Although the tea has lost some heat, the reading is still a reliable representation of the tea's temperature at that point in time.

Practical Implications

In practical terms, the temperature change in the tea is usually minimal, especially if the thermometer is inserted quickly and the volume of tea is significant compared to the thermometer's mass. For example:

  • If you have a cup of tea at 90 °C and a thermometer that quickly equilibrates, the temperature drop might only be a fraction of a degree.
  • In most cases, this slight cooling is negligible for everyday purposes, such as enjoying a hot beverage.

Conclusion on Accuracy

While it's true that the tea cools slightly during the measurement, the thermometer's reading is still a close approximation of the tea's temperature at the moment of equilibrium. In scientific contexts, more sophisticated methods can be employed to minimize this effect, such as using a thermometer with a faster response time or employing non-contact temperature measurement techniques.

In summary, while there is a minor cooling effect, the temperature reading you get from the thermometer is still a valid representation of the tea's temperature at that moment. So, while your reasoning is sound, the practical implications of this effect are often negligible in everyday situations.

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