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Heat and its Measurement - Clinical and Laboratory Thermometers

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Heat is a form of energy that flows from a body at a higher temperature to a body at a lower temperature. Its S.I. unit is the Joule (JJ).

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Temperature is the reliable measure of the hotness or coldness of an object. It is measured using a device called a thermometer.

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The Clinical Thermometer is used to measure human body temperature. It has a range from 35∘C35^\circ\text{C} to 42∘C42^\circ\text{C}. It contains a 'kink' (constriction) near the bulb to prevent the mercury level from falling on its own.

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The Laboratory Thermometer is used for measuring temperatures in science labs. Its range is generally from −10∘C-10^\circ\text{C} to 110∘C110^\circ\text{C}. It does not have a kink, so readings must be taken while the thermometer is in contact with the substance.

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The normal temperature of the human body is 37∘C37^\circ\text{C} (98.6∘F98.6^\circ\text{F}).

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Digital thermometers are preferred nowadays as they do not contain toxic Mercury (HgHg).

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Transfer of heat occurs through three modes: Conduction (solids), Convection (liquids/gases), and Radiation (vacuum/no medium required).

📐Formulae

F=(95×C)+32F = \left( \frac{9}{5} \times C \right) + 32

C=59×(F−32)C = \frac{5}{9} \times (F - 32)

K=C+273.15K = C + 273.15

💡Examples

Problem 1:

A person has a high fever and their body temperature is recorded as 104∘F104^\circ\text{F}. What is this temperature in Celsius (∘C^\circ\text{C})?

Solution:

Using the formula C=59×(F−32)C = \frac{5}{9} \times (F - 32), substitute F=104F = 104: C=59×(104−32)C = \frac{5}{9} \times (104 - 32) C=59×72C = \frac{5}{9} \times 72 C=5×8C = 5 \times 8 C=40∘CC = 40^\circ\text{C}

Explanation:

To convert Fahrenheit to Celsius, we subtract 3232 from the Fahrenheit value and then multiply by the fraction 59\frac{5}{9}.

Problem 2:

Why is a Laboratory Thermometer not suitable for measuring human body temperature?

Solution:

A Laboratory Thermometer does not have a 'kink'. If used on a human, the mercury level would drop immediately upon removing it from the mouth, making it impossible to get an accurate reading. Also, its scale is usually too large (−10∘C-10^\circ\text{C} to 110∘C110^\circ\text{C}) for the small range of human body temperature.

Explanation:

The 'kink' in a clinical thermometer is essential because it breaks the mercury thread, preventing it from falling back into the bulb until the thermometer is jerked.