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Heat and its Measurement - Concepts of Hot and Cold

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. It is measured in Joules (JJ) or Calories (calcal).

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Temperature is the degree of hotness or coldness of an object. It is a reliable measure of the thermal state of a body.

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The human sense of touch is subjective and unreliable for measuring temperature. For example, lukewarm water can feel 'hot' to a cold hand but 'cold' to a hot hand.

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A Thermometer is a device used to measure temperature. The two most common types are the Clinical Thermometer and the Laboratory Thermometer.

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The Clinical Thermometer is designed to measure human body temperature. It has a range of 35∘C35^\circ C to 42∘C42^\circ C. It contains a 'kink' (constriction) near the bulb to prevent the mercury level from falling immediately after it is removed from the mouth.

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

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Normal human body temperature is approximately 37∘C37^\circ C, which is equivalent to 98.6∘F98.6^\circ F.

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The SISI unit of temperature is Kelvin (KK). Other commonly used scales are Celsius (∘C^\circ C) and Fahrenheit (∘F^\circ F).

📐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 patient's body temperature is recorded as 39∘C39^\circ C. Convert this temperature into the Fahrenheit scale.

Solution:

F=(95×39)+32=70.2+32=102.2∘FF = \left( \frac{9}{5} \times 39 \right) + 32 = 70.2 + 32 = 102.2^\circ F

Explanation:

To convert Celsius to Fahrenheit, we multiply the temperature in Celsius by 95\frac{9}{5} and then add 3232. Since 39∘C39^\circ C is above the normal 37∘C37^\circ C, the patient has a fever.

Problem 2:

If a liquid is at a temperature of 313.15 K313.15\, K, what is its temperature in degrees Celsius?

Solution:

C=K−273.15=313.15−273.15=40∘CC = K - 273.15 = 313.15 - 273.15 = 40^\circ C

Explanation:

The Kelvin scale is converted to Celsius by subtracting 273.15273.15 from the Kelvin value.

Problem 3:

Why is a Clinical Thermometer jerked before use?

Solution:

The jerk is given to force the mercury level back below the 35∘C35^\circ C mark.

Explanation:

Because of the 'kink', mercury does not fall back into the bulb on its own. Jerking the thermometer ensures that the reading starts from a point lower than the body temperature for an accurate measurement.