Class 7: Heat - Boojho & Paheli Questions

Class 7: Heat - Boojho & Paheli Questions

Heat

Boojho & Paheli's Questions Answered

Heat is a form of energy that makes things warmer. Understanding temperature and heat transfer helps us in daily life. Let's explore common questions about heat!

B
"My left hand tells me that the water in mug C is hot and the right hand tells me that the same water is cold. What should I conclude?"

This happens because our sense of hot and cold is relative. Your hands were at different temperatures before touching the water.

How our sense of touch works

Our skin senses temperature differences, not absolute temperature. If your left hand was cold and right hand was warm, the same water will feel different to each hand.

What to do

Use a thermometer to measure the actual temperature of the water instead of relying on your sense of touch.

B
Boojho wondered which of the two scales he should read. Paheli told him that India has adopted the celsius scale.

Paheli is correct. In India, we use the Celsius scale (°C) for temperature measurement.

37°C
|
98.6°F

Thermometer showing Celsius and Fahrenheit scales

Celsius Scale

Used in India and most countries worldwide

Water freezes at 0°C and boils at 100°C

Fahrenheit Scale

Used mainly in the United States

Water freezes at 32°F and boils at 212°F

P
Paheli measured her body temperature. She got worried as it was not exactly 37°C.

She need not to be worried because the normal body temperature is approximately 37°C, but it can vary slightly.

Normal body temperature range

Healthy body temperature can range between 36.1°C to 37.2°C (97°F to 99°F). It changes throughout the day and is affected by activity, time of day, and other factors.

When to be concerned

Only be concerned if your temperature is:

  • Below 35°C (feeling very cold)
  • Above 38°C (fever)
B
Boojho wanted to measure the temperature of hot milk using a clinical thermometer. Paheli stopped him.

Paheli was right to stop him. Clinical thermometers are designed only for measuring body temperature.

Why it's dangerous
  • Hot milk can be above 42°C - the maximum limit of clinical thermometers
  • The thermometer might break from high temperature
  • Mercury (if present) could leak and contaminate the milk
What to use instead

Use a laboratory thermometer that can measure higher temperatures safely.

B
Can a laboratory thermometer be used to measure body temperature?

No, laboratory thermometers are not suitable for measuring body temperature.

Clinical Thermometer

Range: 35°C to 42°C

Designed for body temperature measurement

Has a kink to hold the reading

Laboratory Thermometer

Range: -10°C to 110°C

No kink - mercury falls as temperature drops

Not accurate for body temperature

Why it won't work well

Laboratory thermometers don't have the kink that holds the mercury in place. As soon as you remove it from your body, the reading will start changing, making it hard to get an accurate measurement.

B
Why does the level of mercury change when the bulb of the thermometer touches an object?

The mercury level changes due to thermal expansion.

How thermometers work

Mercury expands when heated and contracts when cooled. When the bulb touches a warmer object, heat transfers to the mercury, causing it to expand and rise in the tube.

Cold: Mercury contracts
Hot: Mercury expands
P
"Does it mean that heat will not be transferred if the temperature of two objects is the same?"

Yes! Heat only flows from a hotter object to a colder object.

Heat Transfer Rule

Heat naturally transfers from objects at higher temperature to objects at lower temperature. When two objects have the same temperature, there's no net heat flow between them.

Thermal Equilibrium

When objects at different temperatures come in contact, heat flows until they reach the same temperature. This state is called thermal equilibrium.

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