Notes Class 7 Science Chapter 3 Heat

Notes For All Chapters Science Class 7

1. Hot and Cold

• In daily life, we find some objects hot and some cold.

• We usually decide this by touching, but our sense of touch is not always reliable.

• Sometimes the same water can feel hot to one hand and cold to the other.

• Therefore, we need a proper way to measure how hot or cold something is.

👉 The measure of hotness or coldness is called Temperature.

2. Temperature

• Temperature tells us how hot or cold an object is.

• It is measured using a device called a thermometer.

• The unit of temperature is degree Celsius (°C).

3. Clinical Thermometer

Used to measure human body temperature.

Features

• Made of a long glass tube with a bulb containing mercury.

• Has a Celsius scale marked on it.

• Range: 35°C to 42°C

• Has a kink near the bulb.

  • The kink prevents mercury from falling back quickly so we can read the temperature after removing it from the mouth.

Normal Human Body Temperature

• 37°C (average)

• It may be slightly higher or lower in different people.

Precautions While Using

• Wash before and after use (preferably with antiseptic).

• Mercury level should be below 35°C before use.

• Hold at eye level while reading.

• Do not hold by the bulb.

• Handle carefully — it can break.

⚠️ A clinical thermometer should only be used for measuring body temperature.

4. Laboratory Thermometer

Used to measure temperature of other objects and liquids.

Features

• Range: –10°C to 110°C

• No kink

• Must be read while still in the substance being measured.

Precautions

• Keep it upright.

• Bulb should be surrounded by the substance.

• Bulb should not touch the container.

❌ Cannot be used to measure body temperature conveniently.

5. Transfer of Heat

Heat always flows from a hotter object to a colder object.

There are three methods of heat transfer:

A. Conduction

• Transfer of heat through solids.

• Heat moves from the hot end to the cold end of an object.

• Example: A metal spoon becomes hot when one end is placed in hot water.

Conductors of Heat
Materials that allow heat to pass easily:

• Iron

• Copper

• Aluminium

Insulators (Poor Conductors)
Materials that do not allow heat to pass easily:

• Wood

• Plastic

• Air

B. Convection

• Transfer of heat in liquids and gases.

• When heated, the hot part rises and the cold part comes down.

• This movement forms convection currents.

Examples

• Heating of water in a vessel

• Warm air rising above a flame

Sea Breeze (Day)

• Land heats faster than water.

• Warm air over land rises.

• Cool air from sea moves toward land.

Land Breeze (Night)

• Land cools faster than water.

• Cool air from land moves toward sea.

C. Radiation

• Transfer of heat without any medium.

• Heat from the Sun reaches Earth by radiation.

• We feel heat from a heater through radiation.

All hot objects give out heat by radiation.

6. Why We Wear Different Clothes in Summer and Winter

Dark vs Light Colours

Dark colours

• Absorb more heat

• Keep us warm

• Worn in winter

Light colours

• Reflect most heat

• Keep us cool

• Worn in summer

7. Woollen Clothes in Winter

• Wool is a poor conductor of heat.

• Wool fibres trap air.

• Air is also a poor conductor.

• This prevents body heat from escaping.

• So, we feel warm.

👉 Wearing two thin blankets can be warmer than one thick blanket because air gets trapped between layers.