Class 7 – Science – Notes – Chapter 4 – Heat

Rutven – The Learning Hub

1. Understanding Heat

Heat is a form of energy that makes things warmer. When you touch a hot object, it feels warm because heat flows from the object to your hand.

Heat always moves from a hotter body to a colder body. It stops moving when both objects reach the same temperature.

2. Heat and Temperature – Are They the Same?

Although we use the terms “heat” and “temperature” often, they are different:

Heat	Temperature
A form of energy	A measure of how hot or cold an object is
Depends on size, number of particles, and how fast they move	Does not depend on size or mass
Measured in Joules (J)	Measured in Degrees Celsius (°C) or Kelvin (K)

Example: A bathtub full of warm water has more heat than a small cup of hot tea — even though the tea is hotter.

 3. How Heat Transfers

Heat can move in three ways:

1. Conduction – Through solids
2. Convection – Through liquids and gases
3. Radiation – Through empty space

Let’s explore these one by one.

 4. Conduction – Heat Transfer in Solids

Conduction is when heat moves through a solid material, from one particle to another.

 Examples:

• A metal spoon becomes hot when placed in hot soup.
• Ironing clothes with a hot iron.

 Good Conductors:

Materials that allow heat to pass easily.

• Copper
• Iron
• Aluminium

 Poor Conductors (Insulators):

Materials that do not let heat pass easily.

• Wood
• Plastic
• Rubber
• Air

 5. Thermal Conductivity of Common Substances

Material	Thermal Conductivity (W/m°C)
Copper	401
Glass	1.05
Concrete	1.05
Wood	0.13
Paper	0.05
Air	0.024

6. Uses of Bad Conductors in Daily Life

• Woolen clothes trap body heat in winter.
• Handles of pots are made of wood or plastic to avoid burns.
• Fridge walls use insulation (like cork or glass wool) to keep food cold.
• Double-glass windows with air in between keep homes warm in cold countries.
• Pipes carrying hot water are covered with insulating material to prevent heat loss.
• Fur or hair on animals protects them from cold weather.

7. Convection – Heat Transfer in Liquids and Gases

In liquids and gases, heat moves when warmer particles rise and cooler particles sink.

Activity (In Liquids):

• Add potassium permanganate to water in a beaker.
• Heat it from below.
• The purple water rises on heating – showing convection currents.

 Activity (In Gases):

• Take a box with two glass tubes.
• Place a burning candle below one tube.
• Hold smoke paper above the other.
• The smoke moves in a loop – showing air is circulating.

8. Uses of Convection in Real Life

• Land breeze and sea breeze happen because of heat movement in air.
• Engines are cooled using water that moves heat away.
• Gliders ride rising warm air currents to stay up in the sky.
• Air conditioners are fixed high so that cool air sinks and warm air rises, helping the room cool quickly.

 9. Radiation – Heat Without Contact

Radiation is the transfer of heat energy without any medium. Heat travels in straight lines, even through a vacuum.

Examples:

• The Sun warms the Earth through radiation.
• Sitting near a fire makes you feel warm, even if you are not touching it.

Properties of Radiation:

• Travels like light
• Needs no medium
• Can reflect or be absorbed
• Warmer objects emit more radiation

 10. Measuring Temperature

To measure temperature, we use:

• Thermometer
• Thermocouple
• Thermistor

Let’s focus more on thermometers.

11. Types of Thermometers

 Clinical Thermometer

• Used to measure body temperature
• Range: 35°C to 42°C
• Has a kink to prevent mercury from falling back quickly

 Precautions:

• Wash before and after use (with antiseptic)
• Ensure mercury is below 35°C before use
• Don’t shake or drop
• Hold at eye level to read

 Laboratory Thermometer

• Used in science labs
• Range: 10°C to 110°C
• No kink, as it is not meant for humans

12. Calibration of a Celsius Thermometer

Anders Celsius gave a method to mark the thermometer scale:

1. Place the thermometer in ice water. Mark it as 0°C.
2. Place it in boiling water. Mark it as 100°C.
3. Divide the distance between them into 100 equal parts.

Each part represents 1 degree Celsius.

Today, scientists use the triple point of water (0.01°C) for more accurate calibration.

 13. Callendar’s Gas Thermometer

This thermometer uses gas to measure temperature at constant pressure.

Principle:

As temperature increases, gas expands.

 Formula:

t=(Vt−V0V100−V0)×100t = \left(\frac{V_t – V_0}{V_{100} – V_0}\right) \times 100t=(V100−V0Vt−V0)×100

It can measure temperatures up to 600°C.

 14. How Thermometers Are Made

 Laboratory Thermometer:

• Made of metal or glass
• Filled with mercury or alcohol
• Responds to temperature changes quickly

Clinical Thermometer:

• Used by doctors
• Thin tube with mercury
• Has a bend (constriction) near the bulb to hold reading

15. Common Situations Showing Heat Transfer

• Holding ice: Heat from your hand melts the ice – conduction
• Ironing clothes: Heat from iron moves to fabric – conduction
• Room heating from a heater: Warm air rises – convection
• Sun warming your skin: Radiation
• Cooking food in a pan: All three methods can be involved

 Summary

• Heat is a form of energy that flows from hot to cold.
• Temperature is how hot or cold something is.
• Heat moves through conduction, convection, and radiation.
• Thermometers help us measure temperature accurately.
• Good and bad conductors have many real-life uses.