Echoes Within

🔹 1. Introduction to Thermal Energy, Heat, and Temperature

Thermal Energy

Thermal energy is the total internal energy possessed by the molecules of a body due to their random motion. The faster the molecules move, the higher the thermal energy of the substance.

Heat

👉 Heat is a form of energy that flows from a body at a higher temperature to a body at a lower temperature.
It is a transferred energy, not stored energy, and is measured in joules (J) or calories (cal).

Temperature

👉 Temperature is the measure of the average kinetic energy of the molecules of a body.
It shows how hot or cold a body is. Common units are Celsius (°C), Kelvin (K), and Fahrenheit (°F).

Relationship:

• Heat is the energy in transit,

• Temperature indicates how energetic the particles are,

• Thermal energy is the total energy contained in the body.

🔹 2. Effect of Heat on Molecular Motion and Volume Change

Effect on Molecular Motion

When heat is supplied to a substance, its molecules move faster and farther apart, increasing their kinetic energy.

• In solids, vibration increases.

• In liquids, molecules move more freely.

• In gases, molecules move rapidly in all directions.

Effect on Volume (Thermal Expansion)

Most substances expand when heated and contract when cooled.
This property is called thermal expansion.

• Solids expand slightly.

• Liquids expand more than solids.

• Gases expand the most.

Anomalous Expansion of Water

Water behaves differently between 0°C and 4°C:

• It contracts when heated from 0°C to 4°C.

• It expands when heated above 4°C.

This unusual behaviour is called the anomalous expansion of water.

Importance:

• Prevents ponds, lakes, and rivers from freezing completely in winter.

• Helps aquatic life survive under ice layers.

🔹 3. Specific Heat Capacity

Definition

👉 The specific heat capacity of a substance is the amount of heat required to raise the temperature of 1 kg of the substance by 1°C (or 1 K).

Mathematically,

Q=mcΔT

Where,

• Q = Heat absorbed or released (J)

• m = Mass of substance (kg)

• c = Specific heat capacity (J/kg°C)

• $\Delta T$ = Change in temperature (°C or K)

Example

If 4200 J of heat raises the temperature of 1 kg of water by 1°C, then

c=Q/mΔT=4200/1×1

=4200 J/kg°C

​=4200J/kg°C

Water has a high specific heat capacity, meaning it heats up and cools down slowly — useful for maintaining stable temperatures in nature.

🔹 4. Thermometers and Their Working Principles

(a) Laboratory Thermometer

• Used to measure temperature in experiments (–10°C to 110°C).

• Contains mercury or alcohol inside a narrow glass capillary tube.

• Works on the principle of thermal expansion — the liquid expands or contracts with a temperature change.

(b) Digital Clinical Thermometer

• Used to measure body temperature (35°C–42°C).

• Works on an electronic sensor that detects temperature and displays it digitally.

• More accurate, safe (no mercury), and quick.

(c) Radiation Thermometer

• Measures temperature from a distance by detecting the infrared radiation emitted by an object.

• Commonly used in industry, medical checkups, and thermal cameras.

Calibration of a Thermometer (Experiment)

1. Immerse the thermometer in melting ice and mark the point — this is 0°C.

2. Immerse it in boiling water and mark the point — this is 100°C.

3. Divide the distance between these marks into 100 equal parts to get 1°C intervals.

This process is called calibration, ensuring accurate readings.

✅ Conclusion

Heat and temperature are fundamental to understanding energy transfer and material behaviour. Thermal expansion, specific heat, and precise temperature measurement are vital concepts used in science, engineering, medicine, and daily life.