Tamil Nadu Class 8 Science: Heat

Quick Summary

Heat is part of Term 1 in Class 8 Science. This chapter explains the difference between heat and temperature, thermal expansion of solids, liquids and gases, specific heat capacity, and the three modes of heat transfer: conduction, convection, and radiation.

Key Definitions

• Heat: A form of energy that causes a temperature difference; measured in Joules (J).
• Temperature: The degree of hotness or coldness; measured in Celsius (°C) or Kelvin (K).
• Thermal expansion: The increase in size of a substance when heated.
• Specific heat capacity: The amount of heat required to raise the temperature of 1 kg of a substance by 1°C (or 1 K).
• Conduction: Transfer of heat through a solid without movement of the material.
• Convection: Transfer of heat through fluids by the movement of the fluid particles.
• Radiation: Transfer of heat without any material medium through electromagnetic waves.

Key Formulae

Thermal Expansion

• Solids expand the least when heated.
• Liquids expand more than solids.
• Gases expand the most when heated.
• Applications: gaps in railway lines, pendulum clock correction, bimetallic strip in thermostats.

Modes of Heat Transfer

Conduction

• Occurs in solids.
• Heat passes from particle to particle without the particles moving.
• Good conductors: metals (iron, copper, aluminium).
• Bad conductors (insulators): wood, rubber, glass, air.

Convection

• Occurs in liquids and gases.
• Heated fluid rises (less dense); cooler fluid sinks (more dense), creating convection currents.
• Examples: boiling water, sea breeze, land breeze.

Radiation

• Does not need a medium.
• Travels at the speed of light.
• Dark and rough surfaces absorb and emit radiation better than light and smooth surfaces.
• Example: the sun's heat reaching the earth.

Important Questions

One Mark

• What is the SI unit of heat?
• Which mode of heat transfer does not require a medium?
• Define specific heat capacity.
• Convert 100°C to Kelvin.
• Name one good conductor of heat.

Short Answer (3 marks)

• Explain the three modes of heat transfer with one example each.
• Why are railway lines laid with small gaps between them? Explain with reference to thermal expansion.
• Distinguish between heat and temperature with two differences.

Long Answer (5 marks)

• Explain conduction, convection, and radiation with neat diagrams. Give two applications of each.
• State the formula Q = mcΔT. Define each term. Calculate the heat required to raise the temperature of 2 kg of water from 25°C to 75°C. (Specific heat of water = 4200 J kg⁻¹ K⁻¹)

Common Mistakes

• Confusing heat (energy) with temperature (degree of hotness).
• Forgetting to convert temperature to Kelvin when needed.
• Incorrectly stating that convection occurs in solids.

Revision Checklist

• [ ] Textbook reading completed
• [ ] Q = mcΔT formula practised with numerical examples
• [ ] Three modes of heat transfer with examples listed
• [ ] Thermal expansion applications noted
• [ ] Important questions answered in notebook