Heat — Class 8 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 8 Science, Physics — Chapter 4. Heat, temperature, specific heat and how heat travels.
1. About this chapter
This chapter covers heat and temperature, thermometers, specific heat capacity, the three modes of heat transfer, change of state, and conductors and insulators.
2. Heat, temperature and thermometers
- Heat is a form of energy that flows from a hotter body to a colder one. Temperature is a measure of the average kinetic energy of the particles of a body.
- In a thermometer, the lower fixed point is the ice point (0 °C) and the upper fixed point is the steam/boiling point (100 °C).
- A clinical thermometer reads 35–42 °C; a laboratory thermometer reads a wider range.
3. Specific heat capacity
- Specific heat capacity of a substance is the amount of heat energy needed to raise the temperature of 1 kilogram of the substance by 1 °C (or 1 K).
- Water has a high specific heat capacity, so it heats and cools slowly.
4. Modes of heat transfer and change of state
- Conduction: heat flows through a solid from a higher to a lower temperature without the actual movement of molecules.
- Convection: heat flows in liquids and gases by the actual movement of molecules (e.g., boiling water, sea/land breeze).
- Radiation: heat travels as rays needing no medium (e.g., heat from the Sun).
- Change of state: melting, evaporation, condensation (gas → liquid), and freezing.
5. Worked examples
Example 1. How does heat from the Sun reach the Earth? By radiation (no medium needed).
Example 2. Why are mud houses cool in summer and warm in winter? Mud is a poor conductor (insulator) of heat, so it slows heat flow into and out of the house.
Example 3. What is condensation? The change of state from gas to liquid.
6. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- Convection of heat takes place in — (a) solids only / (b) liquids and gases / (c) vacuum. Ans: (b) liquids and gases.
- In conduction, heat flows from a — (a) colder to hotter / (b) hotter to colder region. Ans: (b) hotter to colder.
- Mud houses are cooler in summer and warmer in winter because mud is a — (a) good conductor / (b) poor conductor of heat. Ans: (b) poor conductor.
- The change of state from gaseous to liquid state is called — (a) evaporation / (b) condensation / (c) melting. Ans: (b) condensation.
II. Fill in the blanks 5. Temperature is a measure of the average kinetic energy of the particles. 6. The boiling point of water is taken as the upper fixed point in a thermometer. 7. The amount of heat needed to raise 1 kg of a substance by 1 °C is its specific heat capacity.
III. Answer briefly 8. Define conduction. (Heat transfer in solids from higher to lower temperature without movement of molecules.) 9. Differentiate conduction and convection. (Conduction: solids, no molecule movement; convection: liquids/gases, actual molecule movement.)
7. Common mistakes
- Mistake: Treating heat and temperature as the same. Fix: Heat is energy in transit; temperature measures the average kinetic energy of particles.
- Mistake: Saying radiation needs a medium. Fix: Radiation needs no medium (it works through vacuum).
- Mistake: Forgetting specific heat capacity. Fix: It is the heat to raise 1 kg of a substance by 1 °C/1 K.
8. Quick revision
- Physics Ch 4 · heat, temperature, specific heat, transfer, change of state.
- Heat flows hot → cold; temperature = average kinetic energy.
- Specific heat capacity = heat to raise 1 kg by 1 °C/1 K.
- Conduction (solids, no molecule movement), convection (liquids/gases, movement), radiation (no medium).
- Change of state: melting, evaporation, condensation (gas→liquid), freezing.
