Notes For All Chapters – General Science Class 8
1. Introduction
- Heat is a form of energy that flows from hot objects to cold objects.
- Temperature tells us how hot or cold an object is.
- The SI unit of heat is Joule (J) and in the CGS system, it is Calorie (cal).
- 1 calorie = 4.18 Joules.
2. Sources of Heat
| Source | Explanation |
|---|---|
| Sun | The biggest source of heat, produced by nuclear fusion of hydrogen atoms. |
| Earth (Geothermal Energy) | Heat from the core of the Earth is called geothermal energy. |
| Chemical Energy | Heat is produced when fuels like wood, coal, and petrol burn. |
| Electrical Energy | Electricity produces heat in heaters, electric irons, and ovens. |
| Atomic Energy | Heat is released during nuclear fission of uranium and thorium. |
| Air | Air around us stores heat, helping in weather changes. |
3. Heat and Temperature
| Heat | Temperature |
|---|---|
| Form of energy | Measures how hot or cold an object is |
| Measured in Joules (J) or calories (cal) | Measured in Celsius (°C), Fahrenheit (°F), or Kelvin (K) |
| Total kinetic energy of atoms in a substance | Related to average kinetic energy of atoms |
| Heat flows from hot to cold objects | Objects at different temperatures exchange heat |
4. Units of Temperature
| Unit | Symbol | Used In |
|---|---|---|
| Celsius | (°C) | Daily life, weather reports |
| Fahrenheit | (°F) | Body temperature measurement |
| Kelvin | (K) | Scientific experiments |
Conversion Formulas:
- Fahrenheit to Celsius: C = (F – 32) × 5/9
- Celsius to Kelvin: K = C + 273.15
5. Thermometers
| Type | Use | Temperature Range |
|---|---|---|
| Clinical Thermometer | Measures body temperature | 35°C to 42°C |
| Laboratory Thermometer | Used in experiments | -10°C to 110°C |
| Digital Thermometer | Measures body temperature using sensors | Accurate and quick |
| Maximum-Minimum Thermometer | Measures daily temperature variations | Used in weather stations |
6. Effects of Heat
- Expansion of Matter – When heated, solids, liquids, and gases expand.
- Change of State – Heat can change solids to liquids and liquids to gases.
- Chemical Changes – Heat causes burning, cooking, and rusting.
7. Expansion of Solids
| Type of Expansion | Definition | Example |
|---|---|---|
| Linear Expansion | Increase in length of a solid due to heat | Metal rods expand when heated |
| Areal Expansion | Increase in surface area of a solid | Metal sheets expand in summer |
| Volumetric Expansion | Increase in volume of a solid | Railway tracks expand and need gaps |
Example Calculation:
- A 0.5m steel rod is heated by 60°C. If the expansion coefficient is 0.000013 (1/°C), the increase in length is:
- Δl = λ × l × ΔT
- = 0.000013 × 0.5 × 60 = 0.039 cm
8. Expansion of Liquids
- Liquids do not have a fixed shape but expand when heated.
- Expansion is given by: V₂ = V₁ (1 + β ΔT)
- where β is the expansion coefficient.
- Example: Water expands when heated, but shows anomalous behavior between 0°C and 4°C.
9. Expansion of Gases
- Gases expand much more than solids and liquids.
- When heated in a closed container, gas pressure increases.
- When heated in an open container, gas expands in volume.
- Expansion formula:
- V₂ = V₁ (1 + β ΔT)
10. Specific Heat
- Definition: The amount of heat required to increase the temperature of 1 kg of a substance by 1°C.
- Formula:
- Q = m × c × (T₂ – T₁)where:
- Q = Heat energy (J)
- m = Mass (kg)
- c = Specific heat capacity
- T₂ – T₁ = Change in temperature
| Substance | Specific Heat (J/kg°C) |
|---|---|
| Water | 4184 |
| Iron | 450 |
| Copper | 385 |
| Gold | 129 |
11. Calorimeter
- A calorimeter is used to measure heat transfer in physical or chemical processes.
- It has two vessels (inner and outer) to prevent heat loss.
- Formula used:Heat lost by hot object = Heat gained by cold object
- m₁ × c₁ × (T₁ – Tₓ) = m₂ × c₂ × (Tₓ – T₂)
12. Practical Applications of Expansion
- Gaps in railway tracks prevent damage due to expansion.
- Bridges have expansion joints to allow expansion and contraction.
- Metal lids on jars expand when heated, making them easier to open.
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