Complete Physics Notes on Basic Concept of Heat and Temperature for BPSC and Other Competitive Exams in 2025
General Science- 3 (Physics)
Basic Concepts of Heat and Temperature
1. Heat
- Concept: Heat is a form of energy that is transferred between objects or systems due to a temperature difference. It is the total kinetic and potential energy of the atoms and molecules in a substance. Heat always flows from a region of higher temperature to a region of lower temperature.
- Nature: It is a scalar quantity.
- SI Unit: Joule (J). Other common units include calorie (cal).
- 1 calorie=4.184 Joules (approximately).
- Kilocalorie (kcal) is also used, especially in nutrition.
- Modes of Heat Transfer:
- Conduction: Heat transfer through direct contact, primarily in solids. Heat is transferred from hotter particles to colder particles without actual movement of the particles themselves.
- Good conductors (e.g., metals) transfer heat efficiently, while poor conductors/insulators (e.g., wood, plastic, air) do not.
- Convection: Heat transfer through the movement of fluids (liquids or gases). Hotter, less dense fluid rises, and colder, denser fluid sinks, creating convection currents. This is how water boils, air is heated in a room, and winds blow.
- Radiation: Heat transfer through electromagnetic waves (like infrared radiation) without requiring a medium. This is how heat reaches us from the sun, or from a fire. Dark, dull surfaces are good absorbers and emitters of radiation, while light, shiny surfaces are poor absorbers and emitters (good reflectors).
- Specific Heat Capacity:
- Concept: The amount of heat energy required to raise the temperature of 1 unit mass of a substance by 1∘C or 1 Kelvin.
- Unit: Joule per kilogram per Kelvin (J/ kg−1K−1) or Joule per kilogram per degree Celsius .
- Key Idea: Water has a very high specific heat capacity, meaning it requires a lot of energy to change its temperature. This property is crucial for climate regulation and makes water an excellent coolant.
- Latent Heat:
- Concept: The heat energy absorbed or released by a substance during a phase change (e.g., melting, freezing, boiling, condensation) without a change in its temperature.
- Latent Heat of Fusion: Heat required to change a unit mass of a substance from solid to liquid at its melting point.
- Latent Heat of Vaporization: Heat required to change a unit mass of a substance from liquid to gas at its boiling point.
- Unit: Joule per kilogram (J/kg).
- Examples: Steam causes more severe burns than boiling water at the same temperature because of the extra latent heat of vaporization it carries. Ice at 0∘C is more effective in cooling than water at 0∘C because it absorbs latent heat to melt.
2. Temperature
- Concept: Temperature is a measure of the average kinetic energy of the particles (atoms or molecules) within a substance. It indicates the degree of hotness or coldness of an object. Temperature determines the direction of heat flow.
- Nature: It is a scalar quantity.
- Units of Temperature:
- Celsius (∘C): Water freezes at 0∘C and boils at 100∘C (at standard atmospheric pressure).
- Fahrenheit (∘F): Water freezes at 32∘F and boils at 212∘F.
- Kelvin (K): The SI unit of temperature. It is an absolute temperature scale, meaning 0 K (absolute zero) is the lowest possible temperature where molecular motion theoretically stops.
- T(K)=T(∘C)+273.15
- Thermal Expansion:
- Concept: The tendency of matter to change in volume in response to a change in temperature. Most substances expand when heated and contract when cooled.
- Types: Linear, Superficial, and Volumetric expansion.
- Anomalous Expansion of Water: Water shows abnormal behavior between 0∘C and 4∘C. It contracts when heated from 0∘C to 4∘C (density increases) and expands when heated above 4∘C (density decreases). Water has maximum density at 4∘C. This property is vital for aquatic life in cold regions.
- Thermometers: Devices used to measure temperature. They work on the principle of thermal expansion of liquids (mercury, alcohol) or other temperature-dependent properties.
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