What is Heat?
Heat is a form of energy that flows from a body at higher temperature to a body at lower temperature due to the difference in temperature.
Heat is not stored in a body like mass or volume.
Heat is energy in transit.
What is Temperature?
Temperature is a physical quantity that indicates the degree of hotness or coldness of a body.
Temperature represents the average kinetic energy (movement) of its atoms or molecules. Faster particle movement means higher temperature, and slower particle movement means lower temperature.
Temperature depends on the motion of particles, and not on the amount of substance. A small object and a large object can have the same temperature.
Temperature determines the direction of heat flow. Heat always flows from a body at higher temperature to a body at lower temperature.
Measurement of Temperature
Temperature is measured using thermometers, based on properties that change with temperature.
Common temperature scales:
- Celsius (°C)
- Kelvin (K)
- Fahrenheit (°F)
The Kelvin scale is the SI unit of temperature.
Heat vs. Temperature
Although closely related, heat and temperature are not the same.
Heat  | Temperature  |
|---|---|
| Energy that flows | Measure of hotness |
| Depends on mass | Independent of mass |
| Measured in joules (J) | Measured in °C, K, °F |
| Can be transferred | Cannot be transferred |
Note: Heat flows because of temperature difference, not because of heat difference.
Thermal Equilibrium
When two bodies at different temperatures are placed in contact:
- Heat flows from hot to cold
- This continues until both reach the same temperature
At this point, the bodies are said to be in thermal equilibrium, and no net heat flows between them.
Microscopic View of Heat
Heat is related to the random motion of molecules inside a substance.
- Higher temperature → molecules move faster
- Lower temperature → molecules move slower
- When a hot object touches a cold object, energy is transferred by collisions between molecules
Specific Heat Capacity
Different substances require different amounts of heat to raise their temperature.
Specific heat capacity is the amount of heat required to raise the temperature of 1 kg of a substance by 1°C (or 1 K).
Q=mcΔT
Where:
- Q = heat supplied
- m = mass
- c = specific heat capacity
- ΔT = change in temperature
Water has a high specific heat capacity, which is why it is used as a coolant.
Latent Heat
Sometimes, heat is supplied but temperature does not change. This happens during a change of state.
Latent heat is the energy absorbed or released by a substance during a change of state, such as melting or boiling, without any change in temperature.
- Heat absorbed during melting → latent heat of fusion
- Heat absorbed during boiling → latent heat of vaporization
Latent heat is used to change the state, not the temperature.
Modes of Heat Transfer
Heat can be transferred (or heat can flow) in three different ways.
- Conduction
- Convection
- Radiation
- Transfer of heat through solids
- Occurs without bulk movement of matter
- Metals are good conductors
Example: Heating one end of an iron rod.
2. Convection
- Transfer of heat through fluids (liquids and gases)
- Occurs due to actual movement of the fluid
Example: Circulation of hot air in a room.
3. Radiation
- Transfer of heat without any medium
- Can occur in vacuum
- Travels as electromagnetic waves
Example: Heat from the Sun reaching Earth.
Study the detailed laws governing heat transfer in Thermodynamics.
Change of State
Matter can change its state due to heat exchange:
- Solid → Liquid (melting)
- Liquid → Gas (boiling/evaporation)
- Gas → Liquid (condensation)
- Liquid → Solid (freezing)
During these changes:
- Temperature remains constant
- Energy goes into breaking or forming molecular bonds
Everyday Examples of Heat
- Cooking food
- Cooling of hot tea
- Sea breeze and land breeze
- Refrigerator working
- Thermal insulation in buildings
Importance of Heat in Daily Life
- Climate and weather
- Cooking and food preservation
- Power generation
- Industrial processes
- Human body temperature regulation
Summary
- Heat is energy transferred due to temperature difference
- Temperature measures hotness
- Heat flows from hot to cold
- Heat transfer occurs by conduction, convection, and radiation
- Heat plays a crucial role in natural and technological processes
Glossary of Key Terms
Recap of the Key Terms in Heat
Change of State: The process by which a substance changes from one physical state to another due to heat transfer.
Conduction: The transfer of heat through a substance without the bulk movement of particles.
Convection: The transfer of heat through the actual movement of fluid particles.
Good Conductor: A material that allows heat to pass through it easily.
Heat: A form of energy that flows from a body at higher temperature to a body at lower temperature due to a temperature difference.
Latent Heat of Fusion: The heat required to change a substance from solid to liquid at constant temperature.
Latent Heat of Vaporization: The heat required to change a substance from liquid to gas at constant temperature.
Latent Heat: The energy absorbed or released during a change of state of a substance without any change in temperature.
Radiation: The transfer of heat in the form of electromagnetic waves without the need for a medium.
Specific Heat Capacity: The amount of heat required to raise the temperature of 1 kg of a substance by 1°C (or 1 K).
Temperature: A measure of the degree of hotness or coldness of a body.
Thermal Equilibrium: The state in which two bodies in contact have the same temperature and no net heat flows between them.
Thermal Insulator: A material that resists the flow of heat.
Thermometer: An instrument used to measure temperature based on a physical property that changes with temperature.
Quiz
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is heat?
Heat is a form of energy that flows from a hotter body to a colder body due to a temperature difference.
How is heat different from temperature?
Heat is energy in transit, while temperature is a measure of the degree of hotness of a body.
In which direction does heat flow naturally?
Heat flows naturally from a body at higher temperature to a body at lower temperature.
What is meant by thermal equilibrium?
Thermal equilibrium is the state when two bodies in contact reach the same temperature and no net heat flows between them.
What is specific heat capacity?
It is the amount of heat required to raise the temperature of 1 kg of a substance by 1°C (or 1 K).
Why does water have a high specific heat capacity?
Because it requires a large amount of heat to change its temperature.
What is latent heat?
Latent heat is the energy absorbed or released during a change of state without any change in temperature.
Why does temperature remain constant during a change of state?
Because the supplied heat is used to change the state of the substance rather than increasing its temperature.
Name the three modes of heat transfer.
Conduction, convection, and radiation.
In which mode of heat transfer is a medium not required?
Radiation.
Give one example of heat transfer by conduction.
Heating one end of a metal rod causes the other end to become hot.
Give one example of heat transfer by convection.
The circulation of hot air in a room.
Give one example of heat transfer by radiation.
Heat from the Sun reaching the Earth.
Why are metals good conductors of heat?
Because they have free electrons that transfer energy efficiently.
Why are woollen clothes worn in winter?
Because wool traps air, which is a poor conductor of heat, reducing heat loss from the body.