1. What Is Heat?
Heat is a form of energy that flows from a region of higher temperature to a region of lower temperature. It moves because particles with more energy share it with particles that have less energy. This flow continues until both objects reach the same temperature, a state called thermal equilibrium.
Heat is measured in joules (J). It is not something a substance contains—it is something that is transferred.
2. Heat Is Energy in Transit
Heat exists only when energy is moving from one body to another. When there is no transfer of energy, there is no heat flow, even if the objects contain large amounts of internal energy.
2.1. Direction of Heat Flow
Heat always flows spontaneously from higher temperature to lower temperature. This is a natural process and does not require any external effort.
Example: A warm hand touching a cold metal surface becomes cooler because heat leaves the hand and enters the metal.
2.2. No Heat Flow in Thermal Equilibrium
When two objects have the same temperature, their particles have the same average energy. In this case, there is no reason for heat to flow either way.
Example: A thermometer settles at a reading only when it reaches the same temperature as the object being measured.
3. How Heat Relates to Temperature
Temperature and heat are connected but not the same. Temperature tells how hot or cold something is, while heat tells us how much energy moves between objects due to temperature difference.
3.1. Different Temperatures Cause Heat Flow
If one object has a higher temperature than another, energy flows from the hotter object to the colder one until temperatures equalize.
3.2. Same Temperature, Different Amounts of Heat
Two objects at the same temperature may contain different amounts of internal energy, but no heat flows between them because heat depends only on temperature difference, not on total energy.
4. Units of Heat
Heat is a form of energy, so it is measured in joules (J). Another older unit is the calorie (cal), defined as the heat needed to raise the temperature of 1 gram of water by 1°C.
4.1. Relation Between Joule and Calorie
\( 1 \text{ cal} = 4.186 \text{ J} \)
This value is called the mechanical equivalent of heat.
5. Heat Capacity and Specific Heat
Different materials respond to heat differently. Some warm up quickly, while others warm up slowly. This behaviour is explained using heat capacity and specific heat.
5.1. Heat Capacity
Heat capacity is the amount of heat required to raise the temperature of a body by 1°C (or 1 K).
\( C = \dfrac{Q}{\Delta T} \)
5.2. Specific Heat Capacity
Specific heat capacity is the heat needed to raise the temperature of 1 kg of a substance by 1°C.
\( Q = mc\Delta T \)
A large value of c means the substance heats slowly but also cools slowly.
6. Processes That Transfer Heat
Heat transfers from one place to another through three mechanisms: conduction, convection, and radiation. Each mechanism works differently depending on the state of the material.
6.1. Conduction
Heat travels through solids by particle vibrations. Good conductors like metals transfer heat quickly.
6.2. Convection
In fluids, warm regions rise and cool regions sink, creating a flow that carries heat with it.
6.3. Radiation
Heat can travel as electromagnetic waves, even through a vacuum. This is how sunlight warms Earth.
7. Heat and Change of State
Heat plays a key role when substances change their physical state. During melting or boiling, heat is absorbed without raising the temperature because the energy is used to change the arrangement of particles.
7.1. Latent Heat of Fusion
Heat absorbed by a solid to become a liquid at its melting point without changing temperature.
7.2. Latent Heat of Vaporisation
Heat absorbed by a liquid to become a gas at its boiling point without temperature change.
8. Everyday Examples of Heat
- A hot cup of tea cools as heat flows to the surrounding air.
- Touching a metal surface feels colder because heat flows quickly from the hand to the metal.
- Stirring soup helps heat spread quickly by mixing hot and cool regions.
- Dark clothes absorb more heat from sunlight than light-coloured clothes.