1. What Is Heat?
Heat is a form of energy that flows from a body at higher temperature to a body at lower temperature. It is not a substance or material—it is simply energy in transit.
Once the transfer stops, there is no ‘heat’ inside a body. What the body has is internal energy, not heat.
2. Heat vs Thermal Energy
Heat and thermal energy are related but not the same:
- Thermal energy is the total internal microscopic energy of particles inside a body.
- Heat is the energy transferred because of temperature difference.
Heat is energy in motion; thermal energy is energy stored inside.
2.1. Example
When a hot cup of tea cools down, heat moves out of the tea. The tea loses heat, and its thermal energy decreases.
3. Direction of Heat Flow
Heat always flows from a hotter body to a colder body. This continues until both reach the same temperature—called thermal equilibrium.
3.1. Everyday Examples
- An ice cube melts in your hand because heat flows from your hand (warmer) to the ice (colder).
- A hot utensil cools down to room temperature because it loses heat to surrounding air.
4. Modes of Heat Transfer
There are three ways heat moves from one place to another. Each mode works in different situations depending on the medium.
4.1. Conduction
Heat transfer through solids without actual movement of particles. Energy travels from particle to particle. Metal rods heating from one end are a common example.
4.2. Convection
Heat transfer in liquids and gases due to actual movement of the fluid. Warm fluid rises, cold fluid sinks, creating convection currents.
4.3. Radiation
Heat transfer through electromagnetic waves. No medium is needed. Sunlight reaching Earth is an example.
5. Heat Transfer and Temperature Change
When heat flows into a body, its temperature can increase. The temperature rise depends on:
- amount of heat transferred
- mass of the substance
- specific heat capacity
5.1. Relation for Temperature Change
The amount of heat required to change the temperature of a mass \( m \) by \( \Delta T \) is:
\( Q = m c \Delta T \)
6. Heat Transfer Without Temperature Change
Sometimes heat flows into a system without increasing its temperature. This happens during phase changes like melting or boiling. The heat is used to change the state instead of raising temperature.
6.1. Phase Change Example
During melting, ice stays at 0°C even though it absorbs heat. This heat goes into breaking molecular bonds, not raising temperature.
7. Role of Heat Transfer in Everyday Life
- Cooking food involves conduction, convection, and radiation together.
- Sea breezes form due to convection differences between land and sea.
- Warm clothing reduces heat loss through conduction and convection.
- Solar panels absorb radiant heat from sunlight.