Temperature and Heat

Temperature and heat are fundamental concepts in thermodynamics and physical science. When two bodies at different temperatures come in contact, energy is transferred between them as heat until they reach the same temperature — a state called thermal equilibrium. This topic helps us understand how energy moves between systems and how matter behaves when heated across its solid, liquid, and gas states.

"Heat is energy in transit. When two bodies at different temperatures are brought in contact, they exchange heat."

"Temperature is a physical quantity that defines the thermodynamic state of a system."

\[E_k=\frac{3}{2}k_BT\]

Where:

• E_k = Average kinetic energy of the molecules (in joules)
• k_B = Boltzmann constant = 1.380649 × 10⁻²³ J/K
• T = Absolute temperature (in kelvin)

Q = mcΔT

Where:

• Q = Heat absorbed or released (in joules)
• m = Mass of the substance (in kg)
• c = Specific heat capacity (J/kg·K)
• ΔT = Change in temperature (T_final−T_initial)

States of Matter — Key Characteristics

When a body is at a different temperature from its surroundings, heat transfer takes place until both reach the same temperature (thermal equilibrium). The source explains this process through the behaviour of particles in each state of matter:​​

In Solids:

• Particles vibrate about fixed equilibrium positions and possess kinetic energy.​
• When heated, particles vibrate with higher energy → temperature increases.
• The energy supplied becomes internal energy (increased kinetic energy of atoms/molecules).
• Near the melting point, heat weakens bonds between particles instead of raising the temperature.

In Liquids:

• Bonds are weaker; particles can move but remain close together.
• Mean distance between particles is roughly the same as in solids.​
• Liquids have definite volume but no definite shape.
• On further heating, kinetic energy increases → temperature rises.

In Gases:

• At the boiling point, particles overcome intermolecular forces and move freely.
• For an ideal gas, there are no forces between molecules (kinetic theory of gases).​
• Gases have neither definite volume nor shape.

• Heat transfer governs everyday phenomena — from warming of cold water to cooling of hot tea.​
• The concept of thermal equilibrium is the basis of the Zeroth Law of Thermodynamics.
• Understanding temperature in terms of average kinetic energy connects microscopic particle behaviour to macroscopic properties like temperature.
• Knowledge of unit conversions (Joule, Calorie, Erg) is essential for solving numerical problems in thermodynamics.
• Dimensional analysis of heat and temperature helps verify equations and solve advanced physics problems.

• Ice-cold water warming up on a table: The water absorbs heat from the warmer surroundings until it reaches room temperature (thermal equilibrium).​
• Hot tea cooling down: The hot tea loses heat to the cooler air around it until both reach the same temperature.​
• A metal spoon in hot coffee: The spoon heats up because heat transfers from the hot coffee to the cooler spoon through molecular collisions.​
• A warm car on a cold day: Heat from inside the car gradually transfers to the cold outside air until equilibrium is reached.​
• Clinical thermometer: Works on the principle that the mercury inside reaches thermal equilibrium with the body, giving an accurate temperature reading.​