Heat, Temperature and Internal Energy — Definition

Definition

Imagine you have a cup of hot coffee and a glass of cold water. When you touch the coffee cup, you feel 'hotness,' and when you touch the water glass, you feel 'coldness.' These sensations are directly related to the concepts of heat, temperature, and internal energy, which are cornerstones of thermodynamics in physics.

Let's start with Temperature. Think of temperature as a numerical value that tells us how 'hot' or 'cold' something is. More precisely, it's a measure of the average kinetic energy of the microscopic particles (atoms and molecules) that make up a substance.

If the particles are jiggling, vibrating, and moving around very rapidly, the substance has a high temperature. If they are moving slowly, it has a low temperature. So, the hot coffee has molecules moving very fast on average, while the cold water's molecules move slower.

Temperature is measured using scales like Celsius ($^circ C$), Fahrenheit ($^circ F$), and Kelvin ($K$). For scientific purposes, especially in NEET, Kelvin is the absolute temperature scale, where 0 K represents absolute zero, the theoretical point where all molecular motion ceases.

Next, consider Heat. Heat is not something a substance 'contains' in the same way it contains mass or volume. Instead, heat is energy *in transit*. It's the energy that flows from one object or system to another *because* there's a temperature difference between them.

When you place a hot object next to a cold object, energy will naturally flow from the hot one to the cold one until both reach the same temperature. This transferred energy is what we call heat. For example, when your hot coffee cools down, it's losing heat to the cooler surrounding air.

When ice melts in your hand, your hand is transferring heat to the ice. Heat is a form of energy, so its SI unit is the Joule ($J$), though calories ($cal$) are also commonly used.

Finally, we have Internal Energy. This is the total energy stored *within* a system due to the random motion and positions of its constituent molecules. Unlike heat, which is energy in transit, internal energy is a property of the system itself.

It includes two main components: the total kinetic energy of all the molecules (due to their translational, rotational, and vibrational motions) and the total potential energy associated with the forces between these molecules.

For an ideal gas, where intermolecular forces are negligible, internal energy is primarily the sum of the kinetic energies of its molecules. When you add heat to a system or do work on it, you change its internal energy.

Conversely, when a system does work or releases heat, its internal energy decreases. Internal energy is also measured in Joules ($J$). Understanding these three distinct yet interconnected concepts is crucial for grasping the First Law of Thermodynamics and solving related problems.