1. What Is the First Law of Thermodynamics?
The First Law of Thermodynamics states that energy cannot be created or destroyed; it can only be changed from one form to another. In thermodynamics, this means that the heat supplied to a system goes into doing work or increasing the system’s internal energy.
It is essentially the law of conservation of energy applied to heat processes.
2. Mathematical Form of the First Law
The First Law is expressed as:
\( \Delta Q = \Delta U + W \)
Here, the heat added to the system is used to increase internal energy and to do work.
2.1. Meaning of Terms
- \( \Delta Q \): heat supplied to the system
- \( \Delta U \): change in internal energy
- W: work done by the system on the surroundings
2.2. Alternate Sign Convention
In some contexts, work done on the system is taken as positive. The equation is then written as:
\( \Delta U = \Delta Q - W \)
3. Understanding the First Law
The First Law explains how heat, work, and internal energy are related. When heat is given to a system, one of two things—or both—can happen:
- The internal energy increases
- The system does work on the surroundings
Similarly, removing heat causes a decrease in internal energy or requires work to be done on the system.
3.1. Heat Added vs. Heat Removed
Adding heat: the system becomes hotter or expands and does work.
Removing heat: the system cools down, or work must be done to compress it.
4. First Law in Different Thermodynamic Processes
The relation \( \Delta Q = \Delta U + W \) takes different forms depending on the type of process. These help understand how heat and work are balanced in real situations.
4.1. Isothermal Process
Temperature remains constant, so internal energy does not change (\( \Delta U = 0 \)).
\( \Delta Q = W \)
4.2. Isochoric Process
Volume remains constant, so no work is done (\( W = 0 \)).
\( \Delta Q = \Delta U \)
4.3. Isobaric Process
Pressure remains constant. Some heat increases internal energy and some does work as the gas expands.
4.4. Adiabatic Process
No heat exchange (\( \Delta Q = 0 \)). So:
\( \Delta U = -W \)
The internal energy decreases when the system does work.
5. Examples from Daily Life
- When air is pumped into a tyre, the pump becomes warm because work done on the air increases its internal energy.
- Steam engines convert heat from burning fuel into work done by the piston.
- A refrigerator uses work from the compressor to remove heat from inside.
- When a gas expands freely without doing work, its temperature drops due to decrease in internal energy.
6. Limitations of the First Law
The First Law tells us how energy is conserved but not how or why certain processes occur in one direction but not the reverse. For example, it does not explain why heat naturally flows only from hot to cold or why 100% efficient engines are impossible. These ideas are part of the Second Law.