First Law

First Law of Thermodynamics

‣ The first law of thermodynamics states that the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system. This law is based on the principle of conservation of energy: energy can neither be created nor destroyed, but it can be transferred and converted from one form to another.

Mathematically: ΔU = Q – W

ΔU – Change in internal energy
Q – Heat added to the system
W – Work done by the system

Sign Conventions:

Heat (Q) is positive when added, negative when removed
Work (W) is positive if done by the system, negative if done on the system
ΔU > 0: Internal energy increases
ΔU < 0: Internal energy decreases
ΔU = 0: Internal energy is constant (cyclical process)

Real-life Example:

The human body is a thermodynamic system.

When you eat, your body gains heat (Q) through metabolism.
If you're resting and doing less work (W), internal energy increases: ΔU > 0.
If you exercise, you do more work and lose heat via sweat, reducing internal energy: ΔU < 0.

Summary of Cases:

ΔU = Q – W
If Q = W ⇒ ΔU = 0 (Cyclical process)
If Q > W ⇒ ΔU > 0 (Internal energy increases)
If –Q – W ⇒ ΔU < 0 (Internal energy decreases)

Practice Problems