First Law of Thermodynamics
The change in internal energy equals heat added to the system plus work done on it: dU = q + w.
What is First Law of Thermodynamics?
The change in internal energy equals heat added to the system plus work done on it: dU = q + w.
Key formula / rule: Internal energy is a state function; q and w are path dependent.
Key points
- Apply the first law to compute internal energy change in a process.
- Classify quantities as state or path functions.
Common exam trap
Mixing sign conventions for work done by and on the system.
Definitions
- Term
First Law of Thermodynamics
- Meaning
The change in internal energy equals heat added to the system plus work done on it: dU = q + w.
- Term
First Law of Thermodynamics — explanation
- Meaning
Energy is conserved but can be exchanged as heat or work, and internal energy is a state function while heat and work are not. Sign conventions decide most numerical answers.
Learning objectives
Apply the first law to compute internal energy change in a process.
Classify quantities as state or path functions.
Formulae
- Key point
Internal energy is a state function; q and w are path dependent.
- Key point
For an isothermal ideal gas process dU is zero.
- Key point
Work of expansion against constant external pressure is -P dV.
Common mistakes
Mixing sign conventions for work done by and on the system.
Treating heat as a state function.
Keywords
First
Thermodynamics
Practice preview
For an isolated system, what is the change in internal energy (dU)?…
easy
In an adiabatic process, 500 J of work is done on the system. What is the change in internal energy (dU) of the system?…
medium
A system absorbs 200 J of heat and performs 150 J of work on the surroundings. What is the change in internal energy (dU) of the system?…
hard