Intro to The 1st Law of Thermodynamics Reference Page
Just Try to Break This Law! I Dare You!

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Intro to The 1st Law of Thermodynamics

The First Law of Thermodynamics is the statement of energy conservation. It is one of the most important ideas in all of engineering, because it governs how energy enters, leaves, and is stored in a system.

In chemical and thermal process engineering, the First Law is used to analyze heating, cooling, compression, expansion, pumping, turbines, reactors, and many other systems. The video on this page introduces the law, defines the major forms of energy used in engineering calculations, and explains how those forms appear in balances.

Just Try to Break This Law! I Dare You!

This video introduces the First Law of Thermodynamics and explains the main energy terms used in engineering analysis, including heat, work, kinetic energy, potential energy, internal energy, and enthalpy.

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Examples and Definitions

Definitions

The First Law of Thermodynamics
The general balance equation applied to energy, assuming that relativistic effects and nuclear reactions are negligible.
Work, \(W\)
Energy transferred across a system boundary by a force acting through a distance or by other non-thermal means. Electrical energy crossing a system boundary is usually treated as work.
NoteWork Sign Convention

In this site we use the convention that work done on a system is positive and work done by a system is negative, as explained in the video
Just Try to Break This Law! I Dare You!.

Heat, \(Q\)
Energy transferred across a system boundary because of a temperature difference.
Potential Energy, \(E_p\)
Energy associated with position in a conservative field. For these videos, the conservative field is gravity, so

\[ E_p = mgh \]

where \(h\) is the height above a reference datum.

Kinetic Energy, \(E_k\)
Energy associated with motion in an inertial reference frame:

\[ E_k = \frac{1}{2} m u^2 \]

where \(u\) is the velocity.

Internal Energy, \(U\)
The sum of all microscopic kinetic and potential energies possessed by the atoms and molecules in a system.
Enthalpy, \(H\)
A thermodynamic property defined as

\[ H = U + PV \]

where \(U\) is internal energy, \(P\) is absolute pressure, and \(V\) is volume. Enthalpy is especially useful for analyzing open systems and processes that occur at approximately constant pressure.

Intensive Property
A property that does not depend on the amount of material present, such as temperature \(T\), pressure \(P\), density \(\rho\), or composition \(x\).
Extensive Property
A property that does depend on the amount of material present, such as volume \(V\), mass \(m\), or total energy \(E\).

It is often useful to define intensive quantities related to extensive ones. These are specific or molar properties and are often indicated by a circumflex, for example specific volume, \(\hat{V} = V/m\), specific internal energy, \(\hat{U} = U/m\), or molar enthalpy, \(\hat{H} = H/n\).