## FIRST LAW OF THERMODYNAMICS

This law is the simplest and one of the most important statements of Thermodynamics.
It
says: “Energy can neither be created nor destroyed.”

**∆**

**U = q + w**

This is the mathematical statement
of First law. Here the signs of q and w become very significant. For this we
have proper sign conventions to be used.

**Sign Conventions for First law**

We will use very simple and
straight forward sign convention. Any change that tends to increase the
internal energy of the system is taken as positive and any change which tends
to decrease the internal energy is negative. That is, heat supplied to the system
and work done on the system by the surroundings are both positive and vice versa.

For example, if 200 J work is done
on the system by pushing the piston inside and 100 J heat is withdrawn from the
system then:

**∆**

**U = q + w = – 100 + 200 = + 100 J**

Internal energy will increase by
100 J in the process.

**“Any energy given to the system is positive and any energy withdrawn from the system is negative.”**

*The sign convention in thermodynamics: w and q are positive if energy enters the system (as work and heat, respectively), but negative if energy leaves the system.*

Sign convention for work done can
be best remembered by observing the volume change.

If volume of the system decreases
positive work is done on the system and if volume increases then negative work
is done.

Back to chemistry notes

Back to chemistry notes