Categories

# Definitions

## Thermodynamics

The science of transformation of energy.

## Energy

The capacity to do work or to transfer heat.

## Work

Work is one means of transferring energy from our system to the surroundings.  Work is given the symbol  in equations. Work has several aspects to it.  There is mechanical work, electrical work, light-energy work and work of expansion or contraction of a gas ( work).  In chemistry, we generally are only interested in  work and will use the symbol  to refer only to that type of work.  All other types of work will be referred to using a prime on the symbol, .

One of the most common definitions for work is when a force  displaces an object by a distance  does work .  (also known as ).  This is a mechanical work and serves as the basis for our definition of  work.

Work can also be done by pushing back the atmosphere (piston in a cylinder). In this case, one pushes back against the atmospheric pressure  and moves the piston a distance . The force needed to push back the atmosphere is actually  where  is the area of the piston. Hence, we can calculate the magnitude of the work done as

We call this atmospheric work.  The only thing left to decide is the sign convention. (Described later)  Since the system increased in volume and hence, did the work of pushing back the atmosphere (positive value for ΔV), it has used up energy so we can now write down a more complete definition for PV work as:

w = -Patm × ΔV

In chemical situations, we are normally interested only in the atmospheric work done when gases are evolved or used up in the reaction.

Kinetic and Potential Energy

Work is organized energy, lifting a book, etc.

Heat (defined later) is random (disorganized) energy that is released or absorbed, often as a result of some work being done, e.g., sound (energy) dissipates through air, when a book drops and hits the floor and the floor gets warmer (molecules vibrate more rapidly) spreading out from the spot where the hit occurred.  Thus, the work done to raise the book (increasing its potential energy) is converted to kinetic energy upon release and then to thermal energy (energy is released as heat) as the book strikes the floor.

One can also transfer energy into or out of a sample via light-energy and electrical work
(We include this type of work in our catchall term w’ ).SI units of energy (work or heat):Let’s start with work: w has dimension of force times distance.
The SI units are N m = kg m2 s-2 = Joules (J). These are the same units used for heat transfer.

## Heat

Heat is the Energy transferred between the system and the surroundings as a result of a temperature differential.  This is the only kind of energy transfer that does not involve doing work. Heat is given the symbol q in equations.