Heat vs work

The concept of Heat vs Work is an important one in the field of thermodynamics. The idea of how heat and work are distinct but interchangeable forms of energy is crucial to how thermodynamic processes work. Although heat can be transformed into work and vice verse (see mechanical equivalent of heat), they aren't necessarily the same thing. The first law of thermodynamics states that heat and work both contribute to the total internal energy of a system, but the second law of thermodynamics limits the relationship between work and heat.

Main Differences

• The Second Law allows work to be transformed fully into heat, but forbids heat to be totally converted into work. If heat could be transformed fully into work it would violate the laws of entropy. The maximum amount of work one can attain from heat is given by the Carnot efficiency.

• Heat is the energy associated with the random motion of particles, while work is the energy of ordered motion in one direction. Therefore heat is "low-quality" energy and work is "high-quality" energy, and this supports the Entropy statement of the Second Law.

Heat and work each have their own distinct properties, and how they differ in how they affect a system. These are listed and compared below:

	Work (W)	Heat (Q)
Interaction	Mechanical	Thermal
Requires	Force and Displacement	Temperature difference
Process	Macroscopic pushes and pulls	Microscopic collisions
Positive value	W > 0 when a gas is compressed. Energy is transferred into system.	Q > 0 when the environment is at a higher temperature than the system. Energy is transferred into system.
Negative value	W < 0 when a gas expands. Energy is transferred out of system.	Q < 0 when the system is at a higher temperature than the environment. Energy is transferred out of system.
Equilibrium	A system is in mechanical equilibrium when there is no net force or torque on it.	A system is in thermal equilibrium when it is at the same temperature as the environment.