Heat and work are two different ways of transferring energy from one system to another. The the distinction between Heat and Work is important in the field of thermodynamics. Heat is the transfer of thermal energy between systems, while work is the transfer of mechanical energy between two systems. This distinction between the microscopic motion (heat) and macroscopic motion (work) is crucial to how thermodynamic processes work. Heat can be transformed into work and vice verse (see mechanical equivalent of heat), but they aren't 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 amount of heat that can be turned into work.

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 they differ in how they affect a system. These are listed and compared below:[1]

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.

For Further Reading

References

  1. R. D. Knight, "Work in Ideal-Gas Processes" and "Heat" in Physics for Scientists and Engineers: A Strategic Approach, 3nd ed. San Francisco, U.S.A.: Pearson Addison-Wesley, 2008, ch.17, sec.2 and 3, pp.471-477