Physics Lesson 13.5.4 - Work Done on a System and Work Done by a System

Please provide a rating, it takes seconds and helps us to keep this resource free for all to use

★★★★★ [ 3 Votes ]

Welcome to our Physics lesson on Work Done on a System and Work Done by a System, this is the fourth lesson of our suite of physics lessons covering the topic of The First Law of Thermodynamics, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Work Done on a System and Work Done by a System

If the piston moves up or down, there is some work done, as the piston has a certain weight, which is equal to the minimum force needed to make it move. (It is known that Work = Force × Displacement).

Although work is a scalar quantity (no direction involved), in this topic it is appropriate to assign a direction to it, in order to understand what is happening to the gas inside the cylinder.

Let's consider a gas in a frictionless cylinder-piston system being slowly heated. The increase in temperature causes the piston to move up slowly, while the pressure remains constant. The gas is expanding against external forces, which are the weight of the piston and the force caused by atmospheric pressure.

Physics Tutorials: This image provides visual information for the physics tutorial The First Law of Thermodynamics

Since the process is slow, we can write

|F_{exerted by the gas on the piston} | = |F_{exerted on the gas by the piston} |

When the piston is displaced by Δx, we have

W_{done by the gas on the piston} = -W_{done on the gas by the piston}

F_{exerted by the gas on the piston} × ∆x = -F_{exerted on the gas by the piston} × ∆x

Thus, we can write:

when the gas is expanding, W_{done by the gas} is positive and W_{done on the gas} is negative

when the gas is contracting, W_{done by the gas} is negative and W_{done on the gas} is positive

You have reached the end of Physics lesson 13.5.4 Work Done on a System and Work Done by a System. There are 6 lessons in this physics tutorial covering The First Law of Thermodynamics, you can access all the lessons from this tutorial below.

More The First Law of Thermodynamics Lessons and Learning Resources

Thermodynamics Learning Material
Tutorial ID	Physics Tutorial Title	Revision Notes	Revision Questions
13.5	The First Law of Thermodynamics
Lesson ID	Physics Lesson Title	Lesson	Video Lesson
13.5.1	Useful Definitions in Thermodynamics
13.5.2	Internal Energy of a Gas
13.5.3	Giving Energy to a Thermodynamic System
13.5.4	Work Done on a System and Work Done by a System
13.5.5	The First Law of Thermodynamics
13.5.6	Special Cases of the First Law of Thermodynamics

Whats next?

Enjoy the "Work Done on a System and Work Done by a System" physics lesson? People who liked the "The First Law of Thermodynamics lesson found the following resources useful:

Work Feedback. Helps other - Leave a rating for this work (see below)
Thermodynamics Physics tutorial: The First Law of Thermodynamics. Read the The First Law of Thermodynamics physics tutorial and build your physics knowledge of Thermodynamics
Thermodynamics Revision Notes: The First Law of Thermodynamics. Print the notes so you can revise the key points covered in the physics tutorial for The First Law of Thermodynamics
Thermodynamics Practice Questions: The First Law of Thermodynamics. Test and improve your knowledge of The First Law of Thermodynamics with example questins and answers
Check your calculations for Thermodynamics questions with our excellent Thermodynamics calculators which contain full equations and calculations clearly displayed line by line. See the Thermodynamics Calculators by iCalculator™ below.
Continuing learning thermodynamics - read our next physics tutorial: The Kinetic Theory of Gases. Ideal Gases

Help others Learning Physics just like you