Pressure-volume work (formula for work) (article) | Khan Academy
Work is the energy required to move something against a force. Work done by gases is also sometimes called pressure-volume or PV work for reasons that .. The relationship between work, heat, and other forms of energy transfer is further . He says "This area is completely different to the one above", but this means nothing. .. Is Pressure of a perpendicular Force applied on the Area of a surface . In this lesson, learn about the relationship between force, area, and pressure, and how changes in pressure can have some serious consequences!.
Different versions of the work equation can be used depending on the type of force involved.
Some examples of doing work include: A person lifting books from the ground to a shelf does work against gravity. A battery pushing electrical current through a circuit does work against resistance.
A child pushing a box along the floor does work against friction. In thermodynamics, we are mainly interested in work done by expanding or compressing gases.
Work done by a gas Gases can do work through expansion or compression against a constant external pressure. Work done by gases is also sometimes called pressure-volume or PV work for reasons that will hopefully become more clear in this section!
Let's consider gas contained in a piston.
- Boyle's law
Pistons are actually used in many mechanical applications all around us, such as inside a gasoline engine. I usually imagine a piston looking very similar to a bicycle tire pump, but here's some more information about what real pistons look like in case you are interested!
Here is a picture of the insides of a gasoline engine that shows two pistons side-by-side: A picture of a petrol-engine that has been cut in half to show the cross section of 2 side-by-side pistons.
The shape of this profile-curve is called a negative exponential decrease compare Figures 1. The density profile behaves in a similar way.Difference between Force and Pressure
The pressure generated by an overlying column of fluid is thus termed the hydrostatic pressure. The upper boundary of the air column that gives rise to atmospheric pressure is the vacuum of space.
Being rather light, the mass of a column of air with a 1 cm2 cross section is almost exactly 1 kg. If a much heavier liquid substance is used to balance this air column, only a relatively small length would be needed. In addition, because the density of liquids does not change with height most liquids are incompressiblesuch an equivalent liquid column has a well defined upper boundary below a vacuumOne of the heaviest liquids at room temperature is mercury Hg and the height of the Hg-column that is equivalent to normal pressure mb is only mm long For this reason, columns of mercury, "hanging" in an inverted vacuum tube, can be used as practical instruments to measure atmospheric pressure see FigureLutgens and Tarbuck, If water were used instead of mercury, the height of the column equivalent to normal pressure would be The Gas Laws The example of the gas-filled balloon can also be used to explore the basic gas laws see also Appendix D, p.