Thermodynamics Work from pressure

• Logan McEntire
In summary, the conversation discusses the determination of work required to pump air into bicycle tires at a specific pressure and volume. It suggests using the simplified formula T=PV/nR and the pressure ratio P1/P2 =V2/V2 to solve for the work per pound mass. This can be done without knowing the value of n.
Logan McEntire

Homework Statement

The tires on a bicycle require an air pressure of 80 psig. When isothermally pumped up the bicycle tires, the volume of the air (that was originally in the atmosphere) is reduced by a factor of 5.7. Please determine the work that must be done on each lbm of air that is pumped into the tires. Please report your answer in lbf-ft/lbm.
.

W=nRT*ln(V2/V1)
Pv=nRT

The Attempt at a Solution

My attempt might be very wrong, but this is what i tried.
I simplified Pv=nRT to T=PV/nR
Plugged that into W=nRT*ln(V2/V1)
W=PV*ln(V2/V1)
I know the standard pressure (14.7psi) and the pressure inside the tire (80psig+14.7psi=94.7psi), but I don't know the volume of to plug in for the V.

Use nRT, with T equal to room temperature.

What do I use for n?

For isothermal compression P1/P2 =V2/V2, instead of using volume, you can use pressure ratio

Logan McEntire said:
What do I use for n?
You are trying to get the work per pound mass. So you don't need to know n. You are solving for W/n.

1. What is the definition of work in thermodynamics?

In thermodynamics, work is defined as the transfer of energy from one system to another due to the application of a force. In other words, it is the process of converting one form of energy into another, such as converting mechanical energy into heat or vice versa.

2. How is work related to pressure in thermodynamics?

In thermodynamics, work is closely related to pressure. Pressure is defined as the force per unit area, and work is the product of force and displacement. Therefore, when pressure is applied to a system, work is performed as the system undergoes a change in volume.

3. What is the formula for calculating work from pressure in thermodynamics?

The formula for calculating work from pressure in thermodynamics is W = -PΔV, where W is work, P is pressure, and ΔV is the change in volume. This formula applies to both gases and liquids, as long as the pressure remains constant during the process.

4. Can work only be done by a gas in thermodynamics?

No, work can be done by both gases and liquids in thermodynamics. However, the formula for calculating work from pressure only applies to cases where the pressure remains constant. In other cases, such as a change in pressure, different formulas must be used to calculate work.

5. How does work from pressure affect the internal energy of a system?

Work from pressure can either increase or decrease the internal energy of a system. If work is done on a system, its internal energy will increase, while work done by a system will decrease its internal energy. This is because work is a form of energy transfer, and any change in energy will result in a change in the internal energy of the system.

Replies
116
Views
5K
Replies
1
Views
778
Replies
3
Views
2K
Replies
4
Views
1K
Replies
4
Views
1K
Replies
2
Views
1K
Replies
3
Views
555
Replies
2
Views
2K
Replies
3
Views
1K
Replies
16
Views
2K