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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Work is F*d
I have a relief valve. There is a gas inlet. The gas hits a plate that is against a spring. When the pressure is high enough the spring compresses, relieving the pressure. Is work being done by the gas while the relief plate stays in one place? I know there is work during the spring compression, but what about the gas pressure required to maintain the spring in a state of compression?
I know this is a classical physics question, but I have forgotten how to resolve this
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
If the spring is being maintained in a state of compression and not changing length no work is being done on the spring.
During the compression work was done on the spring. During the compression the spring was doing negative work on the cylinder.
You can calculate that amount of work by doing an integral of the force over the distance it was compressed. In that case the force with vary according to the equation F = kx and the limitis on the integral are the start and ending length of the spring.
An easier way to calculate the work done would also be to find the total length the spring was compressed and then find the potential energy in the spring using pE = .5kx^2. The potential energy in the spring after compression would be the amount of work done on the spring.
The gas pressure required to maintain the spring in a state of compression just simply has to provide the same amount of force that the spring provides. The spring will provide kx amount of force and the gas pressure will provide PA amount of force. (pressure times area) If those two quantities are equal then nothing moves.
After the compression stops no work is being done on the spring and the spring isn't doing work on the cylinder.
If the spring is not changing length the force exerted on the spring is equal to the force exerted by the spring.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. French Toast Mafia wrote ...
If the spring is being maintained in a state of compression and not changing length no work is being done on the spring.
During the compression work was done on the spring. During the compression the spring was doing negative work on the cylinder.
You can calculate that amount of work by doing an integral of the force over the distance it was compressed. In that case the force with vary according to the equation F = kx and the limitis on the integral are the start and ending length of the spring.
An easier way to calculate the work done would also be to find the total length the spring was compressed and then find the potential energy in the spring using pE = .5kx^2. The potential energy in the spring after compression would be the amount of work done on the spring.
The gas pressure required to maintain the spring in a state of compression just simply has to provide the same amount of force that the spring provides. The spring will provide kx amount of force and the gas pressure will provide PA amount of force. (pressure times area) If those two quantities are equal then nothing moves.
After the compression stops no work is being done on the spring and the spring isn't doing work on the cylinder.
If the spring is not changing length the force exerted on the spring is equal to the force exerted by the spring.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
IF there was no spring, i.e. the enclosure is rigid potential energy would be added to the enclosure due to the stored compressed gas is no work done?
Reminds me, I took into work some N52 magnets and some graphite foil (an earlier listing of eBay item 160934517535 ) to demonstrate diamagnetic levitation, TWO of my engineer colleagues were certain that it was a demonstration of (please forgive me for mentioning it) 'free energy' because a continuous force was operating ... D'oh !
P.S. graphite foil floats almost as well as the much more expensive pyrolytic graphite.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
e.g. part of a tank of compressed air allowed to expand into the enclosure would result in stored compressed air (potential energy) at LOWER temperature, is no work done ?
e.g. part of a tank of compressed air allowed to expand into the enclosure would result in stored compressed air (potential energy) at LOWER temperature, is no work done ?
The total amount of energy in the air, i.e. in the tank and enclosure, stays the same as long as there are no moving surfaces in both. The temperature in the tank would be lower, as you say. The temperature of the air in the enclosure would be higher (after the energy in the air flow is thermalized). In a way, the flow in the connecting tube can be viewed as a receding surface from the point of view of the tank, which is what causes the lowering of the temperature in it.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
This sounds like a steam a blow-down valve. The steam pressure against the open valve does work to compress the spring. The quantity of work is the rate at which it is compressed , while opening, over the distance it moves.
The escaping steam does no work to hold the valve open, however it does work by heating the orifice as it rushes around the stationary blow-down ring.
The spring does work while it closing, against the escaping steam, which volume is reduced as it closes.
It sounds like a cannon when it opens and like a jet engine until it closes.
Opininion of a lowly 4th class steam engineer listening to these bloody things for 20 years.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Work is being done by the gas when it's moving, escaping through gaps, but against a) the atmosphere, pushing it back reversibly (isentropically) b) in the entropic (non-reversible) dissipation in noise, turbulence etc c) against its internal energy (Van der Waals forces, non-ideal gas)
but no work is being done on the spring plate when the plate is not moving
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Is work being done?
