# Energy for pressure equalibrium

• Mechanic
In summary: It's more like the truck is driving on a incline and the brake applies the brakes to the wheels to slow the truck. So it's not really about consuming energy, it's more about using energy to achieve a goal.In summary, a Jake brake is a device that uses compression of the air in the cylinders to slow a truck.
Mechanic
Consider a gas enclosed in a simple piston. Imagine that some type of engine is used to push in the piston until equilibrium is reached – the pressure of the gas exactly countering the pressure/force exerted by the engine. The well known equation “F dot d” describes how work/energy is provided by the engine while the piston is moving the distance d, but how do we describe the energy exerted by the engine to maintain equilibrium? If the engine is turned off the piston will be pushed back out so energy is required to keep the piston from being pushed out. How do we mathematically describe the energy consumption of the engine once equalibrium is reached?

Thanks

At that point you have a situation where there is no output of work, so your device has 0% efficiency. When you are in a car on a hill, you could use the clutch and engine to hold the car stationary on the hill (expending energy and producing none), or you could just put on your brakes. The device you describe could simply have a brake.

Russ - I agree, I could "put on the brakes". But that is not my question. My question is "How do I mathematically describe the energy required (consumed by the engine) to maintain equalibrium?"

You can't. It could literally be anything since the efficiency is zero. Efficiency is always energy out/energy in. so you have 0=0/x. x can be anything, depending on the particulars of the machine.

Not sure I follow. I'll be able to measure/observe the fuel consumption of the engine so I should be able to mathematically modelthe rate at which that fuel/energy is consumed.

Mechanic said:
Not sure I follow. I'll be able to measure/observe the fuel consumption of the engine so I should be able to mathematically modelthe rate at which that fuel/energy is consumed.
You can, of course, measure the input power to the engine, but when you said "mathematically describe", I assumed you meant "calculate". But sure: fuel has a certain energy content, so the flow rate times the energy content is the input energy. My point was that it would be very difficult (near impossible) to predict ahead of time what that energy input (fuel flow rate) is going to be.

I would think that with the piston and engine in equalibrium, and the engine consuming fuel/energy, I could somehow calculate the rate of energy consumption based on measured pressure in the piston and dimensions of the piston. Is this braoching the subject of "virtual work"? Maybe that should be another thread. I've never really understood the differences between normal everyday work and virtual work. Virtual work just seems like regular work only on a really small scale?

Mechanic said:
I would think that with the piston and engine in equalibrium, and the engine consuming fuel/energy, I could somehow calculate the rate of energy consumption based on measured pressure in the piston and dimensions of the piston.
No, all that gives you is force. Force and energy are not directly related, so having the force tells you nothing about what the energy consumption might be.
Is this braoching the subject of "virtual work"?
No. It's a real work of 0 (output).
Maybe that should be another thread. I've never really understood the differences between normal everyday work and virtual work. Virtual work just seems like regular work only on a really small scale?
No, it doesn't have anything to do with scale. Virtual work is just a mathematical tool. What you are looking for is the real work required to generate a force and like I said, there is no relation to be drawn between force and work like that.

Perhaps if I knew what your goal was here, I could better help you: are you trying to design/analyze a real device of some sort? What is it's purpose?

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QuantumPion said:
A Jake Brake is turning the crankshaft and slows the truck via compression of the air in the cylinders. It isn't a static situation like the one being described by the OP.

## 1. What is pressure equilibrium?

Pressure equilibrium is a state in which the pressure exerted by a fluid or gas is equal in all directions, resulting in no net force or movement.

## 2. How is energy involved in pressure equilibrium?

Energy is involved in pressure equilibrium through the conversion of potential energy to kinetic energy. When a fluid or gas is under pressure, it has potential energy stored in its molecules. When the pressure is released, this potential energy is converted to kinetic energy, causing the fluid or gas to expand and equalize the pressure.

## 3. What factors affect pressure equilibrium?

The factors that affect pressure equilibrium include temperature, volume, and the properties of the fluid or gas, such as its density and compressibility. Any changes in these factors can result in a change in pressure and disrupt the equilibrium.

## 4. How is pressure equilibrium used in practical applications?

Pressure equilibrium is used in various practical applications, such as hydraulic systems, pneumatic systems, and scuba diving. It is also essential in maintaining the balance of pressure in our atmosphere, which is crucial for the survival of living organisms.

## 5. What happens when pressure equilibrium is disrupted?

When pressure equilibrium is disrupted, there will be a pressure difference between two or more areas, resulting in a net force or movement. This can cause various effects, such as fluid flow, gas expansion, or changes in volume or temperature.

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