# Question about electrical power compared to mechanical power

• leifh

#### leifh

I am new here and I am seeking for help:) Thank you so much.
In engineering, electrical formula often analogy to mechanical formula.
eg. power[W] = pressure x volume flow rate vs power[W] = voltage x current
My question is could we make the simplest unit between electrical and mechanical?

Reference: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html#c1

1. mechanical pressure [N/m2] = voltage pressure [V]
1a. P=F/A=FR[resistance] ; V=IR
1b. Power[W]= force x velocity = force x distance/time; Could make analogy to electrical?
1c. Energy[J] = pressure x volume = pressure x area x distance;

2. volume flow rate[m3/s] = current[A]/[C/s]
2a. volume flow rate = area[m2] x velocity[m/s] current to ?
For 1a. where force = current = volume flow rate?

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What would you do with thermal, chemical and nuclear energy?

Analogies, if they are taken too far, lead to trouble. Use analogies sparingly, not desperately.

• sophiecentaur, leifh, DaveE and 1 other person
Welcome to PF.

The parallels in the equations for fluids and electricity, are present at the beginning, but they fail when you get deeper into the subject, and so will lead you astray. For that reason, once you have discovered them, they are best avoided.

You might take a look at dimensional analysis, to see how that explains the parallels and analogies you see.
https://en.wikipedia.org/wiki/Dimensional_analysis

• sophiecentaur, leifh and DaveE
The fact that the unit for power is the Watt for both isn't a coincidence. The unit system was designed that way. If that's what you are asking...

• leifh and sophiecentaur
My question is could we make the simplest unit between electrical and mechanical?
There are dozens of linear equations that describe different systems but it's only the form of Maths that they have in common.

You clearly know about the Watt so why isn't that enough? That Hyperphysics page shows an analogy. The mechanical / hydraulic model should be viewed with care as an electrical analogy. Voltage is not Pressure or Force. Personally, I would question whether they should have included it in what, in most other respects, is an excellent source of Physics help.

• leifh
Thank you all so much! Really appreciated. I was thinking some analogy between electrical and mechanical equation so trying to make the simplest and deepest unit or element for them by analogy mindset. It seems not correct to deepest.

• sophiecentaur
I was thinking some analogy between electrical and mechanical equation
I have to come clean about this and I think I must be just like a lot of Engineers / Physicists. For my personal use, I have many quirky analogies - some of them involving bodily movements - but I wouldn't dream of inflicting them on anyone else - certainly not a student. Most analogies really don't translate well from brain to brain.

• • nsaspook and Averagesupernova
The key to any anology is to know when it is no longer useful. If a person is approaching unfamiliar territory and is beginning to become confused, that might be the clue to question whether their old trusty way of understanding something by way of anology is any longer useful. A very sharp engineer once told me that the first thing to do when something isn't making sense is to ask yourself: "What am I doing wrong?" That could include the way it's thought about.

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• nsaspook and russ_watters
The key to any anology is to know when it is no longer useful. If a person is approaching unfamiliar territory and is beginning to become confused, that might be the clue to question whether their old trusty way of understanding something by way of anology is any longer useful. A very sharp engineer once told me that the first thing to do when something isn't making sense is to ask yourself: "What am I doing wrong?" That could include the way it's thought about.

My standard analogy usage warning.
1st year electrical apprentice: "Now tell me again, how does that voltage and current stuff work?"

Electrical instructor: "It's simple. Just think of it like water."

Meanwhile, over at the plumbers apprentice school:

1st year plumbing apprentice: "Now tell me again, how does that pressure and flow stuff work?"

Plumbing instructor: "It's simple. Just think of it like electricity."

• • leifh and anorlunda