electrons per AMP ?

[Besoeker]

The formula, e = -L(di/dt), computes the voltage induced in an inductor, L, by a change in current. di/dt is this change expressed as a rate with the units of amperes/second. The negative sign indicates that the induced voltage opposes the change in current.

Now, I will venture to say that few if any power engineers ever use this formula--and even fewer electricians will do so.

I do so routinely.
Much related to power electronics requires it.

 

[rattus]

The Few:

The Few:

I do so routinely.
Much related to power electronics requires it.

Then, you are one of the few.

 

[gar]

090919-2132 EST

I tend to talk about e = N d phi/dt because it helps to show what happens in the excitation of a ferromagnetic core where L is not constant, or the induced voltage in a motor, or swiping a credit card.

.

 

[mlnk]

Of course moving particles also have a wave component. An example of this is when you get shot by a bullet. There is the damage done by the mass of the bullet and the damage done by the waves created by the impact of the bullet.
I still do not understand all the criticism about "per"
If I drink one glass of water per hour, I am not saying that I drink one glass of water divided by an hour. "per" is an English word with many definitions. All language cannot be converted into math formulas.
I also stated that we use the word amp by itself. The per hour is not stated, it is understood.
I still do not see the problem if someone from the basic electricity class takes a physics class.

 

[rattus]

I still do not understand all the criticism about "per"
If I drink one glass of water per hour, I am not saying that I drink one glass of water divided by an hour. "per" is an English word with many definitions. All language cannot be converted into math formulas.
I also stated that we use the word amp by itself. The per hour is not stated, it is understood.
I still do not see the problem if someone from the basic electricity class takes a physics class.

Mink,

I don't understand why one should understand "per hour" to be attached to a unit of current. if you are saying one ampere is equivalent to a charge flow of 3600 coulombs/hour, you are correct. If you are saying something else, you need to give us an example of the way an electrician would use amperes/hour.

I think one should take basic physics before taking basic electricity. There is no conflict.
To the contrary, they essentially cover the same material.

 

[Mayimbe]

Of course moving particles also have a wave component. An example of this is when you get shot by a bullet. There is the damage done by the mass of the bullet and the damage done by the waves created by the impact of the bullet..

This is absolutly NOT an example of the wave/particle duality. You can find it in wikipedia easily to clear your doubts. You'll be doing yourself a favor.

I still do not see the problem if someone from the basic electricity class takes a physics class.

What do you mean by that?
In my experience, I needed 2 years of physics and calculus courses in order to begin my study in electricity.

 

[winnie]

I still do not understand all the criticism about "per"
If I drink one glass of water per hour, I am not saying that I drink one glass of water divided by an hour. "per" is an English word with many definitions. All language cannot be converted into math formulas.

I believe that you are getting lots of criticism because you are using 'per hour' in a technical setting in a way that is different from its technical meaning.

Language is not dead, 'per' has more meanings then its technical meaning, and not everything can be expressed in equations. However when 'per' is used with units of measure, it has a _specific_ technical meaning, so any casual usage that doesn't fit the technical meaning is going to stick out like a sore thumb.

When you express rates, you always take <quantity> divided by <time>.

Most of the rate units that we deal with don't have special names, instead we say explicitly 'quantity unit per time unit', for example miles per hour, gallons per minute, meters per second, etc. This usage is the specific technical meaning of 'per'.

A few rate units have specific names, and for historic reasons many electrical units are actually rate units. Amp, watt, lumen, curie, etc. are all rate units. The unit itself already includes the concept of some <quantity> per <unit time>. An amp already includes the concept of some number of electrons per second, a watt already includes the concept of so much work done per second, a lumen includes the concept of so many visible photons per second, and the curie so many radioactive decays per second.

Perhaps it would be simpler if we didn't give these units special names, and instead said 'coulombs per second' for amps, 'joules per second' instead of watts, or 'becquerels per second' rather than curies...but we are stuck with the historic fact of these 'rate units'.

I get that you are using 'amps per hour' in a casual sense to convey a quantity. But this usage really is a problem because you are using something that has a specific and known technical meaning in a casual fashion that distorts that specific technical meaning.

The correct way to take a rate unit and get back to a quantity is to multiply. In this way you have divided by a time unit and then multiplied by a time unit, to get back to a simple quantity unit. 1 'amp hour' is a quantity unit, meaning 3600 coulombs.

When you divide twice by time, you have an acceleration unit. Miles per hour per hour doesn't say how far you go; it says how quickly you are changing speed. Because amps are already rate units, and already include the concept of <quantity> /<time>, 'amps per hour' is an acceleration unit.

Imagine that we had a named rate unit for speed. Heck, lets make one up, and define a 'cugnot' to be a speed of 1 mile per hour. So instead of saying '60 miles per hour', we could say '60 cugnots'. With this rate unit defined, a cugnot hour is one mile, and cugnots per hour is a measure of acceleration, equal to about 1.47 feet per second per second, or about 0.046 G.

I also stated that we use the word amp by itself. The per hour is not stated, it is understood.
I still do not see the problem if someone from the basic electricity class takes a physics class.

Only that by saying amps per hour you are confusing a rate unit with a quantity unit, leading to something that will need to be unlearned later.

-Jon

 

[steve66]

I still do not understand all the criticism about "per"
If I drink one glass of water per hour, I am not saying that I drink one glass of water divided by an hour.

As a contractor, you can probably get by with using "per" as a more generic term.

However, in engineering and physics and sciences, there is something called "dimensional analysis". That means almost all numbers get units attached to them. And "per" does specifically mean division. In you "one glass of water per hour" example, the units would actually be glasses divided by hours.

Then, if we wanted to know how many glasses of water you drank in an 8 hour day, we would multiply 1 glass/hour * 8 Hours. We would wind up with glasses * hours/hours. The hours would cancel out (since anything divided by itself is equal to one). We would then be left with "glasses" for the units that go with our answer.

 

[mivey]

If I drink one glass of water per hour, I am not saying that I drink one glass of water divided by an hour. "per" is an English word with many definitions. All language cannot be converted into math formulas.

Then please explain what you mean by saying you drink one glass of water per hour. To most of the English-speaking world that would mean you are drinking one glass of water every hour. Not sure how else you could spin that. There are three main definitions:

1 : by the means or agency of : through <per bearer>
2 : with respect to every member of a specified group : for each
3 : according to ?often used with as<per instructions><as per usual>

You are not using it per #1 because the hour is not the agent of delivery. You are not using it per #2 because you have already said you are not talking about acceleration. You are not using it per #3 as I have done in these sentences. You are just mis-using it, so stop trying to spin it into something that is acceptable unless you can elaborate on something that has been overlooked.

It is OK to admit you are wrong. We already know it, so just unburden yourself already.:grin:

As a contractor, you can probably get by with using "per" as a more generic term.

I doubt it, unless you are talking to the unlearned or those that just went through mlnk's class. Please don't encourage the spreading of this nonsense by saying he can get by with some generic use. Generic for what?

I find it hard to believe a student has never called mlnk's hand on this poor use of terms. Must be some slim pickings for students or maybe an intimidated group at best.

 

[rattus]

Even in the vernacular, "amperes/hour" has no meaning to me. In the technical world, it means the rate of change of current--cbs/sec/sec.

Now, amps/sec and volts/sec are related to the rise times of current and voltage pulses, but we are concerned by and large with steady state AC values, not transient solutions. Certainly, Mink is not using the term in this sense.

To paraphrase Gertrude Stein, an amp is an amp is an amp--not an amp per hour!

 

[VUGear]

Imagine that we had a named rate unit for speed. Heck, lets make one up, and define a 'cugnot' to be a speed of 1 mile per hour. So instead of saying '60 miles per hour', we could say '60 cugnots'. With this rate unit defined, a cugnot hour is one mile, and cugnots per hour is a measure of acceleration, equal to about 1.47 feet per second per second, or about 0.046 G.

Actually, we already have named rate units for speed. One is the "knot", which is equal to 1 nautical mile per hour. Thus, like you say for cugnots, no one says that a boat travels at a speed of "30 knots per hour"; they simply say it travels at a rate of "30 knots".

Good post.