Voltage drop question?

[Square D]

Gotcha. thanks for the input!

 

[LarryFine]

Sorry for the late reply , there has to be a way to reconcile those equations with I squared x R as they both provide the same number.

There is, but I think the simpler basic equations are easier to remember and use when testing.

 

[wwhitney]

E = I x R
P = E x I

Sorry for the late reply , there has to be a way to reconcile those equations with I squared x R as they both provide the same number.

to the OP, so what do you get algebraically if you substitute the first equation for E into the second equation?

Cheers, Wayne

 

[LarryFine]

There's no debate; the shortcut was given in post #2. My intent was to show why the formulae work.

 

[wwhitney]

There's no debate; the shortcut was given in post #2. My intent was to show why the formulae work.

That's right, I'm just trying to help the OP answer his own question.

Cheers, Wayne

 

[LarryFine]

The two basic equations I can remember. The ones with squares and square roots I can not.

In my opinion, once one learns the basics and how to use them, then learn to use shortcuts.

With this chart and a calculator, no problem. Sitting down with neither, I use the basic ones.

 

[kwired]

to the OP, so what do you get algebraically if you substitute the first equation for E into the second equation?

Cheers, Wayne

Yes, but we all seem to remember the E=IxR and P=IxE since was learned pretty early in basic electric studies, and is a base for nearly everything from there on.

 

[LarryFine]

Yes, but we all seem to remember the E=IxR and P=IxE since was learned pretty early in basic electric studies, and is a base for nearly everything from there on.

Exactly. I can explain how and why the two basic ones function with actual electrical properties.

The others, as I said, are merely math shortcuts, but you still have to square or sq-rt the given values.

Ohm's Law states that one volt is capable of forcing one amp through one ohm.

"Watt's Law" states that one volt across a load of one amp will produce one watt.

How do you combine the two verbally, using electrical properties instead of math terms?

 

[wwhitney]

Yes, but we all seem to remember the E=IxR and P=IxE since was learned pretty early in basic electric studies, and is a base for nearly everything from there on.

Certainly, and remembering those is enough, if you know how to combine them, either algebraically or computationally.

But the OP seemed to be asking how to "reconcile those equations with I squared x R." And so my answer was on how to do that algebraically. That's all. I didn't realize it would generate a lot of commentary.

Cheers, Wayne

 

[LarryFine]

I didn't realize it would generate a lot of commentary.

In this forum?!

 

[ptonsparky]

In this forum?!

We're lucky it's on track as it is.

 

[electrofelon]

gar, as Tom points out, the answer the test is looking for has been given, there is no need to confuse the OP with adding anything beyond what was asked.

This part of your analysis is in conflict with the information given in the question. The 40A measurement occurred while the appliance was already in series with the 0.5 ohm splice.

That is "An appliance draws 40 amperes when used on a circuit." is referring to the particular circuit further described, not some other idealized circuit. I.e. it means "There's a circuit. An appliance draws 40 amperes when used on that circuit. . . ."

Cheers, Wayne

Guys, I did not read the question that way. I fully agree with Gar here. It's a bad question and there is nothing wrong to point out the actual correct answer to the OP.

 

[wwhitney]

Guys, I did not read the question that way.

I don't see how it is possible to read "An appliance draws 40 amperes when used on a circuit. A bad splice is discovered on the line at the appliance on one of the phase conductors." as not meaning that the 40 amp measurement was taken with the bad splice in place. Care to expound?

That reads the same to me as "An appliance draws 40 amperes when used on a circuit, on which a bad splice is discovered . . ." Or "A circuit has a bad splice. An appliance on the circuit draws 40 amperes when used. . . ."

The last couple options are clearer, but I wouldn't say the original wording is unclear.

Cheers, Wayne

 

[electrofelon]

I don't see how it is possible to read "An appliance draws 40 amperes when used on a circuit. A bad splice is discovered on the line at the appliance on one of the phase conductors." as not meaning that the 40 amp measurement was taken with the bad splice in place. Care to expound?

That reads the same to me as "An appliance draws 40 amperes when used on a circuit, on which a bad splice is discovered . . ." Or "A circuit has a bad splice. An appliance on the circuit draws 40 amperes when used. . . ."

The last couple options are clearer, but I wouldn't say the original wording is unclear.

Cheers, Wayne

I am very sorry we dont agree. Similar to what bill Clinton said, it depends on what the definition of "a" is . I guess I read it that way because immediately upon reading the question, I knew the the bad connection itself would change the current draw of the appliance, and figured they were giving the "normal" draw.

 

[wwhitney]

I knew the the bad connection itself would change the current draw of the appliance, and figured they were giving the "normal" draw.

So I can agree that the question wording is not as clear as it could be. But I would say in the above you are reading something into the question that isn't there.

And of course as a meta issue, if you assume the 40A measurement is for "normal" operation without a bad splice, the question is unanswerable, as the supply voltage isn't given. So as a test question, the meaning has to be that the measurement is with the bad splice in place.

Cheers, Wayne

 

[electrofelon]

So I can agree that the question wording is not as clear as it could be. But I would say in the above you are reading something into the question that isn't there.

And of course as a meta issue, if you assume the 40A measurement is for "normal" operation without a bad splice, the question is unanswerable, as the supply voltage isn't given. So as a test question, the meaning has to be that the measurement is with the bad splice in place.

Cheers, Wayne

Ok fair enough. I would much prefer your suggested wording: "An appliance draws 40 amperes when used on a circuit, on which a bad splice is discovered . ."

 

[ptonsparky]

FWIW, I first figured as the 240 range drew 40 amps. Did the math for resistance of it and added a .5 ohm resistance in series. Then VD and wattage for both. Nothing was right. Backed up looked at what was given, and added where in the heck did I get range?

 

[electrofelon]

FWIW, I first figured as the 240 range drew 40 amps. Did the math for resistance of it and added a .5 ohm resistance in series. Then VD and wattage for both. Nothing was right. Backed up looked at what was given, and added where in the heck did I get range?

Of course it's a 240V range. What else would it be?

 

[LarryFine]

I think the context of the question should be taken into consideration here.

This is an entry-level electrician's test, not an advanced engineering exam.

Both arguments ave merit, but the OP just wants to become an electrician.

I'm wiling to bet a buck that 588w was not one of the test's answer options.

 

[Carultch]

Ohm's Law states that one volt is capable of forcing one amp through one ohm.

"Watt's Law" states that one volt across a load of one amp will produce one watt.

That's actually closer to being Joule's law. P is proportional to I^2*R is the original form of Joule's law, and it wouldn't be until a consistent set of units was developed (part of which other work of Joule contributed to as well), that it could be promoted to a proportionality to the equation we know and love today.

Watt was a steam engine pioneer, whose work was unrelated to electricity. He coined the horsepower unit to express engine performance in a form his customers could relate. The power unit that bears his name was coined after Watt's lifetime...and now I think in Watts a lot more than I think in horsepower.