Ohms law power problem

Bill Bowden · 2010-07-23T01:34:37+00:00

Using a 12 ohm load and 12 volt supply, what is the power gain whenthe voltage is raised to 12.1 volts?Considering the formula P=E^2/R the power at 12 volts will be 144/12 =12 watts. And, at 12.1 volts, the power will be 146.41/ 12 = 12.2watts, for a gain of 200 milliwatts.But if the current at 12 volts is 1 amp, and the current at 12.1 voltsis 12.1/12= 1.00833 then the increase is 8.33mA and from the powerformula of P=I*E we get .00833*12.1 = 101 milliwatts which is abouthalf as much as the first number.And,considering the increase in current, using the formula P=I^2 * Rwe get .00833^2 * 12 = 833 microwatts, which ain't much.So, which is correct A,B C ?I vote for A-Bill

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Joe 15 years ago

In article , Jon Kirwan

Sheesh... enough already.

The flame of interest can be extinguished by a flood of verbiage. --- Joe, MMX (get the water vs, fire imagery, huh? huh? :))

Or, as Einstein supposedly said,

"Everything should be made as simple as possible, but not simpler."

or as Einstein also supposedly said,

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."

--- Joe

PS: Beware of confusing (A-B)^2 with A^2 - B^2 and give hints to a neophye instead of a tidal wave of ... .

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Jon Kirwan 15 years ago

It sure can be. I suppose it's now up to Bill to say.

Jon

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Tom Biasi 15 years ago

After reading all the responses I got dizzy and I have been teaching this for 40 years. You made a math error. You seem capable of catching it. Check your work.

Tom

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Joe 15 years ago

Did that student finally learn it?

--- Joe

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Tom Biasi 15 years ago

Good one:-)

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Bill Bowden 15 years ago

=3D

I checked the work, but it doesn't add up.

a small change of 8.33mA and 100mV is only 833 microwatts when it should be 200 milliwatts.

I think an analogy might be a freeway traffic jam where you add one more car and everybody stops?

-Bill

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John Fields 15 years ago

Algebraically, you can't just use the change in quantity, you have to use the whole new quantity. JF

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Tom Biasi 15 years ago

I checked the work, but it doesn't add up.

a small change of 8.33mA and 100mV is only 833 microwatts when it should be 200 milliwatts.

I think an analogy might be a freeway traffic jam where you add one more car and everybody stops?

-Bill

You can't just take pieces of what you want. The current produced was

1.00833 Amps not .00833 Amps. The 8.33mA was in addition to the one Amp. Concept of math error. Keep pluggin' its fun.

Tom

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Joe 15 years ago

Okay, let's do this as I would in a decent algebra class:

Power is current x voltage.

Let's make this as a formula P = I*V

Better, from an algebraic understanding, is to write this in FUNCTION NOTATION (dammit!).

P(I, V) = I*V

Where you are confused is thinking that this FUNCTION has the property

P(I+i, V+v) = P(I,V) + P(i,v)

P(I+i, V+v) = (I+i)*(V+v)

Now, compare with :

Gives P(I+i, V+v) = (I+i)*(V+v) = (remember "FOIL") IV + Iv + iV + iv

Gives P(I+i, V+v) = (BOGUS) P(I,V) + P(i,v) = IV + iv which is WRONG!

gives two additional terms: Iv and iV

** NOTE: I never heard of "FOIL" as an acronym until a younger relative told me about it. Some of my students seem to cling to a rule rather than an understanding of what is happening (the distributive law). I sometimes mocked mindless rules by referring to the "FOIL" situation as "Leo Rio". Which actually stood for "LIORIO" (my own "Left Inner Outer Right Inner Outer" Roole). :)

--- Joe

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ehsjr 15 years ago

In other words, what you are saying is that the increase in power cannot be determined by multiplying the increase in current by the increase in voltage. True.

And, even if you don't know that, when you write the equation using functional notation (correctly) the correct answer comes out.

Nice.

Ed

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m II 15 years ago

cough..cough...cough....

mike

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John Fields 15 years ago

news:n1g056hqktafku6r4qdplcqus6s0jhgldr@4ax.com JF

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m II 15 years ago

?

mike

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John Fields 15 years ago

JF

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John Fields 15 years ago

Or maybe: >>>> cough..cough...cough.... >> | >>> --- | >>> | >>> news:n1g056hqktafku6r4qdplcqus6s0jhgldr@4ax.com -+ >> >>> JF >> >> >> >> ? >>mike > >JF JF

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m II 15 years ago

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ehsjr 15 years ago

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m II 15 years ago

Very true.

mike

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John Fields 15 years ago

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m II 15 years ago

I screw up like that a lot. My Dad used to tell me that mistakes are harder to spot when your nose is too close to the work. Taking a step back gives a wider view.

We're all striving to be better. I note with some interest that one or two denizens of this fine group have already attained perfection. A weird thing though..perfection seems to bring with it uncontrolled profanity.

I think I'll stay imperfect.

mike

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Ohms law power problem

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