# question

Discussion in 'Misc Electronics' started by curt, Dec 13, 2003.

1. ### curtGuest

I understand the formula for current is:I=E/R
and I notice some products say the current for 240v is i/2 that of
120v.
Am I asleep or why am I seeing the current increase with voltage
increase (or do I have the formula wrong??
Thanks,
curt

2. ### CWattersGuest

No but I think you might be confusing two different sets of equations....

1) If you have a fixed value resistor then the current flowing in the
resistor is given by the equation: I = V/R Notice that if you double the
voltage V you get double the current I.

2) The power (Watts) consumed by a piece of electric equipment is given by
the equation W = I x V. Lets say you were designing two different toasters -
one designed to work on 240V and one designed to work on 120V. You wouldn't
want the one running on 120V to take twice as long would you? So you design
them both to use the same amount of power. If one operates on half the
voltage it needs twice the current to produce the same power. eg V x I =
(V/2) x (Ix2)

Look at it another way. A fridge needs to burn a ceratain amount of power to
keep the contents cold. If you feed the fridge twice the voltage it only
needs half the current to keep the power the same.

3. ### EugeneGuest

NO: it's right, there are 2(pi)turns unaccounted for in the formula... and
then squared!

4. ### Watson A.Name - Watt Sun, Dark RemoverGuest

If a light is 120W for example, the light will take 1A at 120VAC or
..5A at 240VAC. It's the power that stays the same as the voltage
changes. So the resistance for the 240VAC light is four times the
resistance of a 120VAC light.

Understand, rubber band?

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5. ### Watson A.Name - Watt Sun, Dark RemoverGuest

[snip]
Kind of a sorry example. The fridge could run at four times the
power, but for one fourth the time, since it's a duty cycle type of
appliance.

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http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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6. ### John GGuest

"Watson A.Name - Watt Sun, Dark Remover"

7. ### Thomas C. SefranekGuest

You need to apply the right formula to your observation:

Power = Voltage * Current
The formula is correct, but does not apply to your product observation
in this case, as the product is NOT a pure resistance.

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8. ### CWattersGuest

Ok ok I didn't actually mean the _same_ fridge.

But if you want an argument.... I'm sure fridge pumps must have an optimum
operating duty cycle at which the fridge overall is most efficient. In which
case the fridge should be redesigned to operate close to that point
regardless of the voltage that it's designed to run on.

I note you didn't propose a better example?

A toaster perhaps? Who wants to wait 4 times longer for their toast?

9. ### CWattersGuest

Oops sorry I see you did provide a better example. The light is better you
win.

10. ### Watson A.Name - Watt Sun, Dark RemoverGuest

Or Pop Tarts. )

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Don't be ripped off by the big book dealers. Go to the URL
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Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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12. ### Watson A.Name - Watt Sun, Dark RemoverGuest

Pure resistance? What's that mean? I thinkk you mean the resistance
of the light is non-linear.

Pure? As opposed to unpure? Maybe you need to filter the light to
make sure it's pure.. ;-)

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###Got a Question about ELECTRONICS? Check HERE First:###
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My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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