Re: MMf-EMf(VMf) two ways to cause a current.
- From: John Popelish <jpopelish@xxxxxxxx>
- Date: Fri, 27 Jan 2006 23:55:36 -0500
Xtrchessreal wrote: (snip)
On a 20 amp circuit if you short the line to the neutral you have a current of 20 amps for a moment before the breaker opens. 120 VAC, 2400 watts, 20 amps.
(snip)
Not quite. You have to apply ohm's law to calculate the current before the breaker opens. For instance, if the total resistance in the loop (the source resistance, the breaker resistance and the hot and neutral wire resistance add up to 1/10th ohm, the current is 120/.1= 1200 amperes till the breaker opens.
Of course if the 120 VAC 20 amp circuit was not able to deliver 2400 watts then the supply has the problem of being out of design spec. IOW the supply cannot deliver a 20 amp current as it was specified.
Actually, no single load on a distribution breaker is allowed to use all the current the breaker will pass without opening. A 15 amp circuit can have no legal load that draws more than 12 amperes, continuously.
When designing a new electronic device you need to know the total amount of power dissapation of the device especially if you intend to build a DC power supply for the delicate circuitry within. The DC power supply needs to be designed for the total dissapation and then you need to also know the maximum current that can be supplied by the power supply.
(snip)
And the efficiency of the supply, so you can work back to the power needed from the AC circuit. You may also need to learn something about power factor. The 12 amp limit for a load on a 15 volt branch circuit provides the full 12*120=1440 watts if the current is sinusoidal and in phase with the voltage. Any other wave shape (like the narrow pulses of current drawn by rectifier with a capacitor input filter) or phase relationship reduces the watts that 12 amperes can deliver.
I am just writing out my thoughts as I try to understand these things. I am not going in any particular direction except for one.
The resistor is a device that limits current for a circuit and that is the way I understand it.
Another way to say ohm is volts per ampere. A 1 ohm resistor requires 1 volt across it to have 1 ampere pass through it. A 1000 ohm resistor requires 1000 volts across it before 1 ampere will pass through it.
When its fuction is to create a voltage inside a circuit I get confused.
A resistor relates voltage to current by a proportionality factor (if it is a linear resistor). Its ohms are just that factor of proportionality. If you force a current through it, you multiply that current by the factor (resistance to calculate how much voltage that took. If you connect two resistors in series across a voltage, they divide it up inversely to their relative resistances. For instance, if you connect a 9 ohm resistor and a 1 ohm resistor across 100 volts, the 9 ohm resistor drops (uses up) 9/10ths of the 100 volts and the 1 ohm resistor drops 1/10th of the 100 volts. You could also get there by calculating the total resistance (1+9=10 ohms), divide the voltage by that to know what current is passing through both resistors, and then multiply that current times each of the resistances to know how much voltage drop appears across each.
I get confused because the terminology is twisted and goes against normal thinking.
Specifically, current is not possible without voltage or induction so
If the resistance is zero (superconductor loops) i is possible to have very significant current circulate forever with no voltage any where in the loop.
how can you have a voltage be created by a resistor that is designed to limit current?
(snip)
Voltage is consumed (dropped) as current passes through a resistor. The resistor does not create the voltage, which must come from whatever is pushing that current.
.
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- MMf-EMf(VMf) two ways to cause a current.
- From: Xtrchessreal
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