Unmatched voltages on a transformer's primary coils

[faust9]

ok. so then here's a simpler question: how long does it take a capacitor to charge on a circuit with no load?

Please rephrase your question. Do you mean connect a cap to a source only?

Since I=\frac{V_b}{R}e^{-t/{RC}}

your question is meaningless.

the actual answer to your question is somewhere between 4.6 and 5 \tau depending on how accurate you want to get (you can go greater than 5\tau if you like but you won't see any difference really).

[edit] the above is for a series RC circuit. A parallel RC circuit would be along the same lines though. The charge on a cap follows an exponential function and is considered fully charged at 5 time constants.

 

[Cliff_J]

as for my scenario i asked this because with a high current you could charge teh capacitor much more quickly than with a low current (i think).

No, you can have all the current you want. Once the capactior is at the supply voltage no more current will flow. So the capacitor charges by VOLTAGE and the current is determined by the resistance (see ohm's law).

You are continously posting about getting high voltage and current and are trying to cheat the power out of some switch scenario, but that doesn't work!

If you charge up a cap it takes time yes, and you could figure how many joules that is (or just use 1/2 C * V^2) and that is what you'd have available at discharge.

Yes the shorter time of discharge the larger current you can extract, but as you extract the current the voltage will drop like a rock (just like on the charge curve the discharge follows a similar path). And even if you get 8000A out of some big cap, it doesn't do you much good if it lasts 1msec does it? Unless you're doing an experiment in lightning or a giant camera flash or something like that, most loads like a power source. And regardless of how you skew the equation, you can only expect to extract the same amount of joules you put in.

 

[infamous_Q]

ok...so since I am wrong on everything thus far and my idea(s) won't work...i have a few final questions (for now at least..): say you have an AC generatore, with a relatively low resistance in the circuit (either in the form of an AC circuit, or a DC circuit which uses a converted AC generator for the power source) and you have a voltage of 50 V and a Resistance of 2 ohms (i know its not going to happen..just a question as usual)
1) does Ohm's law apply here? as in...will the circuit actually have 25A runinng through it?
2) and what if the generator can't supply that much mechanical power to be converted? (i'm going to assume that the voltage would simply drop until both values were optimal, i'd just like to know whether I am right or wrong on that)
3) (if i guessed right on #2) what if the power is transformed before hand, will anything change?

and btw, thanks for coping with my idiocy everyone, I am still kinda waiting to graduate high school so i can go get an electrical engineering degree.

 

[faust9]

1) If the generator can supply that much power and if we're talking about shorting the generator to ground then the answer to your question is yes.

2) Yes, the generator would slow down to the point where the mechanical input equals the electrical output---Essentially, Power in will equal power out.

3) No. Nothing will change. Power in will still equal power out. You can't cheat death and you can't make power from thin air. In fact, if you do a conversion then the electrical power out will be less than the mechanical power in because of added losses in the conversion cktry. Conversions=loss of usable power.

 

[infamous_Q]

ok thanks...guess what though, i have another question for you helpful people.

is it correct that emf produced in a coil is proportional to (or the equation is...):
emf = 2pi * number of coils * magnetic field coil is in * frequency coil's turning at * surface area of the coil

 

[willib]

1) does Ohm's law apply here? as in...will the circuit actually have 25A runinng through it?

yes ohms law still applies, and that depends ..if the internal resistance of the generator is low enough, and the generator does not slow down ie. is still producing 50V then yes it could produce 25 A.

2) and what if the generator can't supply that much mechanical power to be converted? (i'm going to assume that the voltage would simply drop until both values were optimal, i'd just like to know whether I am right or wrong on that)

a generator does not supply mechanical power , a generator uses mechanical power to produce electrical power..if the mechanical power supplied to the generator can't keep up , the generator will slow down ,then the voltage will drop yes, ..
ps a load of 2 ohms is not a short to ground ,it is a load of 2 ohms..

 

[infamous_Q]

ok thanks...guess what though, i have another question for you helpful people.

is it correct that emf produced in a coil is proportional to (or the equation is...):
emf = 2pi * number of coils * magnetic field coil is in * frequency coil's turning at * surface area of the coil

so is this true or not?

 

[Cliff_J]

I ask you a followup question to your question - what units is the EMF measured in? How does that relate to the voltage and current output the unit can produce?

In short, the alternator/generator needs more than a simple calc on how it should be wound as the RPM it will be operated at and factors like the inductance/capacitance/conductivity (current capacity, inverse of resistance) need to be balanced as well as the cost and method of construction.

If you can't find what you want in a commerically available generator you are going to need to know an incredible amount more information in order to build one and impressive equipment to create it with tolerances of similar proportion.

On a personal note, you need to look at it this way: If the current efficiency is 85% then the most you could ever hope to achieve is increasing it to closer to 100%. Science is not foolish enough to overlook alternative energy sources, there are many threads on that topic alone in other forums. The big question is cost and practicality. Any person with sufficient funds can have a solar, wind, hydro, maybe goethermal or other source of power for all their energy needs depending on their location. But at best they cost twice as much as burning the stuff from the ground and require big bucks to get started. It should be obvious why it hasn't caught on.