# Doesn't ideal voltage source have different voltages?

1. Oct 2, 2011

### jlpmghrs

"An ideal voltage source is a device that produces V volts across its terminals regardless of what is connected to it" -
this is the definition that I have in my book. Then they have a question in which the voltage across an ideal voltage source varies with time. How is it varying with time, if it's supposed to produce V volts, regardless of what is connected to it? I mean, what is causing it to vary with time? please help, this is really bugging me

2. Oct 2, 2011

### Staff: Mentor

You should post the exact wording, if you want the likely correct answer. But possibly they mean that, in your analysis, the generator producing that v(t) is to be considered ideal, i.e., having zero internal resistance.

3. Oct 2, 2011

### jlpmghrs

this is the question - "Suppose that the voltage v(t) produced by an ideal voltage source is described by v(t) = 10e-t V. Determine the value of this voltage at the instants of time t = 0s, t = 1s, t = 2s and t = 3s ."

As for the definition, I've quoted the exact wording. Hmm...but then I still don't see how it fits in with their definiton.

4. Oct 2, 2011

### Staff: Mentor

Definition of an ideal voltage source: produces at its terminals the exact v(t) desired, regardless of load.

5. Oct 2, 2011

### jlpmghrs

yea..that makes sense :) thanks heaps!

just one thing though, its still a little unclear, what is causing this variation with time? how is it practically happening?

6. Oct 3, 2011

### Staff: Mentor

You want a precisely defined waveform for some application, or for testing, e.g., you might want it to sweep a voltage-controlled audio oscillator across a range of frequencies, periodically, every 10 seconds, to simulate a type of siren.

7. Oct 3, 2011

8. Oct 3, 2011

### jlpmghrs

soo wait, let's see if i got this right

You're saying, for example, it varies with its position in an electric field as a function of time, t? and therefore, is not necessarily depending on what is connected to it?

9. Oct 3, 2011

### Staff: Mentor

I have no idea what that might mean. I didn't say anything about electric fields (though they, too, are fun to play with.)

But instead of hiring a little man to sit beside a power supply and gradually raise its output voltage, and repeating this every 10 seconds, you arrange for the voltage source to do that automatically.

The "ideal" specification simply means that the voltage will be what you intend it to be. Actually, it's a distraction to the problem at hand, and you can ignore it.

Besides, no where does this definition state that V must be invariant with time. In general, V=v(t)

Maybe you are confusing this with a "constant voltage source", which usually would be one that holds its output constant.

10. Oct 3, 2011

### jlpmghrs

oh okay it's clear now! thank you so so much :D