Ideal sources | Circuit analysis | Electrical engineering | Khan Academy

There's two kinds of ideal sources we're going to talk about. One is an ideal voltage source, and the other is an ideal current source. An ideal voltage source, the symbol looks like a circle; like that, we put a voltage indication right inside there. That's called V, and this is a constant voltage. What I've shown here is a constant voltage, and that can come from a power supply, or it can come from a battery.

When it comes from a battery, we have a special symbol for that. The battery symbol looks like this, and the convention for this, we also label it V. The convention for the polarity of a battery symbol is the long line there is the plus terminal, and the short line right there, that guy is the minus terminal. So that's the convention for a battery.

The other type of ideal source is called a current source, and it also has a symbol like a circle. In this one, we put an arrow, and it goes in the direction of the current. The current symbol is I; it can look like that, or we could point the arrow the other way, like that, depending on how the application goes, and that's an ideal current source. Those are the two symbols for constant current.

Now, one of the things we can do is plot these; we can plot these two voltage sources, the voltage source and the current source. We can plot them on a curve that has coordinates voltage and current, so this would be called an IV curve or an IV plot that we're about to do here.

For a constant voltage source, the voltage doesn't change; the current goes up and down depending on what the rest of the circuit demands, but the voltage is the same everywhere. So it plots something like this; that would be the plot, the IV plot of a constant voltage where V equals some constant V.

If we want to plot our constant current source on this kind of IV plot, this would be something where the current is always the same. The current is the same independent of the voltage, and so for that, a positive current would look like this, and we would say something like I equals a constant I. So that's the IV plot of a current source and the IV plot of a voltage source.

All right, these are the two basic ways we deliver power or signals into circuits. Now we have a complete set of elements that we can build things with.