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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 a ideal voltage source the symbol is 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 and 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 the current source and it also has a symbol like with a circle and this one we put an arrow and it goes in the direction of the current the current symbol is I it could 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 and those are the two symbols for constant current one of the things we can do is we can 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 current 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 and 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 a plot of that for 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