About this video
This clip introduces the two ideal sources used in circuits: an ideal voltage source that holds voltage constant, and an ideal current source that holds current constant. It shows their symbols, their polarity conventions, and how each one appears on an IV plot.
You'll be able to
- identify the symbols for ideal voltage and current sources.
- state which quantity stays constant for each ideal source.
- read a constant-voltage and constant-current source from an IV plot.
Key words
- Ideal voltage source
- A source that keeps its voltage constant regardless of the current the circuit demands.
- Ideal current source
- A source that keeps its current constant regardless of the voltage across it.
- IV plot
- A graph with voltage and current as coordinates that shows how a source or element behaves.
- Polarity
- Which terminal of a battery symbol is positive, marked by the long and short lines.
Transcript
- There's two kinds of ideal sources we're gonna 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 and that's called V.
And this is a constant voltage, what I've shown here is a constant voltage. And that come from a power supply or it could 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 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 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. And those are the two symbols for constant current. One of the things we can do, we can plot these, we can plot theses two voltage sources, the voltage source and the current source.
We can plot them on a curve that has coordinates as 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 IV plot of a constant voltage, where V equals some constant V. And if we wanna 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. Alright, 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.
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