In circuit analysis, an ideal voltage source is an active circuit element which holds its terminals at a constant voltage difference. Its behavior is independent of any other element in the circuit. An ideal voltage source is generally represented as:
/-----\ positive terminal
--- positive terminal or | + |
- negative terminal | - |
\-----/ negative terminal
The figure on the right is more general, the figure on the left represents a cell of a battery. A standard AA dry cell battery, for example, contains one 1.5V cell. A 9V battery on the other hand contains six 1.5V cells. The symbols are stacked to represent a battery with multiple cells, or multiple batteries connected in series (as is found in most battery operated electronics, such as remote controls and flashlights). Below is a schematic for a flashlight with two batteries.
A B C
+-----o-/ -o-----/\/\/-----+
| |
--- |
- |
--- |
- |
| |
+--------------------------+
E D
In the above example, points A and E are held at a constant three volts potential difference by the two 1.5V batteries. Points E, D, and C are all at the same voltage potential, and when the ideal switch is closed points A and B are at the same voltage potential. By the principles in Ohm's Law, there is a voltage drop from point B to C across the light bulb, modeled as a resistor. According to Kirchhoff's Voltage Law, the algebraic sums of all the voltages in this closed path must equal zero, so the voltage drop across the resistor must be 3 volts.
The voltage sources shown above are for the DC case. The AC voltage source looks like this:
/-----\
| ~ |
| |
\-----/
The ~ represents a sine wave, the shape AC voltage follows as it oscillates between positive and negative orientation. For this reason, neither of the terminals are considered to be positive or negative because they swap orientations at regular intervals. An AC voltage source is usually defined by its RMS voltage and the frequency at which it oscillates.
A real voltage source is more complex than an ideal voltage source. Realistically, a voltage source has a maximum amount of power and current that it can provide. In the case of a battery, as its stored chemical energy is drained its voltage will begin to drop. In a well designed system, the voltage source is usually powerful enough that these issues can be ignored and is modeled using the ideal case.