* There are some sources that are very good sources and that can be modelled as ideal sources. (And when that happens, be grateful.) Some situations like that include the following.
o A power supply in the lab. Many times you connect a power supply to some electronic circuit, for example, and when you connect the circuit you find that the output voltage from the power supply doesn't change measurably. (After all, power supply designers try to make that happen!) In that case, the power supply might be considered to be an ideal source - at least as long as you are working on that particular circuit.
* There are many sources that do not perform ideally. However, it has proven to be possible to construct models of real sources, and those models often contain ideal source in combination with other ideal elements (like resistors, etc.). Thevinin and Norton equivalent circuits are examples of models of real sources that can account for loading effects (i.e. drawing enough current from the source to change the output voltage) and they are widely used in circuit analysis. You will even find that manufacturers give you parameter values for Thevinin and Norton equivalents on the front panel of many instruments like function generators.
You often have situations in which the sources that you use can be approximated with ideal sources. Shown below is a bridge circuit powered by a battery. Often a battery maintains a pretty constant voltage across the terminals, so you may be able to replace the battery with an ideal voltage source when you analyze the circuit.
Here's the circuit with an ideal voltage source substituted for the battery. At this point, you may know how to do the analysis so you're ready to go.
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