Ideal Voltage Source

An Ideal Voltage Source has several properties which allow the voltage source to always provide the rated voltage independent of the circuit resistance and current drawn.

These properties are:

  • Never goes flat
  • Always maintains the same voltage independent of current
  • Voltage sources can Supply or Absorb power (more on this quantity soon)
  • Wires have zero resistance
  • Has no internal resistance
  • It supplies ANY current necessary to provide its rated voltage

Note: In a non-ideal independent voltage source, which has finite internal resistance, the supplied voltage can decrease significantly when the current draw is large (like draining a battery) due to the IR voltage drop across the internal resistance.

The typical case is shown below:

The voltage source, Vs, provides 3.3V which is dropped across the 3.3kΩ resistor and a current of 1mA is flowing through the circuit (which you can check with Ohm's Law :))

One of the edge cases is shown below. This is a short circuit:

This circuit has the following properties:

  • This zeroes the effect of an ideal voltage source
  • As wires have zero resistance there is no voltage drop across the wires and hence this circuit is logically impossible (would require the supplied voltage value to change which breaks the definition of an ideal voltage source)
  • Short Circuit => R = 0Ω

Note: In reality, wires have a very small  resistance, so to drop 3.3V the current must be very large which will increase collisions in the wire building up heat which would most likely burn out the wire (do not try this at home :) )

The final edge case is shown below. This is an open circuit:

This circuit has the following properties:

  • Logically makes sense, current is 0A
  • As wires have zero resistance, the open point is the difference in electric potential from the terminals
  • Can sense charges through air so there is still a potential difference
  • Open Circuit => R = ∞Ω

Intuitively you can think of the charges on each terminal spreading across the wires up to the open circuit point so we have positive charges on one side, and negative charges on the other thinking in terms of conventional current, eventually reaching a point of equilibrium where no more charges move hence we have 3.3V across the open circuit. The dotted boxes around each terminal up to each end of the open circuit indicate that the same voltage is observed across each length so we would measure 3.3V at any point on the positive side and 0V on the negative side. This action is particularly important for another fundamental component known as capacitors which we will discuss further later :)

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