ANSWERS: 1

Let's look at an ideal DC voltage source. It would supply a set voltage V0. Now attach an ideal wire to the + and  terminals of your ideal voltage source, and Oh dear! The current has gone to infinity. Since we cannot have an infinite number of electrons moving through a wire, there has to be something else going on. In real life, the voltage source is made from real materials that have finite nonzero resistance, or, in other words, the voltage source has an internal resistance that limits the current that can be supplied at that voltage. If the demanded current is too high (low too low in resistance), the voltage of the output will drop to compensate, but it may also cause damage to the voltage source, as it will heat up at the rate P=I^2 R(internal). If the current doubles, the voltage supply heats up four times as fast.
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