In a straight DC circuit, with 3 light bulbs connected serially, is there any even theoretical drop in the relative brightness of the 3 bulbs? This puzzle was put to me by a friend who was coaching his stepson in maths and physics as the boy was coming up to some exams.
The reason I ask is mainly that I vaguely recall the original attempts at installing domestic electricity - using DC - failed because of power loss over large distances. That might be a separate issue from my question, I realise.
cheers
Providing the bulbs are incandescent, and are truely identical, then you have four resistances in series: the three bulbs, and the combined resistance of the wires and the internal resistance of the power source. The brilliance of the lights is proportional (perhaps somewhat non-linearly) to the current flowing in the loop, which is equal to the voltage of the power supply divided by the total resistance. The current through the three bulbs will be equal - must be - so they will all three be equally bright. As the line length increases the fourth resistance goes up, and so the current through the loop decreases, so the three lightbulbs will each be dimmer, but still equal.
The reason DC fails is, as G O R T said, because transformers cannot be used to convert high voltage - low current power to low voltage - high current power. The higher the voltage, the more insulation is required to contain it; the higher the current, the thicker the wires need to be to handle it. High voltages minimize copper costs over long runs (and where the wires can be kept long distances from anything needing to be insulated), but it is dangerous to use in homes, where wires must be very close to each other (and to people, too). On the other hand, generators, motors, light bulbs and other devices work most efficiently at low voltages and high current. It is advantageous to be able to easily and efficiently convert from low voltage to high voltage and then back to low, but transformers only work when their magnetic fields are changing, which is what AC does continuously.
In applications where DC can be generated and used at the same voltage, and doesn't need to be transmitted long distances (trivially, in any battery driven tool; better, in a motor-generator, such as those in diesel rail locomotives), DC works just fine, and is slightly safer for humans at a given voltage. Electronics work on DC exclusively.
