Will AC voltage or DC voltage have higher illumination intensity when provided at the same voltage?

Answer 1

Is this a trick question? It could be. Let's look at AC and DC and see what's up. We'll start with DC. DC is direct current. The voltage of "regular" DC is a fixed value. Like in a car, the battery is about 12 volts. (It's actually a bit higher, but work with me here.) It's a nice, constant 12 volts all the time. The headlights experience a constant 12 volts when they are on. What about AC? Alternating current, or AC, is based on a voltage source that has a changing polarity. For a while it's one way, and then for a while it's the reverse. Also, the voltage isn't constant and then suddenly changing polarity. If we look at the voltage on an AC line, at a certain instant, the voltage is zero. It then rises up over a short time to some maximum value and then decreases to zero again. Then the polarity changes and the voltage goes to some maximum negative value before returning to zero. We usually think of a sine wave, and that's pretty correct. This is how the voltage is actually generated in a generator. It spins, and the voltage output follows a sine wave. Cool so far? Good. Let's jump. With the AC voltage rising to some maximum value, what is that maximum? And since it rises to the value over a short period of time rather than just jumping straight up there, don't we have to "average out" the voltage over the time it takes for it to get to the maximum? And then "average it out" for the back half of that change as it decreases to zero? Yes, we do. And what we end up with is an "effective" voltage, or a "DC equivalent" voltage. In U.S. houses, the 120 volts AC (again, work with me here) is actually the DC equivalent voltage of the AC sine wave. The sine wave actually has a peak at about 170 volts. We could say that a given sine wave has a peak voltage of 170 volts. Or, since it goes negative as much as it goes positive, the total voltage change from the positive peak to the negative peak is twice the peak voltage, or 340 volts peak to peak. But we like to use the "average" or (mathematically) the root mean square voltage, which is the DC equivalent voltage to measure AC. So if an incandescent reading lamp on an end table in a house is plugged into a 120 volt wall outlet, it will have the same brightness as if the lamp was hooked up to a 120 volt DC battery. The effective voltages will be the same because the AC voltage is actually based on the DC equivalent voltage.