Will a 6 volt battery light a 12 volt bulb?

First of all, bulbs are not normally rated in volts, they are usually recognised by how much energy they use per second (measured in Watts). However, in context of the question, there are two basic properties of an electrical circuit; current and voltage. Current is defined as the rate of flow of charge. That means that if a current of 1 amp flows for 1 second, 1 coulomb of charge is transferred. The equation is: Q=IT Charge transferred (in Coulombs) = Current (in Amps) x Time (in Seconds) Voltage is defined as the energy per unit charge: V=W/Q Voltage (in Voltage) = Energy transferred (in Joules) / Charge transferred (in Coulombs) Theoretically, based on these two equations, if you reduce the voltage but keep the current the same, the energy transferred is halved and thus, the light given out by the bulb will be halved. However, depending on what bulb it may not light at all as an LED (light emitting-diode) will only light if there is a bigger voltage than its threshold voltage. Its threshold voltage is the point where as voltage increases beyond that point, the LED's resitance would be almost zero and the bulb will light. A Filament lamp is a non-ohmic conductor (as is an LED) because the resistance is not constant as the voltage increases. This is because as the voltage increases, the energy it transfers is more and the lamp heats up. This heat vibrates the metal lattice of the wire and inhibits electron flow (charge carriers) through the metal and so its resistance increases. Thus depending on the bulb and its resistance, it may light or it may not. A simple answer to all this is, the bulb may light or not depending on the bulb and current across it. This is assuming that the length and cross-sectional area of the wire as well as the number of electron charge carriers (electron number density of the material) is constant.