It's important to understand a few things about electricity in order to get the full picture. You can form a fairly good analogy between water and electricity.
In terms of doing work, a small amount of water under a huge amount of pressure can do an equivalent amount of work of a huge amount of water under very little pressure.
The same applies to electricity. In the electrical world, a resistor is like a small pipe -- it manages to lower the current (quantity) of electricity, that can pass, and whatever comes out the other end builds in voltage (pressure) slowly -- if you're actually consuming electricity on the other side, then the voltage may never build at all.
So, now that that is understood, here is Ohm's law:
- The current (I: amount allowed through the resistor) is equal to the voltage (V: the difference in pressure between different sides of the resistor) divided by the resistance value (R: how much the resistor restricts the flow).
From this, we can deduce a few things... Again imagining this as a water system, we have a water supply (battery) entering an unusually small pipe (resistor), filling a bathtub (an electrical load, say, a lightbulb). The pressure near the water supply is high, but since only a limited amount of water can get through that small pipe, the other side is fairly low pressure -- it just dribbles out into the bathtub. We can calculate exactly how much (current) is flowing through that segment of small pipe (the resistor) by saying this:
voltage (V - measured in volts) = the pressure of the water supply (high) minus the pressure in the tub (low).
resistance (R - measured in ohms) = the smallness of the pipe
current (I - measured in amps) = the amount of water that can pass at any given time, considering the pipe's smaller size.
Now imagine the two extremes:
Imagine a really big pipe (zero resistance) -- the pressure will not change between the different ends of the pipe, so the voltage (pressure difference) is zero. Lots and lots of water (current) can flow through this pipe. This kind of circuit is called a "short" -- a wire that directly connects things.
Or, the alternative, imagine a really small pipe -- say, completely clogged. The flow (current) would be zero -- any pressure on one side will not transfer to the other side. This is called an "open" -- no electrical connection at all.
Using the following formula to calculate watts (a measurement of total energy), you can see that there is no waste in the above two scenarios. "Shorts", such as wires and lightswitches that are on, and "Opens", such as lightswitches that are off, waste virtually zero energy.
In all other cases, some energy is lost in that resistor, and you can use this formula to determine how much.
If you understood my answer, you should now be able to use algebra to figure out the resistance (R) of a 100-Watt light-bulb when it is plugged into a 120-Volt light socket! Good luck!
Let me know if you have any questions.
· 1 decade ago