"Used Up" Electricity and Circuits
Why does electricity have to flow in a circuit? If
the electrical energy is expended at the load, say a light bulb,
what actually gets returned in the neutral wire to where is it
returned? Does it go back to the panel or to the source and why?
Is there some kind of effluent that is produced after the
electrical energy is "expended" at the load? I have looked for an
answer to these questions and have yet to find an explanation.
You can think of electricity as a movement of a property called
charge. In conductors, such as wires, charge is free to move and it
does so with ease. In insulators, like plastic, charge is fixed and
cannot move, and therefore cannot conduct electricity. In things like
light bulbs and toasters, some charge is free and some is fixed. As a
result, those things conduct electricity, but do so at a price.
Charge that moves through light bulbs or toasters must be pushed
harder in order to go through, and consequently, charge through those
items transfers some of the energy used in the pushing. In the light
bulb, it is transferred into visible light and heat, and in the
toaster it is mostly heat. The thing that does the pushing is
something like a battery or a generator. You can think of a battery
or generator as a charge pump. Charge that moves is called current
and is measured in amperes.
Let us just consider the battery for a moment. Current
that comes out of the battery must eventually return to the battery.
Here is a little thought experiment. Think about a pump and a hose.
Fill the pump and a hose with water. Then connect one end of the hose
to the pump outlet and the other end of the hose to the inlet. Turn
the pump on. Water will flow from the outlet, through the hose, to
the inlet, then through the pump and back again to the outlet. This
keeps repeating and becomes a continuous flow of water. In this
analogy, the pump is the battery, the water is the charge, flowing
water is the current and the hose is a conductor, like a wire. The
whole thing is a circuit.
Now, let us modify the circuit by putting a restriction
in the hose, like a half-closed valve. The outlet side of the pump
will build up pressure to force the water through the restriction. If
you measure the pressure in the hose before and after the
restriction, you will find a pressure difference high on the source
side of the water flow and lower on the return side. The pressure
difference is the voltage across the restriction, and the restriction
is some kind of resistance, like a light bulb or toaster.
This analogy points out several things: Charge is
already in the circuit. It exists in the wires, in the battery and in
the toaster, but it will not do anything useful unless you have a pump,
like a battery or generator, to move it. The pump moving the charge
merely takes the charge from the inlet of the pump and pushes it
through the outlet. It does not create any charge, so any charge it
pushes out must have been sucked in through the inlet, so what goes
out eventually has to go back in.
So you see, the energy you spoke of in your question
does not really get expended it simply is transferred from one thing
to another. Within a system, energy is a conserved quantity. Within a
circuit, charge is also a conserved quantity.
Hope this helps.
You seem to have at least a bit of an understanding of electricity, to
believe it is "Used up" in the load. However, some of what you have been
taught is a little inaccurate, so I am going to use an analogy or two to
The voltage created by a generator or a battery is a measure of potential.
Think of this like an elevated trough of water. Lots of pressure, but it is
not going anywhere yet.
Current, or Amperage, is the flow of electricity. In this case, we may think
of it as a thin hose running from the bottom of the trough. Water will run
down it, trying to get towards the ground, but right now it is not moving
because the end is capped, and not going anywhere.
We could in theory allow this hose to fill another trough sitting on the
ground, and the flow would be current. However, unless we put in a pump of
some sort, (Battery or Generator), the upper trough will quickly empty. We
could also get some use out of this flowing water, by sending it through a
waterfall, or perhaps a water wheel so it can do some work for us. All
these diversions however slow the flow of the water from the upper to the
lower trough, and while we could put any number of devices in to take
advantage of our water flow, soon they would each be taking up so
and slowing the flow of water so much that the flow would not be very useful
at all. In this sense, we could say that the load we are using "uses up"
"Electricity" in a circuit has two parts, electrons and electric energy.
The electrons are particles that travel through the wires. Each electron
has an electric charge. The electric energy is transported by the
electrons. Their electric charge allows the electrons to do this.
Batteries and generators are devices that give electric energy to electrons.
The batteries and generators push the electrons through the circuits. As
the electrons move through, each is given a small amount of electric energy.
Light bulbs and motors are given some of the electrical energy from the
electrons. As the electrons pass through such a device, some of its
electrical energy goes to the device. It can then become light, heat,
motion, and other kinds of energy.
The electrons keep traveling through the circuit. Energy is picked up at
the power source and dropped off at the various devices. When there is more
than one device in a circuit, all energy doesn't go to the first object.
When you first turn on a circuit, things aren't perfect. After a few
microseconds, the circuit settles into a balance.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012