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Name: Roger
Status: other
Grade: other
Location: CT
Country: N/A
Date: 12/23/2005


Question:
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.


Replies:
Hi Roger

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.

Robert Froehlich


Roger,

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 explain it.

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 much pressure, 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" our potential.

Ryan B.lscamper


Roger, "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
Physics Instructor
Illinois Central College



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