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Old Jan 29th 2011, 06:26 PM   #1
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The concept behind the electrical circuit.

Ok, I don't have any real problem doing sums in regards to electrical currents, my problem is however, that some of the reasoning/conceptual theory behind electrical currents confuses me.

We know that for a current to flow, there has to be a potential difference, i.e. the Voltage between two points, this is based on an excess amount of positive or negative charge in point A in comparison to point B. We know that Protons, and electrons each have their own electric charge, which is what allows them "Flow". We know realistically that Protons do not actually move, it is the electrons that move.

What I don't understand are the following things, and this is what I need help with.

1)Does the Voltage in a circuit remain constant throughout, or does it vary as you travel further along the circuit. (i.e. the potential energy decrease while electrons gain kinetic energy).
2)What causes the current flowing to power an object, i.e. a Light bulb or motor.
3)Electrons don't actually travel from one end of the circuit to the other, they oscillate, bumping off of each other, but what happens to the last electron at the very end of the circuit, at the battery, does it disappear into the battery? If so, how can a current continue to flow if electrons are being absorbed by the battery, and how does it not run out of electrons and become a positively charged conductor once the battery is removed.
4)How can the current alternate or be direct. Does Direct mean that the individual electron IS making its way to the end, and in the alternating, that is where they oscillate and bump off of each other?
5)Does the kinetic energy of each electron remain constant once the Voltage is applied, or can an electron continue to gain speed. (I understand there is a drift velocity etc.)

These are some of the questions that trouble me the most. By far, out of all my Physics course, I find electricity the most difficult to apply reason to, it always seems like there are holes (no conduction required ) and that some things are left unexplained/highlighted for pure ignorance sake.
Thanks in advance.
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Old Jan 30th 2011, 04:16 AM   #2
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1) The potential difference across two points remains constant so long the power supplied is constant. The potential where the electron is at a particular moment decreases as the electron moves from the negative terminal to the positive terminal. (eg when half the total distance is covered, the potential where the electron is becomes half).

The electrons do not always gain kinetic energy. They accelerate at first, then reach a terminal velocity because there is an opposing force which holds them back. Thus, the electrons will not always accelerate.

2) In the case of a light bulb, when the electrons flow in the filament, they encounter a very big resistance and as such, release a lot of energy, according to the equation:

P = I^2R

The current is the same, while resistance has shot up, hence, the power output is high. The temperature rises and the filament, when being strongly heated will emit light, just as hot bodies emit light when they're very hot.

As for the motors, there are magnets involved. Electricity and magnetism are closely related.

Electricity can make magnets move and when positioned in some way, the magnets can be made to rotate and this creates motion, for example, they can drive wheels of a toy.

3) Lol, no, electrons do not disappear. In a battery/cell, you have a chemical process which occurs. Maybe your chemistry classes can help you if you have any. Chemicals react together and take electrons from one side of the cell, while other chemicals are formed and release electrons at the other end of the cell.

4) Direct current is one where electrons flow in one direction only.
Alternating current is one where electrons change direction periodically. Are you familiar with sine curves? The speed of electrons can be plotted against time in an AC and you'll get the sine curve.

I don't think that electrons bounce onto each other because the change in direction is rather smooth (as per the sine curve), they have a very low mass (hence they are easy to stop and start moving) and they repel each other.

5) Yes, IF the resistance is constant, the kinetic enery is constant.

You can compare this to friction. Let's say you push a box on a smooth floor with a constant force. The box will gain speed until it reaches a maximum. Suddenly, you enter a region where the floor is rough. Friction is greater, and hence, even if you push the box with the same force, the speed of the block will decrease.

Here, you are the potential difference providing the force, the box is the electron, the floor is the wire. When you use wires of different resistances, the electrons have different kinetic energies, if the potential difference is the same.

~~~~~~~~~~
Yes, I find electricity difficult too among the topics in physics and often when you understand something, something else is put into question! Anyway, I also like to compare with what I know from mechanics and some things, you'll be surprised to see are very similar.

I hope it helped!
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Old Jan 30th 2011, 04:30 AM   #3
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2) As the electrons flow in the light bulb, does the potential difference across it remain the same?
3) So the creation of protons and electrons is what allows the current flow in a battery circuit?

Thank you for answering my questions, it frustrates me that I struggle with the theory of electricity so much.
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Old Jan 30th 2011, 04:45 AM   #4
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2) Yes, it does, unless it's connected to a cell, where the cell is becoming weaker as time goes by

3) I wouldn't use the word 'creation'.

Let me take an example; the Daniell Cell

When connected, the zinc in the negative electrode of the cell will lose electrons to become zinc ions. The electrode is the negative terminal of the battery.

At the same time, copper ions near to the copper electrode (positive electrode, also positive end of battery) will gain 2 electrons to become copper metal.

Note that ions are not protons, and only electrons are responsible for the presence of the current here.
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Old Jan 30th 2011, 05:47 AM   #5
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So finally, at the end, or near the end of an electrical circuit, ie. at at the negative end in a conventional circuit, there will be no voltage, or very little, on the electrons?
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Old Jan 30th 2011, 05:58 AM   #6
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There will be the same exact potential difference (or voltage), the electron just doesn't have the same potential as before.

You can see it like this:

A ball on the surface of the Earth has different potentials at different heights above the surface. The difference in potential between the top of the ball and the bottom of the ball is the same throughout though.
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