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Current Electricity
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How we get electric current

When we talk about electric currents, we are talking about electrons in motion. The microscopic buggers racing around the wires at surprisingly slow speeds. How do we get them to move? Here's how.

Imagine a row of happy little electrons, sitting in a wire. (for simplicities sake, I did not draw the protons) Because we know electrons don't like each other, they will spread out as much as possible.

Now, suppose we take away the electron at the right end, and stuff another election in at the left end. As show below, What do you think will happen?

That's right. The electrons on the right side are feeling really uncomfortable that close to each other, and the electrons on the left are seeing lots of free space for the taking. So they all spread out.

If we were to keep taking electrons away from one side, and stuffing them into the other side, we would have a constant flow of electrons through the wire. That's current electricity. Electrons always in motion.

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Centripetal acceleration

Voltage
The most nondescriptive name in physics

Voltage is a slippery character. It goes by many names. Voltage, AKA Potential Difference, AKA Potential Drop, AKA Electrical Potential Difference, AKA Electrical Potential Energy. All these terms lead back to Voltage. Unfortunately, Voltage is the term that tells us the least about what is going on, but people use it the most because it is the shortest. Who would want to say, Electrical Potential Difference all day long, when you could just say Voltage. Then again, I don't know may people who would say either term all day long.... Anyway, I better get back to explaining things.

Voltage is a form of energy, sort of. It is the result of the attraction and repulsion among charged things. What?? OK. sorry 'bout that. I sounded like a textbook for a second there. Let me see if I can slip back into my usual form of writing. Some call it sarcastic. But I don't see it. Oh well. Ready to learn more about voltage? You damn well better be.

In order for the following explanation to work, you have to remember that work and change in energy are same thing. OK.

The above picture is an example of a zero voltage situation. Our two charged people are VERY happy where they are. They are having fun hooking up and doing whatever it is protons and electrons do when no one is looking. That may be fun for them, but we still haven't learned anything about voltage. To do that, we must separate the happy couple.

Now the happy couple find them selves separated and unable to reach each other. (Some of the readers may be making sly comments as to why the two simply don't walk around the barrier. Well, protons and electrons aren't that smart. And if they were, you wouldn't be learning about voltage. So shut yer hole and keep reading).

In order to separate the proton and electron, we had to do work. The work we did was against the force that holds opposite charges together. The work we did to separate the happy couple is equal to the voltage. Since the electron and proton are separated, they have electrical potential energy (voltage). If we were to remove the barriers, the electrical potential energy (voltage) would be transformed into kinetic energy as pair rushed toward each other to continue their make-out session. But when they reached each other, there would be no more electrical potential energy. Just so you know, the farther apart we separate the couple, the higher voltage they have.

For those of you who were unable to follow the above passage, don't despair! It took me forever to understand voltage. I still get tripped up on it occasionally. But you are in luck! I have a more real world analogy to voltage. Voltage in many ways is like gravitational potential energy. Remember how an object has more gravitational potential energy the farther away it is from the earth? That's because the earth is pulling on the object. When you let go of the object, the potential energy converts to kinetic energy until the object hits the ground. Sound familiar? That's right, it's exactly the same thing that happens on the microscopic level with the protons and electrons.

The baby has a lot of gravitational potential energy

Just so you know, voltage also works with like charges. Except with like charges, the voltage increases as the charges get closer together. This is because you have to do work to push them together, and when you let go, the like charges would fly away from each other.

OK, now that we know what voltage is, we need to start assigning it numbers.

The voltage between two points is equal to the magnitude of the electrical field times the distance between the two points.

What is that 'E'? That is electrical field strength. You know how the earth has a gravitational field? Well charged objects have the same thing. It's really annoying to describe, so lets just leave it at, its how hard a charged object pushes or pulls on another charged object. Just like the gravitational field around the earth is how hard the earth pulls on objects.

Voltage is measured in Volts, which are (newton x meters/coulomb or joules/coulomb)

Current

Current is the flow of electrons. With all these electrons flying around through wires, we want to be able to describe them. Well, we don't want to, but we will be tested on it. Current is the amount of charge, measured in coulombs, that passes a point, divided by the time. The symbol for current is an 'I'.

Current is measured in Amperes, after some long-dead french guy. An ampere is one coulomb per second. If you had one ampere go through your body, your heart would stop mighty quick.

We are the borg, lower your shields and surrender your ship. We will add your biological and technological distinctiveness to our own. Your culture will adapt to service us. Resistance is futile.

Resistance, is how much an object resists the flow of electrons. Think of current like water flowing through a garden hose, if there is nothing in the hose, then the water will flow freely. However, if there is a lot of crap in the hose, small pebbles, golf balls, whatever, then it will be harder for the water to flow through.

Resistance depends on several thinks. Primarily it depends on the material. The human body has a low resistance, because we are filled will all sorts of interesting fluids. Plastic, on the other hand, has a really high resistance, because it sucks as a conductor.

Resistance also depends on the length of the conductor. If you have a garden hose several miles long, it is going to be very difficult to push water through it. So the longer a conductor, the larger it's resistance.

Width of a potential conductor also affects the resistance. If you have to push water through a hose the width of a hair, you are going to have a difficult time about it. However, if you need to push water through a barrel, it's not too hard. So resistance decreases as the width of the conductor increases.

The symbol for resistance in a capital R Resistance in measured in ohms, the symbol for an ohm is the Greek letter Omega. It kinda looks like a horseshoe

Speaking of ohms, here is a law discovered by the same guy the ohm was named after.

Ohm's Law
Just like the meditative chant

This is probably the most important thing to remember from current electricity. This is the formula that you can use about anywhere. There is one very important thing to remember. The resistance does not depend on the voltage and current. What do I mean? Resistance is independent of the voltage and current. It is a property of the material itself, not the result of the division of voltage and current. If we were to increase the voltage what would happen? Well, the stronger voltage would pull more current through the resistor, so we would be dividing a larger value by another larger value, and we would get the same number for the resistance.

This is important to remember because teachers love to ask questions like: "If the current is doubled, what it the new value of R?" The answer is nothing! The resistance only depends on the properties of the resistor, what it is made of and its size.

Electrical Power

Electrical power is really the same old power we all got to know and love in the Work and Energy lesson. It's still energy used per time, and it's still measured in watts. I know your disappointed that you don't have a new unit to memorize, but you will just toughen up and move on. That which does not kill us makes us stronger. There is no new unit, but I have a surprise for you. You have not one, not two, but three, that's right, three new formulas to memorize. You can thank me later.

Electrical power is equal to the current squared, times the resistance.

Electrical power is equal to the voltage times the current.

Electrical power is equal to the voltage squared over the resistance.

Series Circuits

Series circuits are the simpler of the two to learn, so I will start with them first.

Amazing isn't it? I didn't think so either. Anyway, lets get started. The battery provides the energy to the system in the form of voltage. There are a lot of electrons (males) on the top who want to get to the protons (females) on the bottom. For unnecessarily complex and rather baffling chemical reasons, they can't just go through the battery. They have to find another way.

The wire provides that other way. By traveling through the wire, the electrons (males) can get to the protons (females) they so desire and to what ever it is that electrons and protons do when no one is looking. For simplicities sake, we assume that the wire has no resistance. It's easy traveling for electrons down a wire, that is, until they meet the resistor.

That's the symbol for a resistor. A resistor can be anything that uses electrical power. A T.V., N64, Computer, what ever. Actually, it can be anything that resists the flow of electricity. Place your brother's head between two wires, and it's magically a resistor.

There are a bunch of formulas that go along with series circuits, and here they are:

The current at any point on a series circuit is the same as the current at any other point on the circuit. Why? Do you really care? I didn't think so, but I'm going to explain it anyway. Why? you ask. Because I don't have a female to distract me at the current time. Current, get it? Anyway, I'll stop stalling. The amount of electrons in a system never changes. Think of them like marbles in a garden hose.

Ugly-Ass Marbles aren't they?

In that situation, if you push any of the marbles, they all move. There is no variation in the density of the marbles. Since current is the amount of charge that passes a given point per time, all the currents have to be the same. Since you have the same amount of charge passing each point, and unless you have a very strong desire to be wrong, the times will all be the same to. So voilá! All the currents are the same. It's like magic, just a lot less interesting.

The total resistance in a circuit is equal to the first resistor, plus the second resistor, plus the third resistor, and so on. If you add more resistors, it's just like adding a bigger resistor.

The total voltage is equal to the first voltage plus the second voltage plus the third voltage, and so on. This is because a little bit of the electrical energy is used to push the electrons past each resistor. The electrical field automatically adjust its self so each resistor gets the proper amount of voltage.

The total current is equal to the total voltage divided by (resistor one plus resistor two plus resistor three and so on). I have never needed to use this formula, but its on the formula sheets (usually), so I thought I would include it.

Parallel Circuits

That is a parallel circuit. Hey! I hear you yell. You just added a resistor! What's the difference?!? They difference, my short tempered friend, it that there are now two ways for the electrons to trave. They go through the right resistor or the left resistor. BUT NOT BOTH! That's the key difference. In a parallel circuit, the electrons have a choice of paths. In a series circuit they do not.

The important thing to remember is that the current is divided up between the resistor. Some electrons go one way, some the other, so the result is less current in each resistor. BUT if you add up those two currents, you would get the total current.

Some parallel circuit formulas are listed below. Enjoy.

The voltage between any two point on a parallel circuit is the same as the voltage between and two other points.

The total current is equal to, the first current plus the second current, plus the third current, and so on.

The inverse of the total resistance is equal to the inverse of the first resistor plus the inverse of the second resistor, and so on.

If any of the above was unclear, or if you have any comments or suggestions, please E-mail me!

ColinGPalmer@hotmail.com

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