Chapter 4: Circuits

Electrons with a negative charge, can't "jump" through the air to a positively charged atom. They have to wait until there is a link or bridge between the negative area and the positive area. We usually call this bridge a "circuit."

When a bridge is created, the electrons begin moving quickly. Depending on the resistance of the material making up the bridge, they try to get across as fast as they can. If you're not careful, too many electrons can go across at one time and destroy the "bridge" or the circuit, in the process.

In Chapter 3, we learned about electrons and the attraction between positive and negative charges. We also learned that we can create a bridge called a "circuit" between the charges.

We can limit the number of electrons crossing over the "circuit," by letting only a certain number through at a time. And we can make electricity do something for us while they are on their way. For example, we can "make" the electrons "heat" a filament in a bulb, causing it to glow and give off light.

When we limit the number of electrons that can cross over our circuit, we say we are giving it "resistance.". We "resist" letting all the electrons through. This works something like a tollbooth on a freeway bridge. Copper wire is just one type of bridge we use in circuits.

Before electrons can move far, however, they can collide with one of the atoms along the way. This slows them down or even reverses their direction. As a result, they lose energy to the atoms. This energy appears as heat, and the scattering is a resistance to the current.

Think of the bridge as a garden hose. The current of electricity is the water flowing in the hose and the water pressure is the voltage of a circuit. The diameter of the hose is the determining factor for the resistance.

Current refers to the movement of charges. In an electrical circuit - electrons move from the negative pole to the positive. If you connected the positive pole of an electrical source to the negative pole, you create a circuit. This charge changes into electrical energy when the poles are connected in a circuit -- similar to connecting the two poles on opposite ends of a battery.

Along the circuit you can have a light bulb and an on-off switch. The light bulb changes the electrical energy into light and heat energy.

Circuit Experiment

As a boy, Thomas Edison built a small laboratory in his cellar. His early experiments helped develop a very inquisitive mind. His whole life was spent thinking about how things work and dreaming up new inventions. The light bulb and movie projector are just two of dozens of inventions.

You can build a very basic electrical circuit similar to what Edison may have crafted as a boy. And you can find out what happens when a current is "open" compared with when it's "closed."

Here's What You need:

1. Penlight bulb
2. Flashlight battery
3. Two 6" pieces of insulated wire (any kind will work)
4. Tape to keep the wire on the end of the battery
5. A small piece of thin flat metal to make a "switch"
6. Small block of wood

Here's What to Do

1. To make a switch:
• Take the block of wood and stick one thumb tack in
• Push the other thumbtack through the thin piece of flat metal.
• Push the thumb tack into the wood so that the piece of metal can touch the other thumb tack (see picture)
2. Connect the first piece of wire to a thumbtack on the switch.
3. Place the light bulb in the center of this wire piece.
4. Tape the end of the first piece of wire to one end of the battery.
5. Tape your second piece of wire to the opposite end of the battery.
6. Attach the end of your second piece of wire to the remaining thumbtack on the switch.

You've created an electrical circuit.

When you press the switch connecting the two thumbtacks, your circuit is "closed" and your current flows -- turning your light bulb on. When your switch is up, your circuit is "open" and your current can not flow -- turning your light bulb off, just like Thomas Edison's may have done.

The number of electrons we are willing to let across the circuit at one time is called "current". We measure current using amperes, or "Amps".

One AMP is defined as 6,250,000,000,000,000,000 (6.25 x 10¹⁸) electrons moving across your circuit every second!

Since no one wants to remember such a big number, that big number is called a "coulomb," after the scientist Charles A Coulomb who helped discover what a current of electricity is.

The amount of charge between the sides of the circuit is called "voltage." We measure Voltage in Volts. The word volt is named after another scientist, Alexader Volta, who built the world's first battery.

You'll remember that back in Chapter 1, we defined energy as the "ability to do work."

Well, one volt is defined as the amount of electrical charge needed to make one Coulomb (625,000,000,000,000,000,000 electrons) do one a specific amount of work -- which is labeled one joule.

Joule is also named after a scientist, James Prescott Joule. Do you remember him from Chapter 2?

Voltage, Current and Resistance are very important to circuits. If either voltage or current is too big you could break the circuit. But if either is too small, the circuit will not be able to work enough to be useful to us. In the same way, if the resistance is too big none of the electrons would be able to get though at all, but if it were too small, they would rush though all at once breaking the circuit on their way.

Parallel Circuits!

When we have only one circuit that electrons can go through to get to the other side we call it a "series circuit."

If we were to set up another circuit next to the first one, we would have two circuits between the charges. We call these "parallel circuits" because they run parallel to each other. You can have as many parallel circuits as you want. Parallel circuits share the same voltage, but they allow more paths for the electricity to go over. This means that the total number of electrons that can get across (the current) can increase, without breaking either circuit.

Electric Motors

An electric motor uses circuits wound round and round. These wound circuits are suspended between magnets. (We send a 'thank you' to How Stuff Works Website for their electric motor graphic.)

A motor works through electromagnetism. It has a coiled up wire (the circuit) that sits between the north and south poles of a magnet. When current flows through the coiled circuit, another magnetic field is produced. The north pole of the fixed magnet attracts the south pole of the coiled wire. The two north poles push away, or repulse, each other. The motor is set up in a way that attraction and repulsion spins the center section with the coiled wire.

Go To Chapter 5: Stored Energy and Batteries

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READ MORE ABOUT IT!

Here are some additional websites to check out.

• Elementary Science Teacher Resource Book (www.uen.org/utahlink/lp_res/TRB032.html)
  
• UCI Summer science - Electric Circuits (http://can-do.com/uci/lessons99/electricity.html)
  
• University of Guelph (www.physics.uoguelph.ca/tutorials/ohm/)