Circuits

Take a battery, a light bulb, and some wire to connect them so that there is a complete path for current to flow. You have a basic electric circuit. In a circuit, an area of high potential is connected to an area of lower potential (such as the two ends of a battery) by a conductor. This allows negative charges (by our sign convention) to flow and this current is able to do work, like light a bulb, heat your toast, or play your CD. Current flow through a wire is a lot like water flowing in a pipe. This is an analogy we'll carry on a bit later. What is current?

Current is the measure of charges flowing past a specific point in a specific period of time. We'll use the letter I to symbolize current, and we'll measure it in Amperes (named for Andre Marie Ampere), or Amps (A). Mathematically:

An amp is 1 Coulomb per second.

Conventional current flow goes from high potential to low potential. Although we don't think about it, we are talking about the movement of positive charges in the direction of the flow. Don't get hung up on this - we'll discuss rules fro determining the direction of current flow in a later lesson. Potential Difference, measured in
Volts, may also be called Electromotive Force (emf). It isn't really a force - it is just potential.

A circuit is formed when a wire is connected from the "high" end of a power source to the "low" end. In our case, we'll spend the next few lessons talking about Direct Current circuits and use batteries. (Direct Current is a type of current that provides a steady, non-varying current. It is used mostly in electronics and simple circuits. Alternating Current is a constantly changing current that is used in power generation and to run large electric loads.) The high end is the positive terminal, the low end is the negative terminal. The battery acts like a pump (back to our water analogy.) The movement of electrons (current) causes work to be done, much like the movement of water in pipes can cause work to be done. In a circuit, energy is conserved. In one complete trip around the circuit, the electric potential energy, or voltage the battery gained equals the voltage used by the loads.

For most conventions, current will flow from the + side to the - side of the battery. We will use simple schematic diagrams to show how our circuits are constructed and what components are in them. Here are the common symbols we will use:

OK - so it's a little rough.  A resistor is a device which opposes or restricts the flow of current. An ammeter is a device that measures current flow and must be connected in series with the circuit where the measurement is desired. A voltmeter is a device that measures potential difference between two points and is connected into the circuit in parallel with the device to be measured. A sample circuit might look like:

There are some additional terms you will want to know:

  • A load is a part of a circuit that uses power. Motors, lights, resistors, etc are all loads.
  • An open circuit is a circuit where there has been an interruption in the path, such as from opening a switch, breaking a wire, or removing a component. No current will flow in an open circuit.
  • A short circuit is a circuit where the load has been bypassed and there is a direct path from the high end of the power source to the low end. These generally result in very high currents, and if there is no protection such as a fuse or a circuit breaker, a short circuit will cause a system to overheat and may cause a fire.

Try the following web sites for further study:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html#c1

http://www.clarkson.edu/~svoboda/eta/dcWorkout/OandKLawsVCVS.html

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elecur.html

http://www.phy6.org/Education/woppos.html

For Practice Problems, Try: Giancoli Multiple Choice Practice Questions (Go ahead - try a few.)