Ohm's Law
(Reference Copy)

Activity  14:  An Electrifying Inverse Proportion

Let's start by examining a basic electrical circuit.  We will first look at a diagram of the circuit, then at the circuit itself.  Some special symbols are used to describe electrical circuit components.  One component is the resistor, something that has the property of dissipating energy.   Anything that uses electricity, from a light bulb to an electric motor, is a resistor.   A resistor is represented by a zigzag or sawtoot-shaped line.  Each resistor in a circuit is identified with such a symbol and with a label consisting of a capital letter R followed by a number.  The numbers are usually assigned in sequence, beginning with one, and thus the resistors in our circuits will be labeled R1.  Resistors come in various sizes and shapes; some are fixed and some are variable.  The amount of resistance a resistor provides is measured in Ohms.  Fixed resistors have constant values and variable resistors can be adjusted to provide a variety of resistances.  Sometimes variable resistors are called rheostats or potentiometers.

Every circuit must have a power source.  What power source is involved when you use a vacuum cleaner?  How about a flashlight?  There are two general types of power source in common use: alternating current (AC) and direct current (DC).  AC/DC is a rock band, not a power source!  Household current in wall outlets in our country is generally 115 volts AC and batteries are usually between 1.5 and 9 volts DC.  Car batteries are 12 volts.  Look at several batteries to determine their voltage.

A circuit must be complete to be operative; that is, there must be a continuous connection from one side of the power source through the components in the circuit and back to the other side of the power source.  Electric plugs have two prongs because current must have a return path to the power source.   Plugs used in AC house current sometimes have a third prong, which is normally used as a "ground."  A ground is just an alternative path back to the source, making the electrical device safe even when the normal path back to the source is blocked.

DC must also have complete circuits.  Sometimes there is a wire returning from the circuit to the battery; other times one side of the battery is connected to a common ground and the return side of the circuit is connected to this common ground.  Cars generally use the frame as a common ground.  This way, only one wire is needed to stretch all the way from the battery to a component, such as a tail light.  The side connected to the battery by this long wire is called the hot side.  A relatively short wire is connected to the return side of the light and is run to a convenient point on the frame to complete the circuit.  This side of the circuit is said to be connected to ground.  The symbol for ground is something like a little upside down evergreen tree.

Two simple circuits follow.  The one on the left uses AC; the one on the right DC.  Other than that, they are equivalent.

We need to know how to measure resistance, current and voltage.  An Ohmmeter measures the resistance of a resistor.  We must disconnect the resistor from the circuit to get an accurate reading.  Then connect the two leads from the meter to opposite ends of the resistor and observe the position of the needle on the meter.  This reading is the resistance of the resistor.  Current can only be measured in complete circuits, so the ammeter is placed between the resistor and the power source in our simple circuit.  Current measurements are in amperes (amps).  The power source voltage may be measured by placing the voltmeter's leads on opposite terminals or leads of the power source.