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OHM’S LAW

Purpose
The purpose of this laboratory activity is to study the relationship between the current in a conducting wire and the electric potential applied across the wire.

Background
For a wide range of materials used in electrical circuits (e.g. metallic conductors), the relationship of the potential difference or voltage drop V across a circuit component, the current I through the component, and the resistance R of the component is given by Ohm’s Law: I = V/R

That is, for a given circuit component which has an "ohmic" resistance (obeys Ohm’s Law), the current is linearly proportional to the voltage.

In this laboratory activity a variable power supply will be used to apply different potentials across the circuit component (a length of metal wire in this instance). Meters will be used to determine the voltage across the length of wire, and the current flowing through the wire. The resulting data will be graphed and analyzed to verify Ohm’s Law and to determine the resistance of the length of wire.

Equipment
The equipment used in this laboratory experiment consists of a board on which wires of varying length and sizes are mounted, a variable power supply, several connecting wires, and two multimeters.

Procedure
1. Connect the circuit as shown in the diagram below, using one of the four longest wires on the board.

2. Now vary the applied voltage in steps from 0 to 3 volts, recording the voltage drop across the wire and the current flowing through the wire. Use a minimum of at least 6 different voltage settings. Do this for each of the four longest wires on the wire board. Your Spreadsheet might look as follows:

3. Next make a graph of the current as a function of the voltage for each of the four wires. If the wires obey Ohm’s Law, each graph should be linear.
4. You should now print out your graphs. Since you will be obtaining information from the graphs, you should make them at least one-half page in size. Next use a pencil and straight edge to draw the best straight line possible through each set of graph points.
5. Now from your graphs determine the slope of each line. Since Ohm’s Law can be written as I=(1/R)V, which has the form of a linear equation (y =mx + b), where the y-intercept (b) = 0, we observe that the slope (m) of the line as measured off the graph should be equal to the reciprocal of the resistance of the wire (1/R).

Thus, using the measured slopes off the graphs, determine the resistance of each of the four longest wires. Enter both the slope and resistance of each wire in your spreadsheet in a manner similar to the one shown below. Be sure to include correct units.