EGN 100 Pencil Experiment

EGN 100 Pencil Experiment

Objectives:

1.  Determine the resistance of a graphite (carbon) pencil.

2.  Determine the resistivity of a graphite (carbon) pencil.

3.  Find the voltage at several points along the graphite.

4.  Gain an understanding of resistance, resistivity, current, and voltage.

Procedure:

You have been provided with a wooden pencil for which the graphite core has been exposed at both ends and for a length along its long axis. See photo below. Tape a millimeter scale near the front edge of the bench top so the scale increases from left to right. Tape the pencil to the bench so that the exposed graphite is parallel to the scale and near to it, with the zero centimeter mark at the intersection of the graphite and wood at the left end of the pencil. Record the length (to the nearest tenth of a centimeter) of the graphite; i.e. from the left end graphite:wood intersection to the right end wood:graphite intersection. Record the pencil brand and type if available, e.g. Sanford American 2 HB.

Pencil length ______cm. Type ______

Turn on a benchtop power supply. Set the voltage to 4.0 volts. Turn off the power supply.

Connect the negative terminal of the power supply to the left end of the graphite using black leads, and the positive terminal to the right end of the graphite using red leads.

Connect the voltmeter black lead to the clip lead at the left end of the pencil.Turn on the power supply. Turn on the voltmeter. Record the voltage and the current as shown on the power supply meter.

Applied Voltage ______volts Current ______amperes

Touch the red probe of the voltmeter to the clip on the left end of the pencil and record the voltage and current at this zero length. Lightly press the voltmeter red probe at the left end of the exposed graphite and record the length to that point, the voltmeter voltage, and the current from the power supply. Repeat at 1 centimeter to the right of this point and at 2, 3, and 4 centimeters to the right and record the lengths, voltages, and current. Repeat with the voltmeter probe tip touching the clip on the right end of the pencil.

Length from Voltmeter Power supply Resistance,

Left end of pencil, cm Voltage, Volts Current, Amperes Ohms=V/I

0

Disconnect the wires from the pencil and remove the tape from the pencil. Using the caliper, measure the diameter of the graphite at the end of the pencil, where you believe the diameter to be the full diameter of the graphite. Calculate the radius, r, of the graphite in centimeters and the cross-sectional area of the graphite: 1”=2.54 cm. A = r2 in cm2.

A = ______cm2

Questions

1.  Using Ohm’s Law, R = V/I, what is the resistance of the graphite from end to end? R = ______ohms

2.  Using Ohm’s relationship R = *L/A, what is the resistivity, , of the graphite, where L is the length of the graphite and R is that calculated in question 1?

 = _____ cm

3.  Using the data obtained above, plot voltage as a function of length of the graphite.

4.  Why doesn’t the current change?

5.  Using the data obtained above, plot resistance as a function of length of the graphite.

6.  What are three ways that the resistance of a resistor can be changed?