Download 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
Left end of pencil, cm
Voltmeter
Voltage, Volts
Power supply
Current, Amperes
Resistance,
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?