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Page 6
Resistance is Futile (student handout)
Objective:
In this laboratory you will investigate how the transmission of electrical energy is affected by the properties of the material that
conducts it.
Background
The transmission of electrons through a conductor is a complicated process to understand at the atomic level. However, we can
investigate how different types of conductors affect the transmission of electrical energy. Different metals transmit electricity with
different levels of effectiveness. Silver, copper and aluminum are the best electrical conductors, while chromium, iron and nickel
are significantly less effective.
In this laboratory exercise, we will use different grades of pencil lead to serve as our conductors. The pencil leads used are 2 mm
diameter drafting leads of varying hardness. The lead hardness depends on the corresponding mixture of graphite and clay used;
note that pencil leads do not contain any lead metal in their composition. Drafting leads are rated on a hardness scale ranging
from 9B for the softest lead to 9H for the hardest. Harder leads are used because they will maintain a finer point longer for use
while drawing. The intermediate lead grade of HB is equivalent to the No. 2 lead used in common wood pencils.
The electrical conductivity of a substance is the inverse of its resistance, and we will compute and use the resistance values in-
stead since this is more common and intuitive. A conductor's resistance (R; units are Ω) depends on three factors: the length (l), the
cross sectional area (A), and the resistivity (ρ; units are Ω•m). The last of these parameters (ρ) is material specific, while the first two
are affected by the geometry of the conductor. The relationship between these parameters is given by the following equation:
R = ρ
A pencil lead is a long cylinder, and the cross sectional area is given by A = π r , and the lead's resistance converts the electrical
energy in thermal energy.
Equipment and Materials
DC Power Supply—Variable Voltage, Knife Switch, Alligator clip leads, Voltmeter, Ammeter, 2 mm diameter graphite drafting
pencil leads (2B, 1H, 3H, and 9H grades recommended)
Experiment Equipment Set-up
Assemble the basic electric circuit shown in the diagram, leaving the knife switch in the OPEN POSITION (BLADE UP). Do not plug
in or turn on the power supply until your instructor has checked your circuit.
L
A
Low Voltage
Power Supply
Voltmeter
Ammeter
Red Alligator Clips
Black
Alligator Clips
