PASCO EM-8812 Resistance Apparatus - Manual (Page 9)

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M o d e l N o . E M - 8 8 1 2 E x p e r i m e n t 2 : R e s i s t a n c e v e r s u s L e n g th
9
Experiment 2: Resistance versus Length
Theory
If a current (I) is flowing through a wire, the voltage drop (V) across a certain length
of wire with resistance R is given by Ohm's Law:
(eq. 2-1)
On a graph of V versus I, the slope is equal to R. In this experiment, you will plot V
versus I to measure R for various lengths of wire. You will then make a graph of R
versus length ( ).
The resistance of a wire depends on , the cross-sectional area (A), and the resistivity
(
ρ) of the material:
(eq. 2-2)
Thus the slope of the R versus graph is equal to
ρ/A.
Setup
1. Measure* diameter of the four brass wires and calculate their cross sectional
areas.
2. Install the largest brass wire in the apparatus (see “Wire Installation” on page 5).
3. Position the reference probe at the 0 cm mark and the slider probe at the 24 cm
mark.
4. Connect the power supply to the power jacks of the apparatus so that current will
flow from right to left through the wire.
5. Connect both galvanometers to your PASPORT interface (or interfaces). If you
are using a computer, connect the interfaces to it and start DataStudio.
6. Set up one of the galvanometers to measure voltage (V): Connect it to the refer-
ence (-) and slider (+) probes of the apparatus.
Equipment Required Part Number
Resistance Apparatus with wire set EM-8812
DC Power Supply PI-9877
Patch Cords (4mm banana plug) SE-7123
Two Galvanometer Sensors PS-2160
Resistor, 0.1 Ω, 3 W (included with Galvanometer)
BNC-to-banana jack adapter (included with Galvanometer)
PASPORT Interface (or interfaces) See PASCO catalog
Micrometer (optional) SE-7337
V IR =
R
ρ
A
--- =
*If you do not have a
micrometer, use these
values of diameter:
0.13 cm
0.10 cm
0.081 cm
0.051 cm