Electricit y/Current and transport of charge
UE3020320
Ohm’s Law
UE3020320
B A S IC P RINCIP L E S
E VA L U AT ION
Georg Simon Ohm was the first in 1825 to show that the current flowing
through a simple conductor is proportional to the voltage applied.
The cross-sectional area A is calculated from the thickness d of the wires:
A=
This means that Ohm’s law applies:
(1)
U = R ⋅I
The constant of proportionality R is the resistance of the conductor. For a
metal wire of length x and cross-sectional area A, the resistance R is given
by the following formula:
R = ρ⋅
(2)
π 2
⋅d
4
The measurements are to be plotted in three graphs of U against I. In
each of these, one of the parameters ρ, x or d will be varied.
x
.
A
U/V
5
The specific resistivity ρ depends on the material of which the wire is made.
In order to verify this fundamental relationship, an experiment is to be
carried out to investigate the proportionality between current and voltage
for metal wires of varying thickness, length and material. The resistivity will
also be determined and compared with values quoted in literature
d = 0.35mm
4
d = 0.5mm
3
d = 0.7mm
2
1
E X P E RIME N T
P R O C E DURE
OBJECTIVE
0
Verification of Ohm’s law.
• Verification of Ohm’s law for a constantan wire and a brass wire.
• Verification of Ohm’s law for constantan
wires of various lengths.
• Verification of Ohm’s law for constantan
wires of various thickness.
d = 1mm
0
1
2
I/A
Fig. 3: Graph of U against I for constantan wires of various thickness
S UMM A R Y
R/Ù
U/V
In simple electrical conductors, the current I which passes through the conductor is proportional to the
applied voltage U. The constant of proportionality, the ohmic resistance R, is dependent on the length x
of the conductor, its cross-sectional area A and the nature of the material. This relationship is to be
investigated using constantan and brass wires.
3
5
4
2
3
2
Re quire d A pparat u s
Quantity Description
Number
1
Resistance Apparatus
1009949
1
DC Power Supply 0 – 20 V, 0 – 5 A (230 V, 50/60 Hz)
1003312 or
DC Power Supply 0 – 20 V, 0 – 5 A (115 V, 50/60 Hz)
1003311
2
Analogue Multimeter AM50
1003073
1
Set of 15 Safety Experiment Leads, 75 cm
1002843
1
1
0
0
1
I/A
2
Fig. 1: Graph of U against I for constantan wire (blue) and brass wire (red)
0
0
1
2
x/m
Fig. 4: Resistance R as a function of length
R/Ù
6
U/V
5
x=2m
4
4
3
x=1m
2
2
1
1
0
0
1
I/A
2
Fig. 2: Graph of U against I for constantan wires of various lengths
3B Scientific® Experiments
0
0
2
4
6
8
10
1/A / 1/mm²
Fig. 5: Resistance R as a function of the inverse of the cross-sectional area A
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