15. A student performed an experiment in which the potential difference V between the ends of a
long straight wire was varied. The current I in the wire was measured at each value of the
potential difference with an ammeter and the results of the experiment are shown in the table
below.
Trial
1
2
3
4
5
6
7
8
V (volts)
5
10
15
20
25
30
35
40
I (amperes)
0.25
0.50
0.75
1.00
1.50
1.65
1.55
1.53
Which one of the following statements is the best conclusion based on the data? Justify your
answer~!
(a) The resistance of the wire is 20Ω.
(b) The wire does not obey Ohm’s law.
(c) The wire obeys Ohm’s law over the range of potential differences between 5 and 30V
(d) The wire obeys Ohm’s law over the range of potential differences between 5 and 20V
Sol) Ohm’s Law I =
, can be rewritten as V = RI. This states that to obey Ohm’s law the voltage
V must be linearly proportional to the current I where the resistance R can be seen as the slope.
If we draw a V vs I graph below, the slope of the graph, which is actually R, is only linear in the
range V=5 ~ 20(V). Hence, the wire obeys Ohm’s Law only over the range of potential
differences between 5 and 20V
V
40
30
20
10
0.25
0.50 0.75 1.00 1.25
1.50
1.75 2.00 I