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Ohm’s Law
Steve MacBride,
Bunny Neil, Victor Roche,
Nadmi Mustapha
Physics 2 Lab
Dr. Roman Kezerashvili
Theory
THE ELECTRIC CURRENT (I) is defined
as the amount of electric charge (q) that
flows through a cross section of wire per
unit of time (t). We represent this as:
I = q/t
Voltage is Proportional to the
Current => I ∝ V
 By
adding a constant of
proportionality, 1/R, the expression
becomes:
I = V/R
Where R is the Resistance of the Conductor…
I = V/R
THE CURRENT THROUGH A RESISTOR
IS DIRECTLY PROPORTIONAL TO THE
APPLIED VOLTAGE V AND INVERSELY
PROPORTIONAL TO THE RESISTANCE
R OF THE RESISTOR
This expression is known as
Ohm’s Law
The relation :
I=V/R
Is known as Ohm’s law and is named
after the scientist Georg Simon Ohm who
discovered it in 1827.
Procedure
We set up an electric current in a wire with
a Resistor…
V
Voltmeter
R
A
Ammeter
ε
Variable Power
Supply
The circuit for
measuring the
current
dependence on
the voltage and
resistance
Verification of Ohm’s Law
We will investigate the variation of
the current with potential difference
when the resistance is constant.
 First, we vary the output voltage in
1V increments,
 Then we raise the
V
Resistance to 1200 Ohms
R
A
and repeat.

We plotted graphs of Current (I)
Versus Voltage (V)…

The Inverse of the slope should have
been equal to the Resistance…

Our percent error was 0% for 1000 Ohm
Resistance
Our percent error was 7.7% for 1200 Ohm
Resistance

Graph 1: Current I vs. Voltage V
0.01
y = 0.001x + 0.0002
0.01
y = 0.0009x + 9E-05
0.008
0.008
Series1
0.006
Linear (Series1)
FOR MORE INFO...
0.004
Current (A)
Current (Amps)
0.012
Graph 2: Current I vs. Voltage V
0.006
Series1
Linear (Series1)
0.004
0.002
0.002
List location or contact for competitive analysis (or other
related documents) here
0
0
0
2
4
6
Voltage (Volts)
8
10
12
0
2
4
6
Voltage (V)
8
10
12
We then varied the current and
kept the voltage constant…

We set the output voltage to 12 Volts

We varied the Resistance from 700
Ohms to 1200 Ohms, in increments
of 100 Ohms .
We graphed the results

We compared the Current I to the inverse
of the Resistance.
 The slope should theoretically have been
equal to the applied Voltage


Our slope was 11.655
The Voltage was 12 Volts
Our results showed
a 2.9% difference
Graph 3: Current I versus 1/R
Current I, A

0.02
0.018
0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
0
Series1
y = 11.655x + 0.0013
0
0.0005
0.001
1/R, Ω^(-1)
0.0015
0.002
Linear (Series1)
PART II
Combinations of
Resistors in
Series and in
Parallel
Finding Equivalent Resistance
Currents through all resistors in series are
the same. The Potentials and, thus, the
Resistances add:
I = IA = IB = IC,
V = VA + VB + VC
Resistors in Series
R = RA + RB + RC
V
RA
A
ε
RB
RC
3 Resistors set up in Parallel…
In Parallel, the Potentials are constant and
the Charges add. Therefore…
The equivalent resistance is equal to the
inverse of the sum of the inverses of the
individual resistances
V
1/R = 1/RA + 1/RB + 1/RC
Resistors in Parallel
RC
RB
A
ε
RA
Resistors in Series



We set the Output Voltage to 12 Volts…
We set the ammeter to the 200 milliAmp range…
We measured the current through each resistor...
Adding all resistances together, our
experimental value was 4920 Ohms
The actual value was 4930 Ohms , a .2 %
difference
We then measured the Voltages

We added the voltages that we
measured at each resistor…
Our total was 11.6 Volts, for a
percent difference of 3.38%
Resistors in Parallel

We set the output voltage to 6 volts
 We set the ammeter to a range of 200
milliAmps
 We measured the current and resistance
across each resistor
Our findings
The actual charge was .065 Amps.
Our experimental value was .065 Amps.
Our percent difference was 0%.
The actual resistance was 180Ω
Our experimental value was 186.47Ω
Our Percent difference was 3.53%
Conclusions
Our resistance followed Ohm’s law…
In each of 4 sets of trials, resistance was
equal to the voltage divided by the
current.
The total current measured in the parallel
circuit was nearly equal to the sum of the
individual currents.
Conclusions, continued
In our series circuit, the total Voltage input
(12 Volts) was nearly identical to the sum
of the Voltage measured at each individual
resistor.
In the series circuit, the current remained
the same at each individual resistor as it
was on the total series network.
Conclusions, Continued
In our parallel circuit, the voltage at each
individual resistor was exactly the same
as the input voltage (12.5 volts)
The sum of currents measured at each
individual resistor was nearly equal to
the total current (.065 Amp) input.
Georg Ohm Was Right
I = V/R