İzmir University of Economics
EEE 205 Fundamentals of Electrical Circuits Lab
EXPERIMENT 2
Ohm’s Law
A. Background
When a voltage over a resistor is applied, there will be a flow of electrons
through the resistor, called current.
Georg Simon Ohm (1789-1854), a German scientist,
discovered that there is a linear relationship between the
voltage over the resistor and the current through, i.e., v = R I
as in Fig. 2.1.
i(t)
+
v(t)
-
v(t) = R i(t)
R
Fig.2.1.
He published his work in the book entitled 'Die galvanische Kette,
mathematische bearbeitet' in 1827. The SI unit of the electrical resistance, the
Ohm, is named after him.
The Ohm’s law may also be written as:
v(t)
i(t)
=
R
or neglecting the time dependence simply
v
=R
i
The units of the quantities in Ohm’s law are given in Table 2.1.
Table 2.1
Quantity
Symbol
Unit
Unit Symbol
Current
i, I
Ampere
A
Voltage
v, V or e, E
Volt
V
Resistance
r, R
Ohm
Ω
2-1
For an electrical component, the graphics of voltage (v) as a function of
current (i) is called v-i characteristics of the component. For a resistor, the v-i
characteristics is a linear curve. The slope of the line is R, as shown in Fig.
2.2.
v
i
+
v
-
slope=R
R
i
Fig.2.2. The v-i characteristics of a resistor
To obtain the v-i characteristics of a resistance value of 1 kΩ = 1 x 103 Ω,
different values to i are assigned and the values of corresponding voltages are
measured (or calculated) as in Table 2.2.
Table 2.2
Current (A)
Voltage (V) (V = R I)
0
0
1 x 10-3
1 x 10-3 x 1 x 103 = 1
2 x 10-3
2 x 10-3 x 1 x 103 = 2
3 x 10-3
3 x 10-3 x 1 x 103 = 3
4 x 10-3
4 x 10-3 x 1 x 103 = 4
5 x 10-3
5 x 10-3 x 1 x 103 = 5
The values listed in Table 2.2. are plotted in Fig. 2.3.
V, volts
5
4
3
2
1
I, 10-3 A
1 2 3 4 5
I, mA
Fig. 2.3. The V-I Characteristics of 1 kΩ Resistor
2-2
B. Preliminary Work
1. For a resistance value of 15 kΩ given in
Fig.2.4., determine the current values I
corresponding to the voltage values given in
Table 2.3. Plot the V-I Characteristics on
Fig.2.5. (Indicate the units of the axis!)
+
V
-
I
10 kΩ
Fig. 2.4
Table 2.3.
Current (A)
Voltage (V)
V, volts
(I = V / R)
0
1
2
4
3
2
1
3
I, ?
4
Fig. 2.5
2. For the series combination of the resistors
given in Fig.2.6., determine the voltage values
corresponding to the voltage values given in
Table 2.4. Plot the V-I Characteristics on
Fig.2.5. (Indicate the units of the axis!)
+
I
15 kΩ
V
15 kΩ
Fig. 2.6
Table 2.4.
Current (A)
Voltage (V)
V, volts
(I = V / R)
1
2
3
4
3
2
1
4
I, ?
Fig. 2.7
2-3
3. For the parallel combination of the resistors
given in Fig.2.8., determine the voltage values
corresponding to the voltage values given in
Table 2.5. Plot the V-I Characteristics on
Fig.2.9. (Indicate the units of the axis!)
I
+
V
-
15
kΩ
15
kΩ
Fig. 2.8
Table 2.5.
Current (A)
Voltage (V)
(V = R I)
V, volts
1
4
3
2
1
2
3
4
I, ?
Fig. 2.9
4. Plot the results you obtained in Question 1, 2 & 3, on the same graph over
Fig. 2.10. Comment on the results. Which has a higher slope., i.e.,
resistance?
V, volts
4
3
2
1
I, ?
Fig. 2.10
2-4
C. Experimental Work
1.
Firstly attach the Y-0016/002 module on your experiment set. You can
make circuit connections as in Fig. 2.11.
Fig 2.11
1.1.
Apply the voltages given below to your circuit respectively and
measure the current values for every step. Write your
measurements onto the Table 2.6. Calculate V/A ratio for every
step and fill in write to table again. (Indicate the unit of V/A!)
Table 2.6
V (Volts)
I (Ampere)
V
A
(………)
1
2
3
4
5
1.2.
Are the ratios invariant? What do you find as the result? Can you
identify the component from this result?
1.3.
Determine the value of the resistor using (i) color codes, (ii)
ohmmeter of your multimeter,
Ohmmeter Result: ……………………………..
Color Code Result: ……………………………..
2-5
1.4.
2.
Are the results obtained in Part 1.2 and 1.3 consistent?
Now connect a 15 kΩ resistor over a bread board.
2.1.
Measure the current values corresponding to the voltage values
specified in Table 2.7. Calculate V/A ratios for each measurement
and fill in the Table.
Table 2.7
V
(Volts)
+
V
-
I
(Ampere)
V
A
(………)
Multimeter
Measurement
Color
Code
1
I
15 kΩ
2
3
4
5
2.2.
3.
Compare the results you have obtained using different ways for
the resistance value?
Now measure the resistance of the series structure you studied in
Preliminary Work using a bread board. Measure the current values
corresponding to the voltage values specified in Table 2.8. Calculate V/A
ratios for each measurement and fill in the Table.
2-6
Table 2.8
V (Volts)
I
+
15 kΩ
I (Ampere)
V
A
(………)
1
2
V
15 kΩ
3
4
-
5
4.
Now measure the resistance of the parallel structure you studied in
Preliminary Work using a bread board. Measure the current values
corresponding to the voltage values specified in Table 2.9. Calculate V/A
ratios for each measurement and fill in the Table.
Table 2.9
V (Volts)
I (Ampere)
I
+
V
-
V
A
(………)
1
15
kΩ
15
kΩ
2
3
4
5
5. Plot the results you obtained in Part 2, 4 & 5, on the same graph over Fig.
2.12.
V, volts
4
3
2
1
I, ?
Fig. 2.12
6. Comment on the graphical results ontained in Part 5.
2-7