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Physics 111
Experiment #11
Resistors Connected in Series and in Parallel
This section of the laboratory exercise will help you get familiarized with electrical resistors
connected in series and in parallel, as well as with Ohm’s Law.
Part I Resistors connected in Series
Procedure:
(a) Consider the electric circuit shown in figure (1) below. Set the resistor box at 400 Ohms and
the Voltage on the DC power supply at approximately 10 Volts. Double check the power supply
setting with a voltmeter connected across it.
(b) Record the Current in the circuit by connecting the ammeter in series with the resistor box as
shown in figure (1), and enter the values in the table below.
(c) Repeat this procedure for applied voltages of about 12 V, 15 V, 18 V, 22 V, and
25 V. Record the values in table 1.
Table 1.
Applied
Voltage V
Measured
Current
Plot the data with Voltage on the vertical axis and Current in the horizontal axis. Perform a linear
fit and record the slope of the line that best fits the data points. The slope of the line represents is
your experimentally measured value of the resistance of the box, i.e, Rexp.
Question 1: How does the computed resistance of the box Rexp compare to the resistance value
setting of Rth = 400 ? Calculate the % error.
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(d) Repeat (a) – (c) above but with the resistance box set to Rth = 800 . Record the values in
table 2.
Table 2.
Applied
Voltage V
Measured
Current
Question 2: How does the computed resistance of the box Rexp compare to the resistance value
setting of Rth = 800 ? Calculate the % error.
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(e) Finally connect the two resistor boxes in series as indicated in figure (2), one resistor box set
at R1 = 400 Ohms and the other resistor box set at R2 = 800 OhmsSet the voltage source at
about 10 V, 12 V, 15 V, 18 V, 22 V, and 25 V. Record the current in the circuit in table 3, and
compute the equivalent resistance Rexp by finding the slope of the line that best fits the data
points (Voltage on vertical axis and Current in the horizontal axis).
Table 3.
Applied
Voltage V
Measured
Current
Question 3: How does the computed (experimentally measured) resistance of the boxes Rexp
compare to the theoretical equivalent resistance value of Rth = R1+R2? Calculate the % error.
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Part II Resistors connected in Parallel:
Procedure:
Connect the two resistor boxes in parallel as indicated in figure (3), one resistor box set at R1 =
400 Ohms and the other resistor box set at R2 = 800 Ohms. With the voltage source set at about
10 V, 12 V, 15 V, 18 V, 22 V, and 25 V, record the current in the circuit in table 4, and compute
the average equivalent resistance Rexp. Again, plot the data with Voltage on the vertical axis and
Current in the horizontal axis. Perform a linear fit and record the slope of the line that best fits
the data points. The slope of the line represents is your experimentally measured value of the
resistance of the box, i.e, Rexp.
Table 4.
Applied
Voltage V
Measured
Current
Question 4: Is the computed equivalent resistance less than, equal to, or greater than the
individual resistances values chosen in the boxes?
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Question 5: How does the computed average resistance of the boxes Rexp compare to the
theoretical equivalent resistance value of Rth = (R1R2 ) / (R1+R2)? Calculate the % error.
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