1
Lab: DC Circuits – Resistors in Series
Theory: Circuits are said to be in series when they are joined, as in the figure below, so that the
current flows without change from one piece to the next.
For resistors connected in series:
1. The total potential difference for a group of resistors in series is equal to the sum of
the potential drop across the individual resistors:
V
T
= V
1
+ V
2
+ V
3
+ …
2. The current in every part of the circuit is the same:
I
T
= I
1
= I
2
= I
3 …
3. The equivalent resistance of the series group is equal to the sum of each individual
resistance:
R
eq
= R
1
+ R
2
+ R
3
…
Objective: To study the relationships among resistance, voltage, and current when resistors are
connected in a series circuit.
Materials needed: D.C. power supply, resistance board, D.C ammeter, digital multimeter,
connecting wire, switch
2
Procedure:
1. Connect one end of the switch to the power supply and the other end to resistor D. Connect
resistors D and E in series. Connect resistor E to the ammeter and finally continue back to
the power supply. Measure the total current with the ammeter.
I
T
= _____________________ A
How much current is running through resistor D?
How much current is running through resistor E?
Explain your previous two answers. Why is the total current the same as the current
through each resistor?
A
To measure total current
Source
3
2. Measure the voltage drop across each resistor (V
E
, V
D
) and the total voltage drop across the
group of resistors, V
T.
V
E
= __________________ V
V
D
= __________________ V
V
T
= __________________ V
V
To measure the voltage around resistor E
To measure the voltage around resistor D
To measure the total voltage
V
V
4
3. Using V
T
and I
T
as determined in steps 1 and 2, calculate the total (effective) resistance, R
T
.
R
eq
= V
T
/ I
T
= _____________ ohms
4. Calculate the value of each resistance:
R
E
= V
E
/ I
E
= ___________________ ohms
R
D
= V
D
/ I
D
= ___________________ ohms
5. Ask Mr. Cozzolino for the ACTUAL resistance of each resistor. Calculate the percent
error for the resistance of each resistor and the equivalent resistance.
% Error = [(Accepted value – Measured value) / Accepted value] x 100
Practice
1. The diagram represents a series circuit containing three resistors. What is the current
through resistor R
2
?
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