Bob Somers
3/7/05
Per. 4
Phil Jenkins, Katherine Kruser
Resistance, Current, and Voltage
Purpose
The purpose of this lab was to apply the concepts of constant voltage and current in
parallel and series circuits to establish the fact that our book-learned knowledge actually
works in the real world. Calculating the predicted values and then measuring the real
values allows us to compare our calculated results with those really present in the circuit.
Theory
There were a whole lot of concepts behind this lab, which allowed us to compute and
observe the results that we did. It was absolutely vital that we knew our voltage and
current rules, otherwise constructing the correct circuits would have been a very daunting
task indeed. The first challenge was figuring out how exactly to design the circuit. We
needed to create a complex circuit with a series chunk and a parallel chunk. There were a
couple of possible ways to wire it, but my group chose to wire a parallel circuit within a
series circuit.
The most difficult part of the lab was making sure that the voltmeter and ammeter were
wired into the circuit correctly, so that they would not disturb the existing current or
potential difference. Voltmeters are constructed with huge resistors inside with the
intention of being connected in parallel with the part of the circuit you want to measure
potential difference across. Their huge resistors prohibits current from flowing through its
branch, however since it is in parallel it still receives all of the potential difference that
every other branch receives. Ammeters are similar in principle, but not in application.
Ammeters are built with literally no resistance to allow current to flow completely freely
through them. Because of this, they are intended to be wired in series where current is the
same among all objects in the series. You have to be very careful when wiring an
ammeter, because if it is in parallel with anything all of the charge will rush through the
ammeter (due to its low resistance) and fry its internal components.
For our potential difference readings, we started by connecting our voltmeter across the
power supply terminals to create a separate parallel branch and read the potential
difference on the entire circuit. We then moved the voltmeter appropriately about to
measure the other potential differences in the circuit. The ammeter was a little more
tricky, but since we double and triple checked our circuitry design before flipping the
switch we didn’t encounter any problems.
Procedure
1. Construct a complex circuit such that there are both a series and a parallel subcircuit within the entire circuit.
2. Connect the voltmeter across the terminals of the power supply and the ammeter
in series with the entire circuit.
3. Slowly increase the amount of potential difference until it reaches a desirable
number and do not touch the dial ever again.
4. Read your and record to a table the measured values.
5. Now connect the voltmeter in parallel with resistor 1 (s) by connecting across the
alligator clips. Leave the ammeter in the same position. Measure and record the
potential difference and current.
6. Now repeat this process of moving the voltmeter and ammeter to be in their
respective locations relative to the last 2 resistors and the entire parallel
combination. Continue measuring and recording potential differences and
currents.
Data
Location
Resistor 1 (s)
Resistor 2 (p)
Resistor 3 (p)
Parallel Total
Circuit Total
Color Code
Green – Blue
Orange – Orange
Red – Red
n/a
n/a
Potential Diff.
5.9 V
1.4 V
1.4 V
1.4 V
7.5 V
Current
0.11 A
0.040 A
0.061 A
0.11 A
0.11 A
Observations
•
The resistors were warm after running a trial.
•
The meters had a tiny bit of “bounce” in them, meaning that when the voltage
knob was adjusted they slightly overshot their mark and quickly settled in.
•
When you don’t have a good connection between the wires and resistors, the
voltmeter and ammeter bounce and jiggle until you have a solid connection.
Calculations
First we calculate the resistance of resistor 1 (s) using the measured voltage and current.
V = IR
R = 5.9 V
0.11 A
R = 54 Ω
Next, we calculate the resistance of resistor 2 (p) using the measured voltage and current.
V = IR
R = 1.4 V
0.040 A
R = 35 Ω
Next, we calculate the resistance of resistor 3 (p) using the measured voltage and current.
V = IR
R = 1.4 V
0.061 A
R = 23 Ω
Next, we calculate the resistance of the total in parallel using the measured voltage and
current.
V = IR
R = 1.4 V
0.11 A
R = 13 Ω
Next, we calculate the resistance of the entire circuit using the measured voltage and
current.
V = IR
R = 7.5 V
0.11 A
R = 68 Ω
Now we calculate the percent error to be analyzed in the following section.
(experimental – theoretical) x 100 = percent error
theoretical
(54 – 56) x 100 = -3.6%
56
By similar processes we obtain +6.1% for resistor 2, +4.5% for resistor 3, +5.1% for the
parallel total, and –1.9% for the entire circuit.
Error Analysis
As always, there is some factor of human error in this lab. However, in this particular lab,
where the science is so exact, human error probably played the largest role in skewing
our answers. We were forced to estimate between lines on both the voltmeter and the
ammeter and were only able to record a precariously small number of significant digits.
Slight reading mistakes on the meters coupled with the large rounding error due to lack of
sig-figs probably can account for two-thirds or more of the error. However, there is a
small amount of error contributed by a source that most would never think about. The
wires themselves were resistors, albeit very small ones. Given that we used, for the most
part, wire longer than necessary to run our tests, it is quite possible that a small amount of
the error recorded was generated by the minute resistance of these wires.
Conclusion
Even when the error is factored in, the results are still quite clear: this stuff actually
works. It was apparent even when we were actually performing the lab, way before we
started calculating anything or pushing numbers around. A lot of the same numbers
started showing up, which at the time was both comforting and disturbing. However, after
applying a small bit of logic, we were successfully able to determine which readings
should match and therefore were able to check ourselves along the way. If something
weird or unexpected showed up, we already had the concepts firmly planted into our head
that told us we must have done something wrong. I personally enjoyed this lab a lot
because it gave us the chance to fiddle with circuits on a large enough scale (as opposed
to integrated circuits or likewise) to actually learn and see where the current is flowing.
I’m already getting excited because half of my Computer Engineering major in college
will be Electrical Engineering and this stuff is just plain fun.