Download Lab 6 - Kirchhoff`s Laws

Lab #6: Kirchhoff's Laws
Copyright R. Mahoney 2013
Name: _______________
The purpose of this lab is to learn how to use Kirchhoff's voltage loop law and
current conservation law to analyze a circuit that by its nature is not easily
reducible by the usual rules for adding resistors in series and parallel.
1. Set up the following circuit:
You will be told what the nominal values are for R1, R2 and R3. V1 is the
voltage from a DC power supply. V2 is the voltage from a lantern battery. You
will be told to what value V1 should be set. V2 will be measured under load
(current flowing). In general, keep the battery (V2) disconnected when you are
not making in-circuit measurements.
2. With the resistors out of the overall circuit (removed one at a time), and using
an ohmmeter, measure the resistors' resistances:
R1 = _______
R2 = _______
R3 = _______
3. With current flowing in both loops of the overall circuit, measure the voltages
across the DC power supply and the battery:
V1 = _______
V2 = _______
Does V2 drift downward?
4. With current flowing in both loops of the overall circuit, measure the voltage
drops across the three resistors:
VR1M = _______
VR2M = _______
VR3M = _______
Carefully observe the polarity of the voltage drop across R2. This will tell you
experimentally which way the conventional current through R2 is flowing. Is it up
or is it down?
5. We identify the currents flowing through R1, R2 and R3 as
I1, I2 and I3 respectively. Using an ammeter, measure these three currents:
I1M = _______
I2M = _______
I3M = _______
Again, carefully observe the polarity of the measured conventional current I2M.
Is it up or is it down?
6. Starting at point A in the overall circuit, use Kirchhoff's voltage loop law to
write down the sum of the voltage increases and decreases for the loop that
includes R1 and R2. For mathematical reasons, assume that the conventional
current I2 is up, regardless of what you found experimentally.
7. Starting at point B in the overall circuit, use Kirchhoff's voltage loop law to
write down the sum of the voltage increases and decreases for the loop that
includes R2 and R3. For mathematical reasons, assume that the conventional
current I2 is up, regardless of what you found experimentally.
8. Draw the directions of the conventional currents I1, I2 and I3 on the schematic
on the first page of this lab, as you used them in steps 6 and 7. Using Kirchhoff's
current conservation law, write down how the three currents are related.
9. Using the simultaneous equations method shown in class, determine what the
theoretical values of the three currents should be. Show your work on the
backside of this lab.
I1T = _______
I2T = _______
I3T = _______
10. Calculate the % errors between the measured and theoretical currents for all
three currents, with the measured currents as true:
% Error, I1 = _______
% Error, I2 = _______
% Error, I3 = _______
11. Using the measured resistances for R1, R2 and R3, and the theoretical
currents in step 9, and using Ohm's Law, calculate the theoretical voltage drops
across the three resistors:
VR1T = _______
VR2T = _______
VR3T = _______
12. Calculate the % errors between the measured and theoretical voltage drops
across all three resistors, with the measured voltage drops as true:
% Error, VR1 = _______
% Error, VR2 = _______
% Error, VR3 = _______
13. State two non-trivial systematic errors for this experiment.