Download Lab 4 Ohms and Kirchoffs Laws

Lab 4
Ohm’s and Kirchoff’s Laws
Background
The important concepts of this lab can be broken down into the following statements:
1) In a parallel circuit, all components see the same voltage.
2) In a series circuit, all components see the same current.
3) In a parallel circuit, the total current is divided up on each path. The sum of the currents for each
path equals the total current.
4) In a series circuit, the voltage drops across each resistor. The bigger the resistor, the bigger the
drop. The total of all the voltage drops is equal to the total voltage.
It sounds pretty straightforward. In this lab, you are going to prove it. These derive from Ohm’s
and Kirchoff’s laws and you can find much more information about them on the internet.
There are also several analogies that may help you to understand them better. Many of them
compare current to water flow and voltage to pressure. I have a few of my own that I will share
with you to help you understand.
The measurements you will be making today will be voltage and current. The voltage
measurements are straightforward. For the current measurements, REVIEW how you measured
current in the previous lab.
Equipment
1 Circuit Prototype Board (We’ll use the same ones we’ve been using, not the ones in the video)
2 100 Ohm Resistors
1 47 Ohm Resistor
1 Multimeter and probes
1 Power Supply
2 Banana-Banana cables (one red and one black)
Short wires, as needed
I.
Circuits
Open the Excel Spreadsheet.
In all of the series circuit experiments, you will use two resistors connected in series. There will be
an experimental setup at the front of the lab and you can bring your components up to the front to
duplicate the setup. By convention, (for series circuits) the Resistor 1 is the one on the left of the
prototype board and Resistor 2 is the one on the right. For parallel circuits, Resistor 1 is the one
furthest from you.
Case 1: 100 Ohm Resistors (Series)
1. Setup two 100 ohm resistors in series.
2. Connect the power supply. Typically, the positive end is connected with a red wire to the left
side of the circuit board. Please note that this is simply a convention that makes it easier for
others (such as your lab instructor) to verify your setup.
3. Turn on the power supply and set the current straight up.
4. Set the power supply voltage to approximately 3 volts. It is not important to set it to exactly 3
volts. Anywhere between 2.8 and 3.2 volts should be fine. Record the power supply voltage in
the Excel spreadsheet.
5. Calculate the equivalent resistance of the circuit and record it in the Excel spreadsheet.
6. Verify that the current drawn (displayed on the power supply) is less than 0.2 amps.
7. Calculate the voltages you expect to measure based on your power supply voltage and record
them in Excel. Note that the resistors are the same so the voltage drops across each resistor
should also be equal. The sum of them should be equal to the power supply voltage.
8. Measure the voltages (V1, V2, and Vtot). V1 is the voltage for the resistor on the left.
9. Calculate the currents you expect to measure based on your measured voltages and the resistors.
Record the calculated currents in Excel. (Hint: Ohm’s Law)
10. Measure the current through the circuit. If this is your first current measurement or you feel
uncomfortable making this measurement, ask the lab instructor or lab supervisor to verify your
setup and the steps that you are taking.
11. Calculate the percent differences, answer all of the questions, and make sure you fill out all of the
cells for the units. No percent signs and do not mix units and numbers!
Case 2: 100 Ohm Resistors (Parallel)
1. Rewire the circuit so that it is two resistors in parallel. Repeat the experiment in case 1.
2. Note that in this case, the calculated voltages should both be equal to the total voltage and the
sum of the currents in each path of the circuit should equal the total current. As you break each
part of the circuit, you should see the current drop. If you do not, you are doing something
wrong.
Case 3: Complex Circuit
1. Modify the parallel 100 Ohm resistor setup used in the previous case by adding a 47 Ohm resistor
in series (to the left of the parallel resistors) and repeat the procedure described in Case 1.
2. Note that this circuit has both series and parallel components. You should find it easier in your
analysis if you first determine the equivalent resistance from the parallel components and then
calculate the total equivalent resistance. From this, you can determine the TOTAL current. If
you need assistance understanding this procedure, please let the lab instructor know.
3. Turn off the power supply.
II.
Conclusion
1. Save your data file in dropbox (either in the morning or afternoon folder). The file name should
be S[Section]S[Station] Lab 1 EField.xls, where [Section] is 1 for the morning and 2 for the
afternoon and [Station] is your station number. Make sure that the names of all of the members
of your group are listed in case 1.
2. Please note that dropbox does not always work. Verify that the instructor has your lab report
before returning your equipment.
3. If you are in the morning class, log off your computer and turn off your monitor. If you are in the
afternoon class, shut down your computer and turn off your monitor.
4. Leave.
III.
Troubleshooting
1. No current measurement: If you get a current on the unfused input but *not* on the fused input,
check to make sure you have the dial set to 200 mA. If it’s still 0, you probably blew a fuse.
2. Current measurement always reads 1: You probably set the dial to 20 mA or 2 mA. It doesn’t
harm the multimeter, but it’s over the limit.
3. Current doesn’t change: Check the HOLD button.
4. No readings at all: The battery on the multimeter is probably dead. Bring it to the lab instructor
or lab supervisor and get a new one. Do not put it back with the rest.
5. No voltage: If you do not read any voltage (or it seems very different from the power supply
voltage):
A) Check that the HOLD button is not set on the multimeter. This will also show up on the
multimeter display.
B) Verify that there is a voltage across both resistors (Vtot).
a. If there is a voltage, think about if the voltage makes sense. Vtot should be about the
same as the power supply voltage.
b. If there is no voltage, measure the voltage across the power supply terminals. If there
is no voltage (and it is plugged in, powered on, and set to 6 to 8 Volts), contact the
instructor. If there is a voltage across the power supply terminals but not the
resistors, you probably have not wired it up properly (usually the case) or one of your
cables is broken (very rare).