# Lab: Ohm's Law

## Lab: Ohm's Law

Objectives: The student should be able to:
1.  Connect and read electrical meters.
2.  Determine resistance in series and parallel circuits.
3.  State the relationship between resistance, voltage and current.
4.  Solve problems concerning electrical resistance, current, and voltage.

Materials:resistors on circuit boards, connecting wires, cells, cell holder, LabQuest with current probe

Background: Ohm's law states that the voltage of a circuit is equal to the product of the current (amps) times the resistance (ohms): V = IR. Ammeters are connected in series with the circuit that is being tested.  The voltage of electrical cells in series is the sum of their individual voltages.  (Actually there is some reduction of voltage due to internal resistance of the cells, but this will be disregarded for our experiment.)  If you do not remember how to connect cells in series, ask the instructor.

I.  Variable Voltage, Constant Resistance:
Set up the LabQuest to graph your data from this part of the experiment by doing the following: Connect the current probe to your LabQuest, then turn the LabQuest on. Click on “Rate:” and switch from “Time Based” to “Events with Entry.” For “Name:” type “Voltage” and for “Units” type “V”. Then you can click on “OK”. You will now be able to record the current at each point in this part. Clicking on the record button (looks like a green “play” triangle) will set the LabQuest ready for recording. Clicking on the “Keep” button after each cell is added will let you graph the data.
Use Ohm’s Law to predict the current produced by 1 cell (1.5V) in series with one resistor (27 ohms) and write it in your data table. Connect 1 cell in series with 1 resistor and the current probe.  The current will likely be reported in milliamps.  Convert milliamps to amps and record in your data table. Be sure to click on “Keep” at this point. Predict the current for two cells in series.  Write your prediction in your data table.  Connect 2 cells in series with the resistor and meter. Again read, convert, record, and “Keep” the data.  Continue by repeating the measurements with 3 cells and then 4 cells. You should now Stop the data recording and save your graph. (“6-X-ohm’s law” with X being your group number would be a good name for the file.)

 # of Cells Approx. voltage (volts) Resistance Predicted current (amps) Measured current (amps)

II.  Connecting Resistors in Series: (Each resistor has a value of 27 ohms)
You can switch to the meter screen for the rest of the experiment. That’s the first tab in the LabQuest.
Using the battery constructedwith 4 cells in series, connect it to 1 resistor in series with the meter.  Read and record the current.  Predict the current with two resistors in series.  Connect 2 resistors in series and record the current.  Repeat the procedure for 3 resistors in series.  The resistance of a series circuit is the sum of the individual resistances in the circuit.
Rtotal= R1+ R2+... + Rn

III.  Connecting Resistors in Parallel:
Connect 1 cell in serieswith a resistor and the meter.  Read the current and record the data.  Predict the current for two resistors in parallel.  Connect a second resistor in parallel with the first.  Read the current and record the data.  Likewise connect a third resistor in parallel with the other two.  Read the current and record the data.  The reciprocal of the resistance of a parallel circuit is equal to the sum of the reciprocals of the individual resistances in the circuit.
1/Rtotal= 1/R1+ 1/R2+... + 1/Rn

Questions:
1.  How did the measured currents compare with your predicted values?
2.  State the relationship you found between voltage, current, and resistance. Does this fit with the graph you produced in part I? Does this fit with Ohm’s Law?
3.  Some measurements were repeated in this investigation.  How did the repeated measurements compare?  Explain any differences.
4.  Calculate the resistance of a circuit that has a current .55 amps at 35 volts.
5.  What voltage would be needed to produce a current of 250 milliamps in a circuit with a resistance of 16 ohms?