Ohm's Law and Circuits
In this lab we will look at the current through and voltage across resistors in series and parallel circuits. According to Ohm's Law, the current through a resistor is proportional to the voltage across it:
V = I R
We will look at three different circuits, one in series, one in parallel and one "mixed". By measuring the current through and voltage across each resistor we will confirm our rules for adding resistors.
Pick three resistors, measure their resistances and record these values in your lab notebook.
A) Series Circuit
Set your power supply to ten volts.
Turn off your power supply. Your instructor will tell you how breadboards work, place the three resistors in series. Knowing the values of the resistances, predict in your notebook the values of the current through and voltages across each resistor.
Place DMM in series with the circuit, between your third resistor and the power supply. See figure 1. Set your DMM to read current. Have your instructor inspect your circuit before you turn on the power. Turn on the power supply and measure the current that flows out of R3. Record this (with error) in your lab book.
Figure 1 - Measuring Current in a Series Circuit
Repeat and measure for R2, R1 and the current flowing out of the power supply. Remember to turn off the power before changing the circuit!
The next step is to measure the voltage across each resistor. To do this we need to set the DMM to voltage and wire it parallel to the element that we are measuring. Measure the voltage from the power supply and the across each resistor. Record these values in your lab notebook.
B) Parallel Circuit
Wire up this circuit. Have your lab instructor inspect it before you turn on the power. Again, predict the current through and voltage across each resistor.
Measure the current through each resistor; this will involve setting the DMM to current and putting it in series with what you are measuring. See figure 2; the solid position gives the total current, the dashed gives the current through resistor 1. Measure the voltage across each of the resistors. Record all of these values in your notebook.
Warning: You should think about the size of your resistors when you do this, keeping in mind that the resistors we use in class are rated for 1/4 watt of power.
Figure 2 - Measuring Current in a Parallel Circuit
C) "Mixed" Circuit
D) Internal Resistance of the Function Generator
Plug in your function generator, set it to 50Hz sine waves, and measure the voltage with a DMM. Make sure that the DMM is set to measure AC voltages. When you measure the voltage with the voltmeter, due to the details of the internal construction of the DMM, you are measuring the voltage of the function generator (in practice we can treat the DMM as if it was a very large resistor, and hence ignore the internal resistance). Next connect a 50Ω resistor to the function generator. You will find that the voltage across the new resistor is roughly half the previously measured voltage. Draw a ciruit diagram with the internal resistance in series with your "load" resistor. Use your voltage measurements and the resistance of the load to verify that the internal resistance of the function generator is what it says it is.
E) Light Bulb
In your conclusion you should compare your measured results to the expected values derived
from the rules we have for adding resistors in series and parallel. Comment on the lightbulb results.
In general, does the DMM change the results you measure? Why must it be placed in series for a current measurement and in parallel for a
Pre-Lab: Ohm's Law
This week in lab we will measure a series circuit, a parallel circuit and a "mixed" circuit. Prior to lab you should play with the DC Circuit Construction Kit.
Needless to say, build a series circuit with two resistors, and measure the voltage across and current through each circuit element. Do the same for parallel and mixed circuits (the mixed case will need three resistors). For the values of the resistors, use the month, day (and for mixed, year) of your birth. Print out copies of the results, and on the hard copies write down the math that shows why your results are correct.