## PHYSICS 220/230 Lab 5: DC Circuits

In this lab you are asked to use your knowledge of DC circuits to determine your own procedure. Your notebook should contain your own description of the objective of the lab and the procedure you used, as well as the usual entries. You are asked to do the following with the equipment provided:

1. Measure the unknown resistances R1, R2, and R3.   Use the resistance mode of the METEX multimeter to make these measurements.
2. Build a series/parallel resistor circuit with the DC Offset of the Function Generator as your power supply and with R1, R2 and R3 in series and Resistor B in parallel with R2. Then make whatever measurements you need to check 1) the details of Kirchoff's rules for this circuit and 2) the ways in which resistors in series and in parallel combine. Use the Pasco program to measure voltage (make sure the sensitivity is set to Low (1X)).  Use the METEX multimeter in its current mode to measure current.
3. Investigate the properties of a "voltage divider". The voltage divider is one of the most ubiquitous electronic circuit components. Build a series circuit with the function generator and Resistors A and B. This circuit is the prototype for the voltage divider. Assume that the voltage across Resistor B is the output voltage of your divider and that the voltage from the function generator is your input voltage. This output voltage would usually be connected to some device (called the load). Before you make any measurements, however, derive the theoretical value of the ratio of the output voltage to the input voltage for this circuit without a load (another resistor) connected in parallel with Resistor B. The result of this calculation will show you why the device has its name.

Now consider what you would do if you wanted to make a device that would allow you to continuously vary the output voltage across Resistor B over a wide range, given the same input voltage. This would be what you would want to do for the sound level of your stereo, the output of your function generator, or the light dimmer switch on the wall. You will quickly come to the conclusion that physically changing Resistor B each time is not what you want to do. A more useful voltage divider can be built, however, from a single variable resistor, called a potentiometer. Several potentiometers will be shown to you in the lab.

Investigate the properties of potentiometers by measuring the resistance of the two potentiometers that are on your lab desk. These potentiometers have three terminals. The two outside terminals give you the fixed resistance of the whole device. The middle terminal allows you to use the resistance across only part of the device. The voltage   input from a different circuit can then be divided. For each potentiometer, place the two outside terminals in a series circuit with the function generator. Adjust the output voltage between the center terminal and one of the outer terminals so that it is about 20% of the input voltage. Then simulate the addition of a "load" on the circuit by connecting Resistor B across the output voltage. What does the addition of the load do to the value of the output voltage? Which potentiometer do you think would make the best voltage divider in a real circuit?