PHYSICS 106

Circuitry, Meters, and Ohm’s Law

Equipment:

• DC power supply (two 2xAA battery holders and 4 AA batteries)
• Alligator clip wires
• Multimeter
• Lightbulb and holder (any voltage - the ones supplies should be about 6V)
• Capacitor (any)
• 2 different resistors (47, 100, or 200 ohm)
• LED bulb

Experiment:

Determine the resistance of each of the following items

• The three resistors included in your lab kit,
• One of the light bulbs included in your lab kit,
• The LED bulb included in your lab kit,
• The capacitor included in your lab kit, 

Be sure to graph voltage vs. current over the limited range you can get with your power supply: You can get at most four data points with four AA batteries, using 1, 2, 3, or all 4 of the batteries. You can vary the voltage by using one or two batteries in a battery holder, and by connecting battery holders. Be sure the connect the positive from one battery holder to the negative of the next battery holder. If the batteries are connected incorrectly, they will get very hot and you may melt the holders!

A note about the AA battery holders: Some have wires, and some do not. You do NOT need wires on any, because you can use your alligator clip wires to clip directly to the battery connectors, and so although it is a 2xAA battery holder, you can just put in one battery, and connect to the holder in the appropriate way.

     To build a circuit, clip one end of one of the alligator clip wires to your battery and the other end to one side of your resistor (or capacitor or bulb). Clip one end of another alligator clip wire to the other side of the resistor (or capacitor or bulb) and then the other end to free terminal of the battery.

    To measure voltage across the resistor, set the multimeter to DC 2V or 20V. Be sure the red probe of the multi-meter is in the port marked "V". Touch the probes to either side of the resistor. If the reading is negative, you simply have the positive and negative probes reversed, just switch which side of the resistor they are touching.

    To measure current, you will need to disconnect the second alligator clip wire from the battery and connect it to one probe of the multi-meter. Use a third alligator clip wire to connect from the other multi-meter probe to the battery. If you follow the wire around the circuit, current will need to flow through the multi-meter. Switch the dial to 10A to start. You may need to adjust the multi-meter to the 20 mA setting to get a reading. To do this, turn the dial and then switch the red probe to the port marked mA.  BE CAREFUL NEVER TO PUT THE MULTI-METER LEADS ACROSS A RESISTOR (IN PARALLEL WITH THE CIRCUIT) WHEN IN CURRENT MEASURING MODE!

Note:  The maximum voltage you can supply to a resistor is determined from its power rating. Power used by a resistor is P = IV. The resistors we are using here are rated at either a maximum of 1/4 Watt or 1/2 Watt. The maximum voltage that can be supplied to the lightbulb or capacitor is typically listed on the device. 

Note: One potential problem can lie in shorting the circuit, so that the current does not run through a resistive element; another potential problem is setting the voltage of the power supply too high (be sure to always start at zero!) In these cases a high current will run and could burn out a component (if it gets hot or starts smoking....)  This is something you always want to be sure to avoid.

Safety Note: To avoid electric shock, or burning yourself with a hot element, you should NOT touch any part of the circuit while the battery is connected. Unclip one end of the plastic covered alligator clips before changing the resistor.

Write-Up

1. If this is a formal lab (as indicated on the lab syllabus), you have been instructed as to whether this is an individual or group write-up, or an oral presentation. If it is a write-up, each person must submit the lab electronically as a word-processed document in Moodle before the next lab meeting. If it is a group write-up, you should all be submitting the same document. For written formal labs, remember to check the "write-up hints" page to be sure everything is included and check your write-up against the grading rubric.
2. If this is an informal lab, work on the results together in your groups, and be sure to have your complete informal lab in your lab notebook and checked by the instructor before you leave.
3. Remember to read the next lab and do the pre-lab before you come to lab next week! You may work on the pre-lab with others, but each person must submit her or his own work.

Department of Physics