PHY 2049L
Lab 2

Ohm's Law

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Updated: 12/27/2009
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Prelab

Answer the pre-lab questions on LON-CAPA after reading the description of this lab.

Comment

This is a pretty straight-forward lab. The main thing you have to be careful about is keeping the power on for very long, particularly with the higher resistance coils. The resistance changes with temperature fast enough to see the voltage change while you are measuring it, which means that you must NOT wait for the voltage to "settle down" to a final value, because it won't. Make your reading as soon as the first few digits of the display are stable. I will remind you of this before the lab.

I will also suggest that you make your measurements for each coil starting with the highest current (1 A) and working your way down. This has two advantages: (a) Most heating takes place at the highest current, so this sequence means that your measurements at lower current will be done with the coil at the same temperature you used for the first one. (b) You can pick the lowest range on the voltmeter that will handle the 1 A case, thus ensuring you get the most significant figures possible in each sequence of voltage measurements. (You should not change voltmeter ranges within a sequence of measurements if at all possible, but can do so between independent sets of measurements.)

Apparatus

This photo shows the equipment the way it may appear when you arrive. (We may or may not use the Keithley multi-meter as a separate ammeter in this lab, so it might not be there.) Please leave it this way when you are done. In particular, be sure to turn off the multimeter when you are done.

meter, resistor coils, power supply, cables

We will probably NOT use the separate ammeter as shown here and in the lab manual. Experience indicates that the amperage reading on the power supply, which you will use to regulate the current at the desired value, is consistent enough from case to case to get excellent data if you set it the same way every time. (You only get two significant figures this way, but that is about all we can expect because we cannot control the coil temperature.)

I have included a separate page showing details concerning how to make the series and parallel measurements with these coils. You might also review the information included with a previous lab on how the power supply controls work with particular emphasis on how to select what quantity is regulated and how to adjust the over-voltage limit. You must push the "V" button and change the voltage until it is greater than the highest voltage output we will need in this lab. We usually only need about 9.6 V, so setting it to a 10 V limit usually works well. Once you do that, push the "A" button and set the current to 1.00 A. In this way the power supply will give you whatever voltage is needed (up to 10 V) to produce a 1.00 A current through your resistive load.

 
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