Resistors convert electrical energy to heat energy. Higher currents cause a higher energy loss as the electrons move through the same resistor. Therefore, if the longer current is allowed to travel through the wires, especially at the higher currents, the warmer they will get. They shouldnot get hot enough to burn you, but you will be able to feel the heat.
There is only one micrometer for use in this experiment. It will be kept at the TA's table. Since the diameters can be measured at any time during the lab period, you can make those measurements at a convenient moment when there is no one waiting to use the micrometer.
We will be using regualated power supplies rather than the variable powersupplies shown in the lab manual. Make sure that the current knob is rotatedas far clock-wise as it will go. This will give you the maximum current forthat supply. Current values will range from 0.30A to 0.43A depending on thepower supply at your station. We are using the regulated power suppliesbecaues they will keep the current constant from wire to wire, thus making ourcomparisons easier. Be sure to record the actual current given by your supply.
Ammeters MUST be connected in series. Make sure that you connect your ammeter so that the current flows in the correct direction through the ammeter.If the needle of the ammeter moves toward the left rather than showing a valueof current, reverse the ammeter connections.
The voltmeter should be set on the 2.0 V, DC scale.
We will be using 1.0 m long wires mounted on meterstick boards rather thanthe small, loose wires described in the lab manual. There are 28 boards, 7 eachfor the four different types of wire. All of the boards will be placed on thecenter table, take only one board at a time and return it to the center tablewhen you finish. Make sure to record which board you used for each wire type.
To connect the power supply to the wires, plug the power leads into thebanana plug recepticles at each end of the mounted wire.
Place one of the leads from the voltmeter at the 10 cm mark on the wire. Tomake the voltage measurements position the 25, 40, 55, 70, and 85 cm marksrespectively. The lab manual calls for 3.0 cm intervals. As you see, we willbe using 15 cm intervals instead in order to reduce the percentage error in thelength measurement.
When you are finished taking data, make sure that all circuits aredisconnected and that everything has been turned off.
Discuss your observations clearly and concisely, stating any conclusions youdraw from those observations. Make sure to support your conclusions byrefering to your data. For example, if you conclude that the voltage across awire is dependant on the length of the wire, you should state the values of thedata you are comparing or at least refer to the tables of data and trends seenthere.
You will be observing the ways in which the material and dimensions of theresistor wires will determine the value of resistance. You should spend moretime in your discussion analyzing the relationship between resistance, andlength and the resistance between resistance and area, rather than indiscussing the percentage error between your resistivity values and those foundin the CRC Handbook.
You will be producing graph of voltage versus length for each of the wiresyou use. If you wish, you may put all sets of data on the same graph- usingdifferent symbols or colors to represent the different situations.
Calculate the slope of each line and tabulate them in the results sectionof your report. The calculated values of resistivity should also be includedin the table.
Also discuss how a plot of Voltage versus length can give information aboutthe Resistance versus length relationship. (HINT: The current should beconstant during the experiment.)
| Section | Points |
|---|---|
| Purpose | 1 |
| Results | 3 |
| Graphs> | 5 |
| Calculations | 3 |
| Analysis | 8 |