PHYS 1402 General Physics II Experiment 5: Ohm s Law Student Name Objective: To investigate the relationship between current and resistance for ordinary conductors known as ohmic conductors. Theory: For ordinary conductor known as Ohmic conductor, the current flowing through a closed circuit is directly proportional to the applied potential difference (voltage) in the circuit as shown by the following relationship. V = RI Where I, is the current following through the circuit and V is the applied voltage across the conductor. The constant of proportionality R is called the electrical resistance of the conductor. It is a constant quantity for the specific conductor at a given temperature. If several data points are collected for I at different values of V and a graph is plotted between V and I, a straight line passing through the origin should be achieved, and the slope of the line should give the value of the resistance R. Plot V along y axis and I along X axis Apparatus: 2 x Digital Multimeter (DMM): use one as ammeter and other as voltmeter Page 1 of 5
Variable regulated DC Power Supply, Circuit board, Resistors: three different resistors available such as 100 Ω, 200 Ω and 300 Ω. Experimental Procedure: Mount 100 Ω, 200 Ω, and 300 Ω resistors between on the circuit board. Construct the circuit shown below using the 100 Ω resistor. Instead of the batteries, use a variable DC power supply as the source of potential difference for this activity. In the circuit diagram the symbol for the potential difference with an arrow drawn through it indicates a variable potential difference. Note that the ammeter should be connected in series to the resistance R and voltmeter should be connected across the resistor R as we discussed in the lecture. One of the DMM will be used as voltmeter and the other as the ammeter. Make sure that the power supply is turned off. Configure a second DMM to be used as an ammeter. by plugging one lead into COM and the other into the appropriate current label. Connect the ammeter in series with the resistor and connect the other DMM configured as a voltmeter across the resistor. With this arrangement of meters, we will measure the potential across the resistor and the current through it. Before turning on the power supply, check your circuit with the instructor. Incorrect circuits will result in blown fuses and possible damage to the DMM. 2. Adjust the power supply so that the potential across the resistor is 1V. Collect and record the potential across the resistor and the current through the 100 Ω resistor. Record your data in a separate, appropriately labeled data table attached to the report. Increase the potential by 1 V and repeat the measurements. Repeat three more times for a total of five or six measurements. 3. Reverse the leads to the power supply only, so that you apply a negative potential difference across the circuit. Don t change any of the other connections. Measure and record Voltage (in V) versus Current (in A) for the the same five values of the potential. Add these data to the table you have for the 100 Ω resistor. 4. Repeat both steps 2 and 3 for the two other resistors. Page 2 of 5
5. Disconnect the circuit and measure the resistance of each of the three resistors for which you recorded data. Record the measured values in your data table. To measure resistance, use the meter with the lead in the V-Ω socket. Place the leads across the resistor (just like measuring potential) and turn to the smallest setting on the Ω scale which gives a reading. Note, if the scale is marked as kω, then you need to interpret the reading as having units of kω. Measured resistance = 100 Ω Measured resistance =100 Ω Measured resistance = Page 3 of 5
Analysis Plot the graph between V and I, with V along Y axis. Compare the slope with your corresponding measured values and determine the percentage error as you did in your previous experiments. Conclusion: Page 4 of 5
Extract the conclusion of the experiment in a short paragraph. Questions: 1. Are all your plots straight line? Do they pass through origin? What does it mean? 2. Are the resistors you use Ohmic? Why? 3. Did your resistors warm up when you applied the voltage? If yes, explain its effect on your measured data. Theoretically, should they warm up? 4. Did you connect the voltmeter across the resistors? What would the effect on your measurements if they were connected as an ammeter? Why? 5. Did you connect the ammeter in series to the resistors? What would the effect on your measurements if they were connected as the voltmeter? Why? Page 5 of 5