Borough of Manhattan Community College Science Department General Physics (PHY 109 and PHY 110) Laboratory Experiment #9 Ohm’s Law OBJECTIVES: To study the relationship among potential difference (voltage), current and resistance in a simple electric circuit. EQUIPMENT NEEDED: 100 Ω Resistor /nominal resistance Rnominal 100 Ω/ (1), 200 Ω Resistor /nominal resistance Rnominal 200 Ω/ (1), Ammeter (100 mA) (1), Cables - black (4) and red (2), Power Supply (1), Tap Key (1), Voltmeter (15 V) (1) THEORY: When a potential difference (voltage) V is applied at the ends of a resistor it results in the flow of a current, I, through the resistor and the resistance is defined as R V I (1) or V  RI (2) Ohm's Law states that in a large class of resistors, R is independent of V or I. In such a case, because of (2), the graph of V versus I is a straight line passing through the origin of the coordinate system. The slope of this line (V/I) is by definition equal to the resistance R. PROCEDURE: Measurements: (Ask your instructor to check the circuit before turning on the power supply.) 1. Set up the following circuit (Figure 1), using a power supply, an ammeter, a voltmeter and one 200 Ω Resistor (R). A digital multimeter may be used instead of an ammeter. Note: It is very important that the Ammeter is connected in series with the resistor R. Connect with a red cable the red outlet of the power supply with the red input on the ammeter. Color code: (+) red (-) black 55 Figure 1. A simple electric circuit to study the relationship among voltage, current and resistance. 2. Set the voltage to 2 V by turning the voltage knob of the power source gradually until you read 2 V on the voltmeter. (Disregard the numbers on the instruments of the DC power supply). 3. Record the voltage V and the current I on the ammeter in Table 1. Note: The ammeter scale is in milliampere (1 A = 1,000 mA; e.g. 15 mA = 0.015 A). 4. Increase the voltage and record voltage and current in Table 1 for 2 V, 4 V, 6 V, 8 V, 10 V and 11 V. 5. Set the voltage to 0 V by turning the voltage knob of the power supply. Turn off the power supply and replace a 200 Ω Resistor by a 100 Ω Resistor. Repeat measurements for a 100 Ω Resistor, for voltages of 1 V, 2 V, 3 V, 4 V, 5 V and 5.5 V and record the data in Table 2. 6. Set the voltage to 0 V by turning the voltage knob of the power supply. Turn off and unplug the power supply. Graph and calculations: 7. Construct a graph of the voltage V (vertical axis) versus the electric current I (A) (horizontal axis) for each resistor (Tables 1 and 2). 8. Draw the "best line fit" in each of these graphs. Note the "best fit" line should go through the (0, 0) origin of the graph. 9. Calculate the slope for each line. The slope is equal to the resistance Rmeasured. 10. Determine the % Error: % Error  Rmeasured  Rno min al  100% , Rno min al where Rnominal is the nominal resistance printed on resistor. 56 Student Name __________________________ Section _______________ Date ___________ Report on Laboratory Experiment “Ohm’s Law” DATA TABLES Table 1: 200 Ω Resistor Voltage Table 2: 100 Ω Resistor Current Voltage Current V mA A V mA A 0 0 0 0 0 0 2 1 4 2 6 3 8 4 10 5 11 5.5 Nominal value Resistance determined from slope of graph Measured value 200 Ω Resistor Resistance from Table 1 =  100 Ω Resistor Resistance from Table 2 =  QUESTION: Do these resistors satisfy Ohm's law? Please explain your answer. 57 % Error Student Name __________________________ Section _______________ Date ___________ Report on Laboratory Experiment “Ohm’s Law” Graph Figure. 58 Student Name ________________________ Section _______________ Date ___________ Report on Laboratory Experiment “Ohm's Law” DATA TABLE Table 1: Rn = 100 Ω resistor Voltage, V Current, I V mA A Table 2: Rn = 200 Ω resistor Voltage, V Current, I V mA 0 0 0 0 1 10,3 2 9,8 2 19,5 4 21,2 3 30 6 30,9 4 39,5 8 40,2 5 50,9 10 50,8 A Note: 1 mA = 0.001 A CALCULATION OF RESISTANCE AS THE SLOPE OF THE GRAPH OF V versus I Nominal value Rn (Ω ) Calculated Rc(Ω) as the slope of the graph of V versus I % error of Rc with respect to Rn 100 200 Notes: Slope = (V2 - V1) / (I2 - I1) % error = [| Rn - Rc | / Rn ] 100 Questions: 1-Do these resistors satisfy the Ohm's Law? Justify your answer 2-What voltage do you expect if 0.2 A goes through the 100 Ω resistor? 3-What current do you expect if the voltage across the 200 Ω resistor is 15 V?
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