Verifying Ohm's Law: Potential Difference vs. Current in Resistors and Light Bulbs, Lecture notes of Law

Download Verifying Ohm's Law: Potential Difference vs. Current in Resistors and Light Bulbs and more Lecture notes Law in PDF only on Docsity! Brooklyn College 1 Ohm's Law Purpose a. To verify Ohm’s law. b. To compare the potential vs. current behavior of a resistor to that of a light bulb. c. To determine the equivalent resistance of combinations of resistors by current-voltage method. Theory The fundamental relationship among the three important electrical quantities current, voltage, and resistance was discovered by Georg Simon Ohm. The relationship and the unit of electrical resistance were both named for him to commemorate this contribution to physics. One statement of Ohm’s law is that the current (I) through a resistor is proportional to the potential difference (V) across the resistor. Ohms law is normally written as RIV  (1) where R is the resistance of the resistor in Ohm () when potential difference (V) is in Volt and current (I) in Ampere (A). Resistance is a measure of how difficulty to flow current through the device. In this experiment you will verify Ohm’s law in several different circuits using a Current Probe and a Voltage Probe. Any device that obeys Ohm’s law showing linear relationship of V and I is called Ohmic device, otherwise non-ohmic device. You will also apply Ohm’s law to determine the equivalent resistance of resistors connected in different combinations. Fig. 1a shows the resistors connected in series and Fig. 1b shows the resistors connected in parallel. Equivalent resistance (Req) in series combination of resistors is given by (2) and equivalent resistance (Req) in parallel combination of resistors is given by (3) Apparatus Vernier Circuit board, LabQuest interface device, current probe, voltage probe, computer with Logger Pro, Power supply, light bulb (6.3 V), digital multimeter. Fig. 1a. Series combination of resistors Fig. 1b. Parallel combination of resistors ...321  RRRReq .... 1111 321  RRRReq a b a b Brooklyn College 2 Description of Apparatus We are going to use a Vernier Circuit Board-2 as shown in Fig. 2 for this experiment. The circuit board contains resistors, capacitors, an LED, switches, battery clips, connections for external power, a resettable fuse, connections for the addition of other components, and turreted test points for easy connection of alligator-clip test leads. The Vernier circuit board is designed for use in the study of simple electric circuits. We will be using resisters and light bulb holders in the circuit board for this laboratory. A DC power supply with variable output, Vernier voltage probe and current probes will be used for measurements. These probes must be connected to a Vernier Lab Quest which may be connected to a computer. Vernier Logger Pro software will be used for collecting and analyzing the data. Basic circuit diagram for this lab is shown in Fig. 3a. A resistor (a and b are terminals of the resistor) is connected to a variable power supply to change the current through the resistor. In order to measure the current an ammeter is connected in series and to measure potential difference a voltmeter is connected in parallel. Why are the ammeter connected in series and voltmeter in parallel to the resistor? As we are using a variable power supply, voltage and current sensors and Vernier apparatus, our real circuit connection will look line in Fig 3b. CS is the current sensor and VS is the voltage sensor. The arrow in the CS indicates the direction of current when the reading is positive. Voltage sensor reads positive when the potential at the red terminal is higher than that at the black terminal. Fig. 2. Vernier Circuit board Fig. 3a. Basic Circuit diagram + - Red Black CS Power supply Red Black VS LabQuest Fig. 3b. Sketch of circuit connection a b Brooklyn College 5 2. Measure the resistance of the combination using the digital multimeter and record in table 2. 3. Now connect the terminals of this combination in your previous circuit to measure potential and current. Have your instructor check the circuit before proceeding. 4. Repeat Steps 2–7 of Part I. Record the value of slope in table 2. b. Parallel combination 1. Now, remove all the wires used to prepare the previous combination of resistors and prepare a parallel combination of three resistors (two 10  and one 51  using connecting wires. Make sure to identify the terminals of the combination. You may connect two wires at the terminal for terminals a and b. 2. Measure the resistance of the combination using the digital multimeter and record in table 2. 3. Now connect the terminals of this combination in your previous circuit to measure potential and current. Have your instructor check the circuit before proceeding. 5. Repeat Steps 2–7 of Part I. Take the data only up to 3 V. Record the value of slope in table 2. c. Mixed combination 1. Now construct a new circuit on the circuit board with a combination of resistors as shown in the Fig. 4. Measure the resistance of the combination across the terminals a and b using the digital multimeter. 2. Connect the terminals a and b to your previous circuit to measure potential and current. 3. Repeat Steps 2–7 of Part I. Take the data only up to 3 V. Record the value of slope in table 2. COMPUTATION AND ANALYSIS Part I 1. From your observation, is there a proportional relationship between voltage and current? Compare your observation with the resistors and the light bulb. 2. In case of the resistors, how close is the y-intercept to zero? What is the significance of having the y-intercept close to zero? 3. Compare the value of the slope you determined in each fitting to the resistance of each resistor. Resistors are manufactured such that their actual value is within a tolerance. For most resistors the tolerance is 5% or 10%. Check with your instructor to determine the tolerance of the 10  51  51  68  a b Fig. 4. Combination of resistors Brooklyn College 6 resistors you are using. Calculate the range of possible values for each resistor. Is the value of slope within the appropriate range of values for each resistor? 4. Do your resistors follow Ohm’s law? Base your answer on your experimental data. Part II 1. Describe what happened to the current through the light bulb as the potential increased. Was the change linear? 2. Since the slope of the linear regression line is a measure of resistance, describe what happened to the resistance as the voltage increased. 3. How does, you think, the resistance vary with temperature? 4. Does your light bulb follow Ohm’s law? Base your answer on your experimental data. Part III 1. For all combination, calculate the equivalent resistance and tabulate in table 2. 2. Compare the calculated resistances with measured values from digital multimeter and V-I curve. Are those values within the appropriate range? Questions 1. If one lamp of a group of five identical lamps in series is removed, the voltage across the empty socket becomes equal to the line voltage, rather than remaining one-fifth of it. Why? 2. The circuits, P and Q, show two different ammeter-voltmeter methods of measuring resistance. Suppose the ammeter has a resistance of 0.10 ohms and the voltmeter has a resistance of 3.0 ohms; the ammeter reads 2.5 amps and the voltmeter reads 6.0 volts. What is the uncorrected value of the resistance in each case? What is the true value of the resistance in each case? 3. A piece of copper wire of resistance R is cut into 10 equal lengths. These parts are connected in parallel. How does the joint resistance of the parallel combination compare with the original resistance of the single wire? Brooklyn College 7 Data Sheet Date experiment performed: Name of the group members: Table 1. Potential vs. current Part I and II Slope of regression line (V/A) Y-intercept of regression line (V) Resistor  Resistor  Light bulb (low current) Light bulb (high current) Table 1. Equivalent resistance Part III Resistors used Combination Resistance from multimeter () Resistance from the slope of the V-I curve () Calculated resistance () 10  10  51  Series 10  10  51  Parallel 10  68  51  51  Mixed