PHYS 2426 Engineering Physics II EXPERIMENT 4 OHM'S LAW I. INTRODUCTION The objective of this experiment is to examine Ohm's law by measuring the potential dif- ference, V, across various circuit elements and the electric current, I, through them. Ohm's Law states that the potential difference, V, across a resistor is directly proportional to the current I through it with the resistance R being the proportionality constant. Mathematic cally this is written as V = IR. (1) Also Ohm's law states that R is a constant independent of V and I. Circuit elements which have a constant R are called ohmic and those which have a non-constant R and called non-ohmic. II. APPARATUS Decade resistance box, circuit board with a lamp and a two semiconductor diodes, 2 DMMs, power supply, and connecting wires. III. EXPERIMENTAL PROCEDURE Procedure (1): Constant R 1. Note: although you are not directly measuring the resistance R, assume the uncer- tainty is OR = 0.50. 2. Wire the circuit as shown in figure (1) (on the back page). With the power supply off, set the resistance box to 200 2 and the ammeter to 40 mA. Important note: you should always set the ammeter to the largest possible scale and move it down to ac- commodate the measured values of the current and give you the maximum precision (either a 3 or a 4 digit display). 3. Have the instructor check the circuit before you turn the power on. Now turn the power on and increase the voltage until the current is about 5 mA. Measure the current and the voltage across the resistor and record the values in the data table. The 5 mA given here is a nominal value and you should not spend valuable time trying to get the current to be exactly 5 mA. 4. Repeat this process for a total of 7 data points going up in increments of approxi- mately 5 mA. Each time record the current and the voltage in the data table. 5. Notice the linear relationship between V and I. 6. Turn the power supply off.Procedure (2): Constant V 1. Set the power supply at V = 7.50 V. With the decade resistance box set to 200 0, measure the current and enter it in the data table. 2. Repeat the above step for a total of 7 data points each time increasing the resistance by 100 02. You should check the voltage across the resistance box and maintain it at 7.50 V. This may require you to make small adjustments in the voltage of the power supply. 3. Turn the power supply off. Procedure (3): Constant I 1. Set the resistance box to 50 0. 2. Turn the power supply on and adjust the voltage until the ammeter reads 20 mA. 3. Measure the voltage across the resistor and record it in the data table. 4. Warning: At no time (not even for an instant) should you have zero resistance in the circuit while the power supply is on. 5. Repeat the above step for a total of 7 data points each time increasing the resistance by 50 0 and increasing the voltage of the power supply keeping the electric current constant at 20 mA. 6. Turn the power supply off.\fExperiment (4) Data Table Constant R+ SR = 200 + 0.50 Constant V = 7.50 V Current Voltage Resistance Current (mA) (Volts) (12) (mA) 4. 96 10. 005 0. 9920. 005 200 0.6 35. 120.05 10 10, 1 10. 05 2. 03 20. 005 300 + 0.5 2 3 61 0.05 15 14. 97 0.05 2. 98 to. 005 400 + 0.5 17.6- 0.05 20 20. 07 0. 05 4. 0010. 005/ 500 to 14. 140. 05 25. 0+ 0.05 4. 99+0. 005 600 * 0.5 11. 8 2 0. 05 30 29. 71 0.05 5.92+ 9005 700 + 0.5 10. It 0.05 35 35. 1+ 0.05 6.98 + 0.005 800 10.5 8 . 8 2 + 0. 005 Constant I = 20.0mA 19.9mA Light Bulb Resistance Voltage Current Voltage V+SV VA SV (12) (Volts) (mA) (Volts) 50 to.5 0. 995 10.0005 10 11. 91 0.05 0. 635+ 0.0005 100 to.5 2.00 10.005 20 79. 6+ 0.05 0.050to.0005 150 to.5 2. 99+0.005 30 30. 61 0. 05 0.095+0.0005 200 to.5 4.01+ 0.005 40 396 5 +0.05 0. 12910.0005 250 to.5 14.98+ 0.005 50 49- 4+/ 0. 05 0. 174 +0.0005 300 + 0 .5 16.07+ 0.005 60 60.0+0.05 0 . 239 1 0-0005 350 0.5 6.99+ 0.005 70 70. 04 0. 05 0. 226 +0.0005IV. ANALYSIS 1. For procedure (1), plot Von the vertical axis vs. I on the horizontal axis. Draw the best straight line "fit" for the data. Calculate the slope of this line and compare it to the constant R by calculating the % difference. Calculate the slope and the % difference on the same sheet as the graph. slope - R %difference = X 100 slope + R (2) 2 2. For procedure (2), plot I on the vertical axis vs. 1/R on the horizontal axis. Draw the best straight line "fit" for the data. Calculate the slope of this line and compare it to the constant voltage by calculating the % difference. Calculate the slope and the %% difference on the same sheet as the graph. Notice that the horizontal axis is (1/ R).3. For procedure (3), plot W3. R on the horizontal axis. Draw the best straight line \"t\" for the data. Calculate the slope of this line and compare it to the constant current by calculating the % difference. Calculate the slope and the % difference on the same sheet as the graph