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. Mathemati 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.Experiment (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 +0.0005Procedure (4): Light Bulb 1. Replace the resistance decade box with the tungsten lamp which becomes the element (resistor) under study. See figure (4.2). Our objective is to see if the tungsten filament of the lamp obeys Ohm's law. We would like to obtain 3 data points before the lamp starts to glow and 4 data points after. 2. Gently increase the voltage of the power supply until the ammeter reads around 10 mA. Measure the voltage across the filament and record the current and the volt- age in the data table. 3. Repeat the above step for a total of 7 data points each time increasing the current by 10 mA and measuring the voltage. Keep an eye on the light bulb to see when the filament begins to glow. Indicate the current and the voltage when this happens. Do not exceed the maximum filament voltage which is 12 V. 4. Turn the power supply off. 24. For procedure (4), plot V on the vertical axis vs. I on the horizontal axis. Draw a smooth curve through the data. Notice the V - I characteristics of the light bulb filament is not linear. (a) Calculate the filament "cold" resistance by using voltage and current values before the filament started to glow preferably close to zero current. Rcold = (3) (b) Repeat the above calculation for the part of the data after the filament started to glow preferably towards the end of the data set. This is the hot filament resistance.