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.Procedure (5): Solid State Diode: Silicon or Germanium 1. On the same circuit board as the light bulb, there are two diodes, one is a silicon diode and the other is germanium. You can use either one. 2. Replace the lamp from the last procedure with one of the diodes. See gure (4.3). The circuit symbol for a diode is a triangle (like an arrow). Under normal operations, the diode allows current to pass only in the direction of the \"arrow\". This happens when you wire the circuit as shown in gure (4.3). In this situation, the diode is forward biased and it starts to conduct current when the voltage across it exceeds a certain minimum value. This minimum value is about 0.20 V for germanium and about 0.60V for silicon. Below this value, the diode does not conduct the current and is said to be \"oft\". 3. Note that the power supply is connected to the diode through a currentlimiting 100 Q resistor. 4. Turn the power supply on. Increase the supply voltage until the voltage across the diode is around 0.2 V. Measure the electric current. If this reading is zero, record zero. . 5. Repeat the above step for a total of 7 data points each time increasing the voltage across the diode by approximately 0.1 V and recording the current and the voltage. 6. Do not excwd 0.7 V for the germanium and 0.9 V for the silicon. 7. If time permits, reverse the power supply connection to the diode and take 5 data pomts each tune increasmg the voltage by 0.1 V and recording the current and the voltage. You are done with the experimental procedure. , , >7 8. Turn the power supply off. \f5. For procedure (5), plot I on the vertical axis vs. V on the horizontal axis. Draw a smooth curve fit for the data set. The I - V graph of a diode is nearly piece-wise linear. 6. Calculate the "off" resistance, V Roff (4) using data in the early part of the data set when I ~ 0. Note that this is the reciprocal of the slope of that part of the curve. 7. Calculate the "on" resistance using data in the latter part of the data set when the diode is "on". The dynamic resistance of a (p-n) junction diode is defined as dV Rdynamic = di (5) Note this is the reciprocal of the slope of the tangent to the curve