Basic Physics II

Lab 4 : Ohm’s Law and Resistors  (October 7 & 8)

In this lab you will examine Ohms law for resistors and see if it works for diodes.  You will also examine the behavior of a couple of “exotic” resistors, photosensitive resistors and a thermistor.  We want you to become familiar with multimeters and making simple electrical measurements.

Part I:  Ohms Law for Resistors

        Set up a circuit like figure 1 below, with the power supply off.  You will measure the current through a resistor at the same time you measure the voltage across it.  This means that the voltmeter will be connected across the resistor (Parallel with it) and the ammeter will be in series with the resistor.  With the meters connected, it will look like the figure 2. below.

Fig 1.

 

Fig. 2

 

Have me check your circuit before you turn on the power supply!

        Simultaneously measure the current through the resistor and the voltage across it for several voltages from -10V to + 10V, say in 2V increments.  To get negative voltages, just reverse the connections to the power supply.  If

        V = IR                                                                                                                             1.

a plot of voltage versus the current should be linear and the slope should be the resistance.  (What should the intercept be?  Is it?)  Do this and find the slope using the linear regression routine.  Compare this value to the value you get on your multimeter using it as an Ohmmeter.  (You should see if the difference between them is greater or less than 2 standard deviations for your slope.)  What should the intercept be and why?  Is it?

Part II:  Diodes

        Now you will replace the resistor with a diode.  Put the diode in so that the end with the band on it is toward the negative terminal of the power supply.  The diode only “wants” to conduct current in this direction and the current and voltage are not linearly related.  A diode is said to be non-Ohmic.  The current is exponential in the voltage, i.e.

                                                                                                       2.

 

 

Measure the voltage across the diode for currents of 3mA, 10mA, 30mA, 100mA, 300mA and 1mA.  To do this you will have to put a resistor in series with the power supply as shown at the right.  For the smaller currents, R = 330,000W is appropriate.  Then switch to a 10,000W for the larger currents.  Let me see your circuit before you turn the power supply on!

        Plot I vs. V.  Is this linear?  Now plot the natural logarithm (Ln) of the current vs. the voltage.  (Note the interchange of the axes!)  (For the currents above, exp(heV/kT) >> 1 so the −1 can be neglected in eqn. 2.)  You only need to consider the terms in eqn. 3 below.

                                                                                                   3.

This should be a straight line.  Is it?  Again, find the slope of this line.  The slope should be

                                                                                                                                         4.

Where e is the fundamental charge, 1.6x10-19C; k is Boltzmann’s constant, 1.38x10-23J/K; T is the absolute temperature in Kelvin, about 293K for room temperature; and h is a constant (it is between 0.3 and 3) dependent on the type of diode.  How does your slope compare to that given by equation 4?  What would h have to be to make them equal? 

        Now reverse the polarity on the battery and try to drive a current through “backwards”.  If you apply –1V to the diode, what is the current?  What about –10V?

Part III:  Thermistor

        Thermistors are resistors whose resistance is strongly dependent on the temperature.  Use the multimeter as an Ohmmeter to measure the resistance of a thermistor at room temperature.  Then measure the resistance when it is near, but not touching the ice.  Then put is near the hot water.  Record these values.  Typical thermistors are good at measuring temperatures in the range of – 40oC to around 100oC or a little above.  (Specialized ones can go to lower or higher temperatures.)  Does it vary a lot?  You might compare it to your resistor in part I.