CURRENT - VOLTAGE RELATIONSHIPS


PURPOSE:	The current - voltage relationships for three different electrical devices will be 
determined experimentally.

WHAT'S THE POINT?	To show that some materials do not obey Ohm's Law in a simple 
fashion; that is, some materials possess resistance which changes under different circumstances.

BACKGROUND READING: Cutnell and Johnson 20.2, 20.3.  Note Equation 20.5.

THEORY:	A material can be characterized electrically by measuring the current (I) that 
flows through the material as a result of an electric potential difference (V) applied across it.  
This information is most readily displayed in an IV curve (called the characteristic curve) which 
is a graph of the resulting current flow versus applied potential difference.  A material is said to 
be OOhmicO, or to obey OhmOs law, if the ratio of the voltage V to the current I is a constant.  
This constant is called the resistance R and has the units of volts / amperes, or ohms («).  
Graphically,  an ohmic device is characterized by a linear IV curve.  Most metals are ohmic 
provided the temperature is not allowed to vary appreciably.  For materials that are nonohmic, 
the current - voltage relation can be a complicated function of the voltage.  With some nonlinear 
IV curves, the definition of resistance given above is of little use in allowing a determination of 
the current for given potential difference.  A more useful quantity is the dynamic resistance

R = ?V/?I

which measures the change in current resulting from small changes in the 
potential difference.  Graphically, this is the reciprocal of the slope of a tangent 
line drawn to the characteristic curve.

PROCEDURE:	During the course of the experiment, do not allow the current to exceed the 
range of the ammeter (500mA).  Before you construct each circuit, turn the coarse and fine 
adjustment knobs on the power supply to their minimum positions, fully counterclockwise.

Part I.  Ohmic Resistor

Hook up the circuit shown in Figure 1 without connecting the power source 
until checked by the instructor.  Do not put more than two connecting leads on 
the same terminal unless really necessary.  Use the decade box as the resistor to 
be studied.	
	 

Set the decade resistance at 6 ohms.  Have the circuit checked by the instructor.  
You will be able to vary and record the voltage in 1/2 volt steps from 0 to 3 
volts.  For each value, measure and record the current.  Be sure that both meters 
are read to the maximum number of significant figures.  Plot current versus 
voltage and from the slope determine the resistance.  Calculate the percent 
difference from the decade box value.  Note:  The slope is not equal to the 
resistance if the graph is plotted as instructed.

Part II.  Semiconductor Diode

Remove the decade resistance box and substitute the silicon diode (see Figure 
2).  Connect the diode so that its end with the silver band is closest to the 
negative terminal of the power supply. In this case it is important that the 
voltage does not exceed 1.2 volts.  When you begin this part, be sure that the 
voltage is at 0 and proceed in steps of 0.1 volt, using the "fine" adjustment knob 
only.  If the diode is biased in the forward direction you should obtain a 
measurable current at about 0.6 volt.  From this value proceed in steps of 0.1 
volt until you reach 500mA or 1.2 volts, whichever comes first.  If the diode is 
biased in the reverse direction you will not obtain a measurable value for the 
current in the range from 0 to 1.2 volts.  DO NOT APPLY VOLTAGES 
GREATER THAN 1.2 VOLTS for either biased condition.

Plot the current versus the voltage for the diode in the forward bias situation.  
Determine the slope of the (approximately) linear portion of the curve and 
calculate the forward resistance (dynamic resistance) of the diode.  How does 
the resistance vary as the voltage applied to the diode increases?


Part III.  Light Bulb

Referring to Figure 1, replace the 0 - 3 volt voltmeter with a 0 - 10 volt 
voltmeter and replace the decade box with a standard light bulb and obtain 
current - voltage data for voltages from 0 to 8 volts in increments of 0.5V.  Is 
the resistance of the bulb constant over this voltage range?  Plot the current 
versus voltage.  Calculate the slope of the tangent line to your curve at the 5V 
position.  What value of the dynamic resistance can you assign to the light bulb 
for a voltage  of 5 volts?  Since the filament of the bulb is metallic, it might be 
expected to behave as an ohmic resistor.  Clearly the shape of your IV curve is 
inconsistent with this notion.  Explain in your notebook why the IV curve is not 
linear.





















	CV-2