Download Measuring Resistance and Equivalent Resistances in Circuits: Ohm's Law Experiment and more Study notes Physics in PDF only on Docsity! Experiment II Ohm's Law and Resistor Circuits Introduction In this experiment you will test Ohm's "law" for a carbon resistor. Then, using this "law", you will determine the equivalent resistance of 2 or more resistors connected in series and parallel. Theory Ohm's law states that for an ohmic conductor, the current I through the conductor is directly proportional to the voltage V applied across the conductor. That is, I % V or I = CV where C is a constant. (2.1) The constant of proportionality C is written as 1/R so that I = V/R and R is called the resistance. (2.2) Thus, the higher the resistance the lower the current for a given applied voltage. R has units of volts/amps or ohms (Ù). Ohm's law, V = IR is only an approximation for the electrical behavior of certain materials under certain conditions. The resistance of many conductors such as metals increases with increasing temperature. When a current I flows through a resistance R, heat is generated at the rate, I2R (Joule heating). Thus, if enough current flows through a resistor to cause it to heat up appreciably, it will behave in a non-ohmic way and one cannot speak of the resistor as having a certain fixed resistance for all currents. Procedure Part I. A study of Ohm's Law Your instructor will discuss with you the use of ammeters and voltmeters. The main points to remember are that a voltmeter has a high resistance and is attached across the ends of a circuit element to measure the voltage between the ends of the element. An ammeter has a low resistance and is never placed across the ends of circuit element. It is always wired into a circuit so that it acts as a connecting wire to the circuit element whose current is to be measured. Construct the circuit below to study Ohm's law for the resistor. Experiment II - Ohm's Law and Resistor Circuits 4 Figure 2.1 - Ohm's law. Figure 2.2 - A 56 kÙ resistor. Figure 2.3 - Resistors in series. The element on the left is a power supply set at 5 VDC. The 340 Ù rheostat is connected as a voltage divider. By moving the rheostat wiper, the voltage across R can be varied from 0 to 5 V. Use one of the three carbon resistors on the board given you as R. Note that the voltmeter V is connected across the ends of R. V and R are said to be connected in parallel. On the other hand, the ammeter A connects the rheostat to the resistor and is said to be in series with the resistor. Measure the current I through R for at least 5 voltages across R between 1 and 5 volts. Note that if you change to a new ammeter scale after you have set the voltage, you will need to reread or reset the voltage because the ammeter resistance changes with a change in scale. Make a linear plot of V versus I. You may do this on the computer using the program “Quattro Pro”. Assume that the meters are accurate to a few percent in estimating your error in V. (We will ignore the fact that the meter readings for small deflections tend to be less accurate than those for large deflections.) Do your data support a straight line fit? That is, does Ohm's law V = IR appear to be obeyed? What value do you obtain for R? Compare the value of R obtained from your analysis with the value given by the color codes on the resistor. (See Figure 2.2.) Then compare your value for R with the value obtained by a direct measurement of R using an ohmmeter furnished by your instructor. Note on determining the value of a resistor using its color bands; Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Gray=8, and White=9; Silver=10% and Gold=5%. Part II A. The Equivalent Resistance of two or more Resistors connected in Series. Wire the series circuit as shown in Figure 2.3 using two of the carbon
