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# COLLEGE PHYSICS B PHY 2054C

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This 4 page Class Notes was uploaded by Garett Kovacek on Thursday September 17, 2015. The Class Notes belongs to PHY 2054C at Florida State University taught by Staff in Fall. Since its upload, it has received 66 views. For similar materials see /class/205541/phy-2054c-florida-state-university in Physics 2 at Florida State University.

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Date Created: 09/17/15

Experiment II Ohm s Law and Resistor Circuits Introduction In this experiment you will test Ohm s quotlawquot for a carbon resistor Then using this quotlawquot 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 0lt V or I CV where C is a constant 21 The constant of proportionality C is written as lR so that I VR and R is called the resistance 22 Thus the higher the resistance the lower the current for a given applied voltage R has units of voltsamps or ohms U Ohm s law V IR is only an approximation for the electrical behavior of certain materials under certain Jquot The 39 of many J such as metals increases with increasing temperature When a current I ows through a resistance R heat is generated at the rate 12R Joule heating Thus if enough current ows through a resistor to cause it to heat up appreciably it will behave in a nonohmic way and one cannot speak of the resistor as having a certain xed resistance for all currents Procedure Part I A study of Ohm39s 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 Ohm39s law for the resistor Experiment 11 Ohm39s Law and Resistor Circuits 4 The element on the left is a power supply set at 5 VDC The 340 U 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 5vnc2 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 Figure 21 Ohm s law 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 1 You may do this on the Blue orange computer using the program Quattro Pro Assume that the Green i S meters are accurate to a few percent in estimating your error in V K ver We will ignore the fact that the meter readings for small de ections tend to be less accurate than those for large de ections A 4 Do your data support a straight line t That is does Ohm s law V 5 6 X l 03 i 10 Q IR appear to be obeyed What value do you obtain for R Figure 22 A 56 kU resiston Compare the value of R obtained from your analysis with the value given by the color codes on the resistor See Figure 22 Then compare your value for R with the value obtained by a direct measurement of R using an ohmmeter lmished by your instructor Note on determining the value of a resistor using its color bands Black0 Brownl Red2 Orange3 a IA Yellow4 Green5 Blue6 Violet7 4 Gray8 and White9 Silver10 and Gold5 Palt II A The Equivalent Resistance of two or more Resistors connected in Series Wire the series circuit as shown in Figure 23 using two of the carbon Flgure 23 39 Resmors 1 senes Experiment 11 Ohm39s Law and Resistor Circuits 5 resistors furnished Measure the current along ab cd and ef Next measure the voltage across afVT bc V1 and de V2 Note that the dashed circles indicate the different places you will need to put the ammeter and voltmeter to make the required measurements It is generally best to wire up your circuits rst without any meters and then to insert and remove them as needed Attach the voltmeter to measure V Set the power supply so that VT 5 V Then measure the voltages V1 and V2 across each of the resistors Does VT V1 V2 within experimental error Now measure the current entering R1 on the side the current owing between R1 and R2 and the current leaving R2 How do these currents compare What can you say about the current anywhere in a series circuit Since you have measured VT and IT you can determine the equivalent resistance Req of your circuit from Ohm39s law VT ITReq Similarly from IT V1 and V2 you can determine R1 and R2 Compare the sum R1 R2 to Req as determined above Does it appear that R1 and R2 in series have an equivalent resistance Req R1 R2 Note that once you have found Req for two resistors in series the addition of a third series resistor R3 gives atotal equivalent resistance of Req12 R3 R1 R2 R3 R1 R2 R3 Hence the equivalent resistance of any number of resistors in series is the sum of the individual resistances B The Equivalent Resistance of two or more Resistors connected in Parallel Wire the two resistors that you used above quot into parallel circuit as shown in Figure 24 Note that in drawing a circuit it is always assumed that the connecting wires have a R A R negligible resistance compared to any resistors 5 V DOE I I in the circuit This being the case here both R1 quot V I I and R2 have the same voltage VT across them A A By de nition two in parallel have the same voltage across them Insert the voltmeter to measure this voltage VT Adjust the power supply so that VT 5 V Now measure the total current IT and then the individual currents I1 and Figure 24 39 ReSiStOTS in PaI 31191 I2 through R1 and R2 Does IT 11 I2 within experimental error Experiment 11 Ohm39s Law and Resistor Circuits 6 The equivalent resistance of R1 and R2 is again de ned by VT ITReq What value do you nd for Req What values do you nd for R1 and R2 Compare your value for lReq with the sum lR1 lRz Are your measurements consistent with the statement that for two resistors in parallel 1 RC ii7 23 q R1 R2 I Note that if a third resistor R3 were to be added in parallel to Req of R1 and R2 the new equivalent resistance would be given by L 1 i 2 4 Req Reql2 R2 39 But this is just 2 Req R1 R2 R3 I so that the extension to any number of resistors in parallel is now evident Conclusions Summarize what you have learned about the voltages currents and equivalent resistances in series and parallel resistor circuits N i If Part 111 If time permits connect a small light bulb in the circuit 51 wlt shown below Vary the power supply from 1 to 5 V and record the current I owing through the bulb Check to see if the light bUIb Obeys Ohmis laW Figure 25 Temperature dependent resistance

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