Phys 351 Lab 4

Phys 351 Lab 4 - Lab 4: RC, RL and RLC circuits...

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Lab 4: RC, RL and RLC circuits Observations and Results Section A-1. RC Circuit. In this section, we observed the response of simple circuits to square waves. The capacitor was observed charging and discharging in order to find what the time constant was. The time constant was then compared to the expected time constant for the RC circuit which was equivalent to 1/RC. The Resistance value was 15kOhms, and the capacitance value was .01uF. The voltage at any point for a charging capacitor is ) 1 ( ) ( / RC t e Vo t Vc - - = and the equation for the voltage at an point of a discharging capacitor is given by ) ( ) ( / RC t e Vo t Vc - = . By observing when the voltage drops to ½ the initial voltage, we can easily find the observed time constant using the formula ) 2 ln( / ) 2 / 1 ( T = τ where T(1/2) is the observed time where the voltage dropped by ½ the initial value. With an input voltage of 5.44 volts, we then observed the oscilloscope for where the voltage was equal to 2.72 volts and then found the time difference from the start of the discharge to this point, thus finding T(1/2). This value was observed to be 140x10^(-6), which gave the time constant value of 2.02E-4. The expected time constant was found to be 1.5E-4 using RC / 1 = . The percent difference in these two values was found to be 31%. The circuit used in this section is shown in figure 1.1
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Figure 1.1, RC circuit used in part A-1 Section A-2. Inductors In section A-2 nothing was done experimentally but we read about inductors and their application in labs. Section A-3. RL Circuit In section A-3, a similar procedure to A-1 was performed. An RL circuit was set up. In an RL circuit, the current changes according to the equation ) 1 ( ) ( ) / ( t L R e Io t I - - = . Using the relation that Io=Vo/R the time constant was then found. The circuit was set up with initial values of resistance of 2kOhms and L of 25mH. The oscilloscope was used to observe one full period of exponential growth and decay and the pattern was sketched. The pattern is observed in figure 1.2
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Figure 1.2, Pattern observed for the RL circuit. The input voltage was 5.2Volts, and V/2 was 2.6. Thus, when the voltage was at V/2, or 2.6 volts, the time was found from the peak to this point, and then from this value, the time constant was found. The half life time was 12x10^-6seconds, and the observed time constant was 17.3x10^-4. The expected value for the time constant was 12.5x10^04. The percent difference was found to be 38.4%. The measured resistance for L was found to be 40.7Ohms and the measured value of R was 2kOhms and the measured value of the function generator was found to be 50.5Ohms. L was expected to be very small because
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Phys 351 Lab 4 - Lab 4: RC, RL and RLC circuits...

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