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Lab #3 and Lab 4: RC and RL Circuits

Lab #3 and Lab 4: RC and RL Circuits. Remember what capacitors and inductors are Remember why circuits containing them can have currents that change with time. No lab report, just excel spreadsheet. Capacitor. Two conducting plates with an insulating dielectric in between.

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Lab #3 and Lab 4: RC and RL Circuits

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  1. Lab #3 and Lab 4: RC and RL Circuits • Remember what capacitors and inductors are • Remember why circuits containing them can have currents that change with time No lab report, just excel spreadsheet

  2. Capacitor Two conducting plates with an insulating dielectric in between

  3. RC Circuit: close switch Charge will flow to the capacitor, charging it and raising its potential. The potential will asymptotically approach V0. The current will be biggest at the beginning, and will get smaller as the capacitor charges. Note: Vc+Vr = constant! RC time constant (t): time for cap to charge to .63 of Vo Current to drop to .37 of max value

  4. RC Circuit: Open Switch Discharge with same RC time constant

  5. Experiment • Be careful with the grounds! Outer shield on BNC cable from scope is at ground. Make sure it goes on ground side of cap when measuring it • instrumentation amplifier: needed when measuring voltage across R • calculate RC from the values of the components • measure the RC time constant (t), which is the time to drop to 37% of maximum signal • compare

  6. Sprague-Goodman Inductor A coil of wire Minus sign means sense of the voltage will be to oppose the change in current.

  7. Inductors Instead of a step function change in voltage, the inductor will develop a voltage across it due to the change in current which will partially cancel the voltage in the battery and reduce the current. Experiment: similar to the last lab, but with L/R circuit L/R time constant

  8. 174 refresher If you have made two measurements of the same thing, how do you check to see if they agree within errors -> Is their difference zero within errors? Calculate chi2 and prob of having a difference that big or bigger…

  9. Comparing more Use the weighted average as your theory. ndof=#data points -1

  10. Least Significant Bit and Sqrt(12) When you have an LSB, what is the random error? Imagine a digital step with width ‘a’ centered at zero. Remember:

  11. Hints • When wiring a circuit, use black wires only for portions of the circuit at ground. • When wiring the circuit, first wire everything except the scope. Add it last. • Be sure scope is DC coupled (AC coupling adds an extra capacitor, beyond the one you want to measure) • Make sure CH1 and CH2 are on the “x1” setting • Make sure they have the same Volts/Division scale • When you measure R, L, and C, make sure they are not embedded in the circuit. If you put an ohm meter across a resistor in a circuit, you measure the resistance of the circuit, not of the resistor, etc.

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