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AC Circuits

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Spacer Gives an overview of AC circuits. In particular, the following theoretical predictions concerning AC circuits are checked by the student:

  1. the current in a circuit and the voltage across a resistor are always "in phase";
  2. the current in a circuit leads the voltage across a capacitor by 90 degrees;
  3. the current in a circuit lags behind the voltage across an inductor by 90 degrees;
  4. the ratio of the voltage across a capacitor to the capacitive reactance, 1/wC, gives the current in that circuit element;
  5. the ratio of the voltage across an inductor to the inductive reactance, wL, gives the current in that circuit element;
  6. for an LRC circuit with a square wave driving voltage:

    a. when R < 2*(L/C)1/2, the current oscillates with an angular frequency given by w = (1/L*C)1/2;

    b. when R = 2*(L/C)1/2, the current does not oscillate but rather returns to zero in the fastest possible time; and

    c. when R > 2*(L/C)1/2, the current does not oscillate but rather returns to zero slower than the critically damped case.

  7. for an LRC circuit with a sine wave driving voltage:

    a. when w2 << the natural oscillating frequency squared, wo2, the current leads the driving voltage by approximately 90 degrees;

    b. when w2 = wo2, the current is in phase with the driving voltage; and

    c. when w2 >> wo2, the current lags behind the driving voltage by approximately 90 degrees;

  8. a plot of the angular frequency of the driving voltage vs the current is that of a typical resonance curve; and
  9. the ratio of the driving voltage to the impedance gives the current in the circuit.

Quantitative results tend to be within 10% of the theoretically predicted values.

Course Level

Introductory level - P4, P14, P16

Student Handouts

Equipment

Number of set-ups available: 20

Per lab station:

1 oscilloscope
1 function generator
1 frequency counter
1 multimeter
1 resistance substitution box
1 capacitance substitution box
1 inductor
8 banana leads

Charges apparatus

Notes


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