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PHYS 241 Recitation

PHYS 241 Recitation. Kevin Ralphs Week 11. Overview. HW Questions Alternating Current RMS Reactance Impedance Phasors. HW Questions. Ask Away…. Alternating Current (AC). Alternating current describes a current that periodically reverse direction Terminology

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PHYS 241 Recitation

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  1. PHYS 241 Recitation Kevin Ralphs Week 11

  2. Overview • HW Questions • Alternating Current • RMS • Reactance • Impedance • Phasors

  3. HW Questions Ask Away…

  4. Alternating Current (AC) • Alternating current describes a current that periodically reverse direction • Terminology • Waveform: The shape and form of a signal • Frequency: The number of times the waveform repeats in a second; measured in hertz (inverse seconds) • Period: The length of time it takes for the waveform to repeat; measured in seconds • There are multiple shapes that a waveform can take (square, sawtooth, triangle, etc.), but sinusoidal is most common since it is easily generated mechanically and all other periodic waves can be made by an appropriate mixing of sinusoidal waves

  5. AC Practical Considerations • The instantaneous currents or voltages are not always needed to understand the behavior of a circuit • We can use averaging to get the overall idea of how the circuit is functioning, but a simple average of a waveform will often be zero

  6. AC- Root Mean Square (RMS) • What does it tell me? • RMS is a type of averaging • First square the wave form, then we average and take the square root • Why should I care? • This allows us to keep a form of the Joule heating law

  7. AC - Reactance • What does it tell me? • Capacitors and inductors resist changes in the state of the circuit – Reactance is a measure of this • Why should I care? • Calculating the voltages on capacitors and inductors in an AC circuit can be complicated • Reactance give you a direct link between the across these components and the current in an Ohm’s law type format • It also shows how the frequency of the applied voltage affects the system

  8. AC - Impedance • What does it tell me? • It represents the relationship (magnitude and phase difference) between the applied voltage and the current • Why should I care? • Impedance provides a compact way to carry a lot of information about your circuit

  9. AC - Impedance • Since the impedance carries phase information, it is a complex number • The circuit is at resonance when the impedance is a real number • This corresponds to maximum power transfer to the resistors

  10. AC - Phasors • A phasor is a graphical representation of the relationship between voltage and current in a system • This exploits the power of complex numbers as both vectors and rotations • The phasor rotates through the complex plane and the real projections of the phasor give the measured value • See Demonstration

  11. Quiz Question

  12. Quiz Question

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