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# lecture21 - Physics 2102 Jonathan Dowling Lecture 21...

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Lecture 21 Lecture 21 Physics 2102 Jonathan Dowling Alternating Current Circuits Alternating Current Circuits

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Alternating Current: To keep oscillations going we need to drive the circuit with an external emf that produces a current that goes back and forth. Notice that there are two frequencies involved: one at which the circuit would oscillate “naturally”. The other is the frequency at which we drive the oscillation. However, the “natural” oscillation usually dies off quickly (exponentially) with time. Therefore in the long run, circuits actually oscillate with the frequency at which they are driven. (All this is true for the gentleman trying to make the lady swing back and forth in the picture too).
We have studied that a loop of wire, spinning in a constant magnetic field will have an induced emf that oscillates with time, Ε = Ε m sin( w d t ) That is, it is an AC generator. Alternating Current: AC’s are very easy to generate, they are also easy to amplify and decrease in voltage. This in turn makes them easy to send in distribution grids like the ones that power our homes. Because the interplay of AC and oscillating circuits can be quite complex, we will start by steps, studying how currents and voltages respond in various simple circuits to AC’s.

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AC Driven Circuits: 1) A Resistor: 0 = - Ρ ω εμφ v
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## lecture21 - Physics 2102 Jonathan Dowling Lecture 21...

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