S13Phys2BaLec31A

# Note i max only occurs for a small amount of time the

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Note: I max only occurs for a small amount of time, the instantaneous current is likely a different value. P = i 2 R

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AC Circuits Which average current value should you use for calculating the energy dissipated? The average value of current is zero. But we actually see power in the circuit, so this can’t be right. Since direction is not important, the current values should always contribute to power.
AC Circuits Squaring the current, will eliminate this directional dependence. So, we define a new value to help us. We call this the rms current (I rms ). rms stands for Root of the Mean of the Square. I rms = I max 2 = 0.707 ( ) I max

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AC Circuits The alternating voltage can also be discussed with respect to rms values. Δ V rms = Δ V max 2 = 0.707 ( ) Δ V max We can now define the average power dissipated by a resistor in an AC circuit carrying a current I as: P avg = I rms 2 R Ohm’s Law can either be expressed as: Δ V rms = I rms R Δ V max = I max R
Clicker Question 31A-1 Which of the following statements about average power and average current can be true for a resistor connected in a simple series circuit to an operating AC generator? A) P avg = 0 and i avg = 0. B) P avg = 0 and i avg > 0. C) P avg > 0 and i avg = 0. D) P avg > 0 and i avg > 0. E) P avg < 0 and i avg < 0.

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AC Circuits with Cap. Consider a simple circuit containing a capacitor and an AC source. Initially the current charges the capacitor plate without hinderance. This initial value of current will be a maximum. As the charge on the plates increases, the voltage across the capacitor increases and the current flowing in the circuit decreases.
For Next Time (FNT) Finish reading Chapter 31 Start the homework for Chapter 31
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