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the resistor. Because this rate is proportional to the square of the current, it makes no max is not the same as that produced by a direct current equal to I max . This is because the alternating current is at this maximum value for only an instant during each cycle (Fig. 33.5a). What is of rms current. As we learned in Section 21.1, the notation rms stands for root-mean-square, . Because i 2 varies as sin 2 # t and because the average value of i 2 is I 2 max (see Fig. 33.5b), the rms current is 1 (33.4) This equation states that an alternating current whose maximum value is 2.00 A ! av " I 2 rms R I
rms " I
max √ 2 " 0.707I
max 1 2 I
rms " √ i 2 S E C T I O N 3 3 . 2 • Resistors in an AC Circuit 1037 Figure 33.5 (a) Graph of the current in a resistor as a function of time. (b) Graph of the current squared in a resistor as a function of time. Notice that the gray shaded regions under the curve and above the dashed line for I 2 max /2 have the same area as the gray shaded regions above the curve and below the dashed line for I 2 max /2. Thus, the average value of i 2 is I 2 max /2. 1 That the square root of the average value of i 2 is equal to can be shown as follows. The current in the circuit varies with time according to the expression i " I max sin # t, so i 2 " I 2 max sin 2 # t. Therefore, we can find the average value of i 2 by calculating the average value of sin 2 # t. A graph of cos 2 # t versus time is identical to a graph of sin 2 # t versus time, except that the points are shifted on the time axis. Thus, the time average of sin 2 # t is equal to the time average of cos 2 # t when taken over one or more complete cycles. That is, (sin 2 # t) av " (cos 2 # t) av Using this fact and the trigonometric identity sin 2 & $ cos 2 & " 1, we obtain (sin 2 # t) av $ (cos 2 # t) av " 2(sin 2 # t) av " 1 (sin 2 # t) av " When we substitute this result in the expression i 2 " I 2 max sin 2 # t, we obtain (i 2 ) av " " I 2 rms " I 2 max /2, or I rms " I max / . The factor is valid only for sinusoidally varying currents. Other waveforms, such as sawtooth variations, have different factors. 1/ √ 2 √ 2 i 2 1 2 I
max
/ √ 2 I max I 2 i 2 I 2 1 2 t t (a) (b) i = i 2 max max 0 0 rms current Average power delivered to a resistor |