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(b) When the current is at a maximum, its derivative is zero. Thus, Eq. 30-35 gives ε L = 0 at that instant. Stated another way, since ( t ) and i ( t ) have a 90° phase difference, then ( t ) must be zero when i ( t ) = I . The fact that φ = 90° = π /2 rad is used in part (c). (c) Consider Eq. 31-28 with / 2 m = − . In order to satisfy this equation, we require sin( ω d t ) = –1/2. Now we note that the problem states that is increasing in magnitude , which (since it is already negative) means that it is becoming more negative. Thus, differentiating Eq. 31-28 with respect to time (and demanding the result be negative) we
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