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Can anybody help me in this question in A2 phy red book by miles hudson Watch

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    It is page 59 Q 3 b
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    Since current is decreasing according to the graph, using V=IR states that voltage will also decrease. Once the voltage drops bellow 4v the hand dryer will no longer work. Use the known values of V0 (6v, initial voltage) and V (4v, final voltage) in the discharge equation

    V = V0e^-(t/RC).

    This will give the answer in seconds. Hope this helps.


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    Apologies, it asks to use the graph. V= IR , 4 = I x 200 therefor I = 0.02. Use the graphs line of best fit which gives approx 8 seconds.


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    (Original post by x11chickling)
    Since current is decreasing according to the graph, using V=IR states that voltage will also decrease. Once the voltage drops bellow 4v the hand dryer will no longer work. Use the known values of V0 (6v, initial voltage) and V (4v, final voltage) in the discharge equation

    V = V0e^-(t/RC).

    This will give the answer in seconds. Hope this helps.


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    but why is this wrong ? (I know it asks for a graph but theoretically i mean)
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    (Original post by x11chickling)
    Apologies, it asks to use the graph. V= IR , 4 = I x 200 therefor I = 0.02. Use the graphs line of best fit which gives approx 8 seconds.


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    I did that but the answer is in the range of 50 to 55 so you'll have to deduct 60 from 8 you have any idea why? :confused:
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    Dont think it is unless ive messed mytheory up, but it gives the same answer as interpretation. Although, as you actually sub in T = RC as you value for RC as your given a time constant. The answer actually comes out as about 8.1s. So your using V = V0e^-(t/T). But non the less, I over complicated it with that method as it wanted you to interpret the graph. Apologies.


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    The books answer is from 50-55 :O ? seems strange considering the math behind it.
 
 
 
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