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How do I input this kind of word problem into Wolfram Alpha Pro?

Posted 9 years ago

The current i (in ?A) in a DVD player circuit is given by the equation i = 4.0 - 7.0t where t is time (in ?s) and 0 ? t ? 30 ?s. If q(subscript 0) = 0 C, excluding the initial charge, for what value of t is q = 0 C?

POSTED BY: Arch Yeomans
4 Replies
Posted 9 years ago

Let me double check the problem just to make sure the entry I made was correct.

POSTED BY: Arch Yeomans
Posted 9 years ago

If you draw a graph of the current then I think from your description that will be a straight line with y intercept at 4 and a slope of -7.

From t=0 to t=4/7 there will be current (charge) flowing into the circuit.

To find where the cumulative charge is zero the area in the triangle above the x axis and below the line must equal the area in the triangle below the x axis and above the line. It seems that will happen when x=2*4/7.

The WolframAlpha plot of the integral shows two orange points where the charge is zero, one when t=0 and the other when t=8/7 or 1.1428 microseconds which seems to match this description.

Is there any chance there is something in the description of the problem that needs to be changed? Or did I misunderstand something?

POSTED BY: Bill Simpson
Posted 9 years ago

I think there is something missing. The result should be 0 C at t = 11 ?s.

POSTED BY: Arch Yeomans
Posted 9 years ago
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