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December 3rd, 2017, 02:31 PM   #1
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Finding the value of A and b from a log graph.

This is the question in my book:
https://image.prntscr.com/image/eoWE...tBk7l6Z6WQ.png
I made on paint how far I got so far.

However I'm having trouble with the last 2-3 questions.
I believe "A'' is 1,9 because for the differential equation I took: 0,74-0,175 = 0,565
and 0,003-0,298 = 0,298 and 0,565/0,298 = 1,9

However, can someone verify that that's correct?
And if someone could help me with the other two, I'll very much appreciate it.
Thank you.

Last edited by skipjack; December 3rd, 2017 at 02:57 PM.
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December 3rd, 2017, 03:10 PM   #2
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Is 0,45 in the table supposed to be 0,55? Were you given the table or the graph?

Your equation "0,003-0.298 = 0.298" should be "0,003 - (-0.295) = 0.298".

Try to be consistent with the variable names... is it U or V, a or A, b or B?

I would think that A = 1,9 is close enough. The value of B should be slightly less than 0,74 (anything from 0,734 to 0,737 would be acceptable).

Last edited by skipjack; December 3rd, 2017 at 03:38 PM.
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December 3rd, 2017, 03:24 PM   #3
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Yes, 0,55 my apologies.

I was given the non-log table

V, 1.5 2,5 3,5 4,5 5,5
I, 0,51 0,66 0,79 0,9 1,005

Last edited by skipjack; December 3rd, 2017 at 04:03 PM.
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December 3rd, 2017, 04:05 PM   #4
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There are minor inaccuracies in your log table. Were you told to use logs?
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December 3rd, 2017, 04:18 PM   #5
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Quote:
Originally Posted by skipjack View Post
There are minor inaccuracies in your log table. Were you told to use logs?
True; I rounded it up a bit, but it shouldn't matter much. I just need to learn the steps on how to find A and b.

Yes. We got a standard table. Turn it into log. Draw a graph, put the dots on it,
and then solve for A and B
to make the formula U = b*I^a,

so all I need is A and b.

Last edited by skipjack; December 4th, 2017 at 01:43 PM.
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December 4th, 2017, 03:06 PM   #6
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Having found a = 1.9 correctly, b = 0.74 - 1.9*0.003 = 0.7343.

Using b = 0.175 + 1.9*0.295 would give about the same result, especially if you hadn't rounded up the logarithms.
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