# Math Is Fun Forum

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## #1 2021-01-02 07:33:06

don.farr
Member
Registered: 2020-11-24
Posts: 3

### I'm in trouble of 10 and exponent math.

A)    VL =    Δi /Δt
(200*10-6) ((200*10-3)/2*10-6)
0.002       ((   2        ) / 0.00002)
0.002 * 1OOOOO
200V

B)    VL =    Δi /Δt
(200*1.0e-6) ((200*10-3)/2*10-6)
0.02       ((   2        ) / 0.00002)
0.0002 * 1OOOOO
20V

Where VL = Instant voltage on an inductor
Δi = Change in amperage
Δt – Change in time

The answer given in the textbook is “20 volts” However, using the book’s formula (A) I come up with 200V instead of the answer of 20V
So I created process “B” and I get the correct answer, but I cannot not explain why there is a difference between using the “Micro value (10-6) and using the exponent value 1.0e-6”

Last edited by don.farr (2021-01-02 07:36:06)

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## #2 2021-01-02 20:52:47

Bob
Administrator
Registered: 2010-06-20
Posts: 10,542

### Re: I'm in trouble of 10 and exponent math.

hi don.farr

I'm confused in trying to compare your two methods.

In B

(200*1.0e-6)

What does this mean?

1.0 x e^-6 is a different thing altogether.  e raised to the power -6 would not be a simple power of ten.

Are you using the 'exp' key on a calculator here.  That would make this 1.0 x 10^-6 which makes this the same as A.

Also

0.02       ((   2        ) / 0.00002)
0.0002 * 1OOOOO

Here you have changed 0.02 into 0.0002.  I cannot see why.

This physics is not my area but I can substitute values into a formula and evaluate.  Please go back a step and say where the values have come from.  Then I'll try to see what the correct evaluation is.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

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## #3 2021-01-03 03:39:52

don.farr
Member
Registered: 2020-11-24
Posts: 3

### Re: I'm in trouble of 10 and exponent math.

Good morning Bob and thanks for your help.  Yes I am using the exponent key on a calculator.
No matter how I do this, problem (A) is supposed to resolve to 20A.  I get 200A using the equation given in (A)

The textbook equation is give in (A) without any changes.  So I ask, what am I doing wrong in my evaluation of the formula?   Is it possible that the formula is incorrect or the answer given is incorrect?

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## #4 2021-01-03 05:06:40

zetafunc
Moderator
Registered: 2014-05-21
Posts: 2,436
Website

### Re: I'm in trouble of 10 and exponent math.

Hi don.farr,

Welcome to the forum. I believe you're missing out a factor of 10 in your multiplication -- note that:

This might help:

I'm getting 20 as the correct answer when I run through your calculation in A. Let us know if you need any more help!

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## #5 2021-01-03 05:36:48

don.farr
Member
Registered: 2020-11-24
Posts: 3

### Re: I'm in trouble of 10 and exponent math.

Got it.  I was using the wrong button on the calculator.  I was using the exponent button instead of power button.  Thanks!!!!!

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