[sdiy] Newbie Electronics Math problem (I need help!)

[Veronica Merryfield]

The question is oddly phrased.

"The current through a wire is equal to 100mA." rather than "the  
current through the wire is 100ma"

"The energy being used by the voltage source is" which would imply  
that it is not providing energy but receiving it.

On 2009-09-07, at 9:46 PM, David G. Dixon wrote:

> Sam,
>
> You're missing nothing here.  You are correct.  Either the textbook is
> wrong, or it's a "trick question" (which is what I always tell my  
> students
> when I've f***ed up a problem!).
>
> P = VI = E/t
>
> Ergo, V = E/I/t = E/Q
>
> I.e., no t, no V.  V could be anything, since it is just the driving  
> force.
> If it takes 17 years for that 0.0025 J to be expended, then  
> obviously the
> driving force needn't be very high!  If it takes a nanosecond, then  
> it is
> pretty high!
>
> You should probably know that student TA's write most textbook  
> problems.
>
> Cheers,
> Dave
>
> David G. Dixon
> Professor
> Department of Materials Engineering
> University of British Columbia
> 309-6350 Stores Road
> Vancouver, B.C. V6T 1Z4
> Canada
>
> Tel 1-604-822-3679
> Fax 1-604-822-3619
>
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>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
>> bounces at dropmix.xs4all.nl] On Behalf Of Sam Ecoff
>> Sent: Monday, September 07, 2009 8:48 PM
>> To: sdiy DIY
>> Subject: [sdiy] Newbie Electronics Math problem (I need help!)
>>
>> Hi All,
>>
>> I'm attempting to solve a problem for my electronics class. I have  
>> the
>> permission of the professor to ask for help, so here I am... I have
>> put about an hour and a half into this, attempting to solve it  
>> myself,
>> researching it on the internet, but I am stumped. The problem is from
>> Page 41 of Electronic Technology Fundamentals 3rd ed. by Paynter and
>> Boydell. It reads as follows:
>>
>> 18. The current through a wire is equal to 100mA. The energy being
>> used by the voltage source is 2.5 x 10^-3 J. What is the value of the
>> voltage source?
>>
>> I understand that 1 Amp = 1 Coulomb per second, but given that they
>> don't give an amount of time, that formula is not particularly
>> helpful. I also get that 1V = 1Joule/Coulomb. I feel that without
>> specifying a time period, the problem doesn't provide enough info to
>> answer the question.
>>
>> What am I missing here?
>>
>> Sam E.
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>
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