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

[David G. Dixon]

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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