- #1
Acuben
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First of all this isn't really Homework problem, but since it is based on homework problem, I'll post on this section of the forum. Feel free to move if I was supposed to post on different part of the forum.
Either way, I'll use homework as an example since that is what brought this up..
Regardless of how long this post looks, it's really a simple question =p
This question involves circuit system using...
Power, Voltage, Current, and Resistance (internal and load resistance), and EMF
This is probably really basic question for most of you
Question is when measuring Current and Power of any part of the circuit, do I use EMF? or Voltage? (So it's two questions. 1. when measuring current, 2. when measuring power)
say...
P=V^2 / R or P=E^2 / R ?
and I= V / R or I = E / R ?
and when using other Power formula such as P=I*V, P=I^2 * R
do I use the current that is derived from using EMF ? or Voltage?
assuming I use I = E/ Rt
P=I*V=V^2/R=I^2*R
I=V/R
V=E-Ir (or V=E+Ir)
where...
P-power
I-current
V-voltage
R-Resistance
Rl-Load Resistance
Ri- Internal resistance
Rt-total resistance
E-EMF (Electro Motive Force ithink...)
This is a conceptual question
Don't read this part, unless you feel it's neccessary...It's not really quotation, I just wanted to box it somehow ...
Is there something that I don't know and should know? (maybe about V vs V(terminal)?)
Either way, I'll use homework as an example since that is what brought this up..
Regardless of how long this post looks, it's really a simple question =p
Homework Statement
This question involves circuit system using...
Power, Voltage, Current, and Resistance (internal and load resistance), and EMF
This is probably really basic question for most of you
Question is when measuring Current and Power of any part of the circuit, do I use EMF? or Voltage? (So it's two questions. 1. when measuring current, 2. when measuring power)
say...
P=V^2 / R or P=E^2 / R ?
and I= V / R or I = E / R ?
and when using other Power formula such as P=I*V, P=I^2 * R
do I use the current that is derived from using EMF ? or Voltage?
assuming I use I = E/ Rt
Homework Equations
P=I*V=V^2/R=I^2*R
I=V/R
V=E-Ir (or V=E+Ir)
where...
P-power
I-current
V-voltage
R-Resistance
Rl-Load Resistance
Ri- Internal resistance
Rt-total resistance
E-EMF (Electro Motive Force ithink...)
The Attempt at a Solution
This is a conceptual question
Don't read this part, unless you feel it's neccessary...It's not really quotation, I just wanted to box it somehow ...
it seems like there is a controversy in the textbook or I simply have wrong idea about this Voltage vs EMF deal...
Let's not worry about significant figures and units in detail...
Book Physics for scientists and Engineers with Modern Physics by Serway Jewett Volume 2, 6th ed (older edition afaik)
page 861
one of the example they gave was...
E= 12.0V
Ri=0.05
Rl=3.00
I= E / (Ri + Rl) = 120 / 3.05 = 3.93
(not I=V / (iR+Rl) ...)
page885 (homework section, number1)
battery with E= 15.0V
V(terminal)= 11.6V delivering 20.0W of power to Rl
a) what is value of Rl?
Rl=V^2 / P
so I have two choices...
1. If V= Voltage
Rl= 11.6^2 / 20
2. If V= EMF
Rl= E^2 / P
Rl= 15^2 / 20
so I was thinking. Of course it should be of course the answer should be E^2/P therefore 15^2/20 > 11.25 Amps
but the answer shows as 6.73 Ohms... = 11.6^2 / 20
so in example it used EMF
and in Homework it used terminal voltage.
So one of them should be wrong?
or is there something that I am not understanding?
Oh by the way, my professor did mention that he'll purposely assign homework with wrong answers just because :P
Is there something that I don't know and should know? (maybe about V vs V(terminal)?)