I'm trying to learn a little more about basic electronics and am using
Success In Electronics by Tom Duncan. There's a diagram, which I've
posted at
http://homepage.eircom.net/~gerfmcc/circuit.html.
It just shows a battery, 12 ohm resistor, and a switch in series, with
a voltmeter connected across the battery terminals. The text reads:
In the figure, V is a high resistance voltmeter. It reads 6V when the
switch is open and 4.8V when the switch is closed.
a) Calculate the internal resistance of the battery.
That part is okay, I got this:
The current in the circuit -
I = V / R
I = 4.8 / 12
I = .4A
Then -
e.m.f = useful voltage + lost voltage
6 = 4.8 + I * r
1.2 = .4r
So the internal resistance of the battery is 3 ohms.
--------------------------------------------------------------------------------
b) What value of resistor must replace the 12 ohm resistor to give
maximum power output? What will be that power output?
I thought it was necessary to match the load resistance to the source
resistance for maximum power transfer, so a 3 ohm resistor would be
required here. But that's probably wrong, because the answer at the
back of the book says Power output is 3 watts.
Thanks for any help.