- #1
Karol
- 1,380
- 22
Homework Statement
a conduction coil with 2 resistors are connected, according to the drawing. write the expression for the potential difference Vac over the coil after t seconds after the switch is closed.
Homework Equations
The current at any time: (a) i=I(1-e-Rt/L)
The electro motive force (votage) at a pure coil: [itex]V=L\frac{di}{dt}[/itex]
And together with the resistor: (b) [itex]V=L\frac{di}{dt}+iR[/itex]
[tex]\Rightarrow\mbox{ (c) }\frac{di}{dt}=\frac{V}{L}-\frac{R}{L}i[/tex]
The voltage is the sum of the one on the coil plus the one on the resistor R0:
(d) Vac=Vab+Vbc
The Attempt at a Solution
The current in the hole circuit is:
[tex]\mbox{(e) }i=\frac{V}{R+R_{0}}\left(1-e^{-(R+R_{0})t/L}\right)[/tex]
I use the last aquation (e) and insert into (c), instead of the current i:
[tex]\mbox{(f) }\frac{di}{dt}=\frac{V}{L}-\frac{(R+R_{0})}{L}\frac{V}{(R+R_{0})}}\left(1-e^{-(R+R_{0})t/L}\right)=\frac{V}{L}\left(1+e^{-(R+R_{0})t/L}\right)[/tex]
Finally, to get the required voltage on the coil+resistor R, i use, again, (b) and insert into (d) equations (e)+(f):
[tex]V_{ac}=V_{bc}+V_{ab}=V\left(1+e^{-(R+R_{0})t/L}\right)+\frac{VR}{(R+R_{0})}}\left(1-e^{-(R+R_{0})t/L}\right)[/tex]
This does not lead to the required result, according to my book, Sears-Zemansky, 1965:
[tex]V_{dc}=\frac{V_{dc}R}{\left(R_{0}+R\right)}\left(1+e^{-(R+R_{0})t/L}\right)[/tex]