Rc circuit multiple choice don't understand why this is the correct answer

[madah12]

Homework Statement

http://physics.ku.edu.kw/LectureNotes/TheoryCourses/PHYS102/ExamPapers/y0910-2-md2.pdf
page 6 number
5

Homework Equations

The Attempt at a Solution

I don't understand how they got the answers ? i applied the loop and junction rule but they only give me more variables to work with
but shouldn't I after a long time be zero? i mean the only answer i can think of is infinity...

 

[madah12]

I thought of trying to use ohms law as emf=I(total resistance) but the problem is i don't know how to treat the capacitor
also by loop rule i know that whatever change of voltage occurred in the capacitor it is the same as I through the resistance *3R) but still all of that is useless

 

[gneill]

At time t=0 the capacitor "looks like" a short circuit. So what is the expression for the current I_o?

After a long time, the capacitor will have charged up to its final value and will "look like" an open circuit (no further current will flow through it). So what's the expression for the current then?

 

[madah12]

I don't know what is a short circuit i know that when the capacitor is full it will be I=e/5R because there is no current in the capacitor , cna you teach me what is a short circuit? it isn't in the book
unviersity physics by young and freedman

 

[gneill]

A short circuit implies a zero ohm (or at least very low resistance) path, often bypassing current around other components. In your circuit, when the switch is first closed and the capacitor is initially uncharged, it "looks like" a zero ohm path for the current -- it "shorts" the 3R resistance so no current will flow through it, all of the current going through the capacitor instead.

 

[madah12]

how can I recognize a short circuit? btw now i see why the answer if 5/2 but i now want to learn how to recognize a short circuit

 

[gneill]

As I stated, a short circuit is a zero resistance path between nodes.

Any unbroken length of wire is a short circuit between its end points. In a DC circuit an uncharged capacitor is initially a short circuit, eventually an open circuit, while an inductor is initially an open circuit and eventually a short circuit.

 

[madmike159]

You use short circuits and open circuits to look at what would happen to some parts of a curcuit. You normaly see this when you study thevenin's theorem, kirchhoff's laws etc.