Please help me solve this circuit

Thread Starter

Beginner0001

Joined Sep 29, 2017
16
Hey guys, can you help me with this?

So i need to solve it in two ways, first when switch is on, and second when switch is off.
I guess when switch is off i can just ignore that line, but what to do whan switch is on, will that change something?
Because i learned at school that if there is no resistor that line has no function.

I hope you understood me, so please help me..

Thank you!
 

Attachments

WBahn

Joined Mar 31, 2012
25,762
Hey guys, can you help me with this?

So i need to solve it in two ways, first when switch is on, and second when switch is off.
I guess when switch is off i can just ignore that line, but what to do whan switch is on, will that change something?
Because i learned at school that if there is no resistor that line has no function.

I hope you understood me, so please help me..

Thank you!
I hope that's not what they tried to teach you.

If the switch is closed, what is the voltage across R4? What is the current through R4?
 

Thread Starter

Beginner0001

Joined Sep 29, 2017
16
Maybe i misunderstood professor.
I think that R4 and R5 is serial connection and then with R3 is parallel, but that works for other case.

Question is which connection is when switch is on.
 

WBahn

Joined Mar 31, 2012
25,762
R4 and R5 is in series WHEN the switch is open because whatever current is flowing in one has to flow in the other -- that's the definition of being in series.

But when the switch is closed, they are NOT in series since the current flowing in R5 does not have to be the same current as the current flowing in R4. Current can flow through the closed switch and then through R5, too. So they are not in series.
 

crutschow

Joined Mar 14, 2008
24,978
So in that case i can ignore R4 cause there is line with switch and no resistance, so current will not go through R4?
From Ohm's law, the current through R4 is the voltage across R4 divided by the value of R4 (Vr4/R4).
So when the switch is closed, what value of current does Ohm's law give you?
 

MrChips

Joined Oct 2, 2009
21,134
Just for my curiosity, what year/grade/level school/college/university course/class is this assignment?

This is not the type of question I would expect to find about circuit theory and Ohm's Law.
 

WBahn

Joined Mar 31, 2012
25,762
So in that case i can ignore R4 cause there is line with switch and no resistance, so current will not go through R4?
Yes, but it's important that you understand why there will not be any current (which you may, but it's not clear that's the case).

With the switch closed the voltage across the switch will be zero volts (assuming an ideal switch). Because the switch and R4 are in parallel, the voltage across the switch is also the voltage across R4. With zero voltage across R4, Ohm's Law dictates that there will be no current through R4 (it will all go through the switch). Thus R4 is effectively removed from the circuit when it is shorted out by the switch.
 
Top