# Voltage drop across a resistor in an open circuit?

#### sayeo87

Joined Aug 30, 2007
3
Hi, as seen in the problem below, I need to find the voltage drop across the resistors when the switches are open. I know that voltage across a break is equal to the supplied voltage. But how about across a resistor? Thanks for any help.

#### beenthere

Joined Apr 20, 2004
15,819
E = IR. Since I = 0, so does E.

#### mrmeval

Joined Jun 30, 2006
833
When the switches are open the resistors are not connected to anything so they don't get anything. Answer is 0

#### Distort10n

Joined Dec 25, 2006
429
For this exercise the answer is 0V. Practically, the switch is not infinite impedance when open, so there will be a VERY tiny voltage drop.

#### mOOse

Joined Aug 22, 2007
20
Hi, as seen in the problem below, I need to find the voltage drop across the resistors when the switches are open. I know that voltage across a break is equal to the supplied voltage. But how about across a resistor?
The kind of break you are talking about is a completely different situation.
For instance, consider the break caused by the open S1. There is voltage
across it because the terminals of S1 are connected appropriately with a
voltage source.

However, both sides of the resistors are not in contact with the voltage
source when the switches are open, so no voltage can appear across them.

BTW, none of questions a, b, or c actually ask what happens when both
switches are open!

#### recca02

Joined Apr 2, 2007
1,212
for the first case.
v1 = 30/3 v2=0
for case 2
v1=0; v2=20
for case 3 (apply superposition)
v1= 10/r
v2=20/r
i solved this by mere observation if any correction by any one please reply

#### techroomt

Joined May 19, 2004
198
all good mr recca, except i like V1 = 10v and V2 = 20v for case a.

#### recca02

Joined Apr 2, 2007
1,212
yeah i see that now,
wonder whether it was negligence or a typo on my part.
thanks anyways.

#### steeve_wai

Joined Sep 13, 2007
47
there is no voltage across that resistor simply because it is not connected "PROPERLY" to any voltage source in this circuit.hence no voltage is "experienced" by that resistor. i=v/r,v=0.so i=0.

#### cheenu

Joined Apr 11, 2007
1
there are 2 loops........
consider 2 loop current flowing thru each.......

[B]case1[/B]

the 2nd loop is opened hence there is no current thru it .......
hence consider only the 1st loop.......with R and 2R resistors......and Vs1.......
see the resistance......
one is double of the other one.
hence vltge drop across 2R will be DOUBLE of that across R....

since V1 = IR...........(1)

and V2 = I*(2R)

HENCE.... V2/2= IR..........(2)
FROM (1) & (2)

V1=V2/2
2V1=V2
APPLY KVL....
VS1=V1+V2
30= V1+2V1
30=3V1
THEREFORE V1=10V

HENCE V2=20V...........SINCE V2=2*V1

CASE2

SIMILARLY.
FOR THE SECOND CASE....
1ST LOOP WILL NOT BE PRESENT......
AND R IS PARALLEL WITH VS2...
HENCE V2=VS2=10 OR 20 WATEVER IS GIVEN.....

#### 85db

Joined Mar 2, 2008
1
Since the drop across 2R is 0, does it mean that if I measure the voltage across the open switch S2, I'll get all 20v?

#### thingmaker3

Joined May 16, 2005
5,083
Yes. The open switch is an insanely large resistance, so it drops all the voltage.

#### recca02

Joined Apr 2, 2007
1,212
Thats a much better explanation than "the whole voltage appears at the terminals" for people who are confused.

#### japj

Joined Sep 17, 2009
3
there is no way to figure it if you do not know what the resistance or amps are. unless you figure R and 2r as anything you pick out. E/I*R so you have to know 2 of these