# finding current

#### phantomvs

Joined Jan 19, 2016
39
R3 amps is 0 , 0 volts ?
R2 would be 30 v . 3 amps

I think i gotta go to the dentist

#### WBahn

Joined Mar 31, 2012
25,304
R2 and R3 are in parallel. What do we know about the voltage across elements that are in parallel?

#### phantomvs

Joined Jan 19, 2016
39
Man , i knew that should of sticked with my gut feeling.

Voltage in parallel stays the same.

So voltage across R2 & R3 is 0 volts

#### WBahn

Joined Mar 31, 2012
25,304
Man , i knew that should of sticked with my gut feeling.

Voltage in parallel stays the same.

So voltage across R2 & R3 is 0 volts
So how much current is flowing in R2 and R3?

#### pigpen

Joined Jan 26, 2016
23
View attachment 99374

I am stuck on this question, that asks me the following :

If R2 is short circuited , the amps in R3 is : ?

Here is what i did :
I redrew circuit without R2.
What am i doing wrong ?
Hey phantomvs, when you "removed" R2 from the circuit, you turned R2 into an open circuit/branch, and applied the behaviour of open circuits (current is 0 and resistance is infinite) to solving the problem... But the instructions ask you to do the exact opposite: short R2, don't open it. What is the basic behaviour of a short circuit? That is, what is the value of current in ANY short circuit?

#### phantomvs

Joined Jan 19, 2016
39
This is still bugging me.
I have been doing some reading

So what i have jotted dowm is the following.
So i shorted R2.

I get R1 at 6 volts and 0.6 Amps
R2 becomes 24 volts at 0 Amps
since R3 also becomes shorted R3 becomes 24 volts at 0 amps.

Any of this logic good ? Or am i way way off again ?

#### Jony130

Joined Feb 17, 2009
5,145
Can you tell us why there is only a 6V across R1 ?

Also try to find IR2 for this circuit (R2 = 0.1Ω)

#### WBahn

Joined Mar 31, 2012
25,304
This is still bugging me.
I have been doing some reading

So what i have jotted dowm is the following.
So i shorted R2.

I get R1 at 6 volts and 0.6 Amps
R2 becomes 24 volts at 0 Amps
since R3 also becomes shorted R3 becomes 24 volts at 0 amps.

Any of this logic good ? Or am i way way off again ?
We are going round and round.

We've agreed that the voltage across a short is 0 V. So, if you short R2, how can it have anything other than 0 V across it?

If you DID have 24 V across R2, how can you have 0 A through it?

If you DID have 24 V across R3, how can you have 0 A through it?

#### JoeJester

Joined Apr 26, 2005
4,197
In the reference, you saw this image

Jony's problem could be setup like

Fill in the missing information, using scientific notation, and tell us how you solved for the unknowns.

Last edited:

#### phantomvs

Joined Jan 19, 2016
39
If this is not right someone explain this to me
I

#### Attachments

• 55 KB Views: 7

#### WBahn

Joined Mar 31, 2012
25,304
If this is not right someone explain this to me
I
I can't keep track of the various problems here. Does this problem have a short across R2 or not?

If so, then how can the total resistance be 10.10 Ω

If not, then where is the 3 A in R1 going if the currents in R2 and R3 are both zero?

#### JoeJester

Joined Apr 26, 2005
4,197
I did err when I posted R2 as 1e-2 when it should have been 1e-1, as the TS posted. Even though the TS changed the value, he did NOT follow through with the calculations. I think Jony's problem is just a drastic lowering of the resistance, which most would commonly call a short, as the resistance is 0.005 times it's original value.

The TS continually rounded up each answer starting with the total current.

I would recommend to the TS to recompute keeping everything to three significant decimal places. Just maybe he will discover the obvious.

Last edited:

#### phantomvs

Joined Jan 19, 2016
39
Ok guys, i don't like to give up. I am REALLLY sorry about this. I am REALLY trying to understand this.

So this is what i know so far.
In original question i posted .. I said IF R2 is shorted what is amps in R3 ?

Now i know that in a parallel circuit voltage remains the same.
Current is split in the branches and equals the total current of circuit.
Series circuit current is the same throught circuit and voltage varies but sum of all voltage equals supply voltage.

Now i learned that a "short" across a resistor is equal to replacing the resistor with a straight piece of wire.

From what i see in the circuit is that i have 30 volts as my power..
It hits the first resistor in series so would give me 30v on R1 with 3 amps of current. Using V=IxR

Now where i am stuck is if R2 is "short circuited" why does R3 short as well.
I am guessing its the increase of amps on R3 kills it or cause its in parallel with R2 ?
Cause doesn't current take the path of least resistance.
So shouldn't just go to R2 to get to the other side 0v

#### WBahn

Joined Mar 31, 2012
25,304
Ok guys, i don't like to give up. I am REALLLY sorry about this. I am REALLY trying to understand this.

So this is what i know so far.
In original question i posted .. I said IF R2 is shorted what is amps in R3 ?

Now i know that in a parallel circuit voltage remains the same.
Current is split in the branches and equals the total current of circuit.
Series circuit current is the same throught circuit and voltage varies but sum of all voltage equals supply voltage.

Now i learned that a "short" across a resistor is equal to replacing the resistor with a straight piece of wire.

From what i see in the circuit is that i have 30 volts as my power..
It hits the first resistor in series so would give me 30v on R1 with 3 amps of current. Using V=IxR

Now where i am stuck is if R2 is "short circuited" why does R3 short as well.
I am guessing its the increase of amps on R3 kills it or cause its in parallel with R2 ?
Cause doesn't current take the path of least resistance.
So shouldn't just go to R2 to get to the other side 0v
You need to do the one thing that you haven't done yet -- draw the modified circuit.

Consider the case below in which the red line is the wire.

Is it shorting R2 or R3?

Again -- what is a wire other than a zero ohm resistor?

Regardless of the current flowing through a zero ohm resistor, what is the voltage across it?

By definition, components in parallel have the same voltage across them. Since R2, R3, and the short are in parallel, they have the same voltage across them.

So what must the voltage be across both R2 and R3?

Given that voltage, what must the current through both R2 and R3 be?

Therefore, any current that enters the node on the right must go through which branch?

#### phantomvs

Joined Jan 19, 2016
39
You need to do the one thing that you haven't done yet -- draw the modified circuit.

Consider the case below in which the red line is the wire. Which case below ?

Is it shorting R2 or R3? In my original diagram it Shorts R2

Again -- what is a wire other than a zero ohm resistor? A conductor or a jumper

Regardless of the current flowing through a zero ohm resistor, what is the voltage across it? 0 Volts

By definition, components in parallel have the same voltage across them. Since R2, R3, and the short are in parallel, they have the same voltage across them.

So what must the voltage be across both R2 and R3? 0 Volts

Given that voltage, what must the current through both R2 and R3 be? 3 Amps

Therefore, any current that enters the node on the right must go through which branch?
Parallel Branch

#### WBahn

Joined Mar 31, 2012
25,304
Ah, my file didn't upload successfully. Here it is:

#### WBahn

Joined Mar 31, 2012
25,304
A HUGE part of your problem is that you don't seem able to look at what you've just said and ask if it makes sense in light of what you said immediately before it.

Consider:

So what must the voltage be across both R2 and R3? 0 Volts

You just said that the voltage across R2 and R3 is 0 V.

Okay, fine.

Given that voltage, what must the current through both R2 and R3 be? 3 Amps

But now you are saying that R2 and R3 both have 3 A of current in them.

How can that possibly be since you've just stated that neither resistor has any voltage across it?!

Circuit analysis (and many things in life) require that you determine one constraint on the solution to a problem and then solve the problem consistent with that constraint. But you immediately forget and/or ignore any and all constraints that you've just figured out!

This is like trying to plan a trip across a long desolate stretch of highway that doesn't have any gas stations for 300 miles from the starting point, determining that your gas tank only holds 10 gallons of gas, and then figuring that since your car gets 25 miles to the gallon you will need 12 gallons of gas and, forgetting that you've already established that you only have a 10 gallon tank, setting off down the road into the desert.

If a non-zero valued resistor has zero volts across it, then it has zero current in it. Conversely, if a non-zero valued resistor has any amount of current in it, then it does NOT have zero voltage across it.

Once you declare that such a resistor has zero volts across it, then all kinds of red flags and warning bells need to go off if you then try to state that there is any non-zero current in that same resistor.

#### phantomvs

Joined Jan 19, 2016
39
Thanks WBahn

I thank you for your time and help this forum provided me.

#### pigpen

Joined Jan 26, 2016
23
Given that voltage, what must the current through both R2 and R3 be? 3 Amps
Ooops!!! Pls tell me if this is yet another big hole in my knowledge... The way I remember this type of problem being explained in class is (of course the jumper wire will have 3 Amperes but) that compared to the zero resistance in the jumper wire/short, the resistance of R2 and R3 and any other paralleled resistor, would be so great that it might as well be infinite, so the current will always only pass through the jumper or the short. So we had to redraw that part of the circuit with only the jumper / red wire in your diagram, no more R2 and R3. In effect the only resistance left in the system is R1.

If I encountered the TS question in an exam I would have answered : R2 and R3 no longer exist because of the short, but that branch now has 3 amperes.

#### WBahn

Joined Mar 31, 2012
25,304
Ooops!!! Pls tell me if this is yet another big hole in my knowledge... The way I remember this type of problem being explained in class is (of course the jumper wire will have 3 Amperes but) that compared to the zero resistance in the jumper wire/short, the resistance of R2 and R3 and any other paralleled resistor, would be so great that it might as well be infinite, so the current will always only pass through the jumper or the short. So we had to redraw that part of the circuit with only the jumper / red wire in your diagram, no more R2 and R3. In effect the only resistance left in the system is R1.

If I encountered the TS question in an exam I would have answered : R2 and R3 no longer exist because of the short, but that branch now has 3 amperes.
I don't see any problem with your knowledge -- this is the line of reasoning that I am trying to lead the TS to one step at a time. My concern now is that, by spelling it out, the TS will read your answer, think it makes sense, but not really grasp it and, given another similar problem (say on an exam) will not be able to work it because they didn't earn it themselves and so something still hasn't clicked in THEIR knowledge of the topic. That is why we don't just supply answers in Homework Help, but rather try to lead the TS into figuring it out for themselves.