Executing Circuit: Troubleshooting R_c Short-Circuiting

[Rectifier]

Hey there!
I don't understand why you can't execute following circuits:

It looks like $R_c$ is short-circuited in $a$. But I can't see what the problem is in $b$.

Can someone please help me?

 

[gneill]

Passive components are never a problem, do with them as you will. Pay attention to sources though.

 

[Rectifier]

I have been sitting with this problem an hour now and I am pretty sure that it is something obvious, but I can't really see what the problem is with the sources :(

 

[SammyS]

Hey there!
I don't understand why you can't execute following circuits:

It looks like $R_c$ is short-circuited in $a$. But I can't see what the problem is in $b$.

Can someone please help me?

There is more wrong with circuit (a) than the fact that R_C is short circuited.

Do you know Kirchhoff's Voltage Law ?

 

[Rectifier]

Do you know Kirchhoff's Voltage Law ?

Yep, the sum of voltages around a loop is 0.

$a$
Starting from 1V source - voltage across $R_A$ is 1 V according to KVL.

Starting from 2V source - voltage across $R_B$ is 2 V according to KVL.

Wich means that the inner loop containing $R_A$ and $R_B$ is not satisfied because equation (according to KVL) in that loop is $1+(-2)=0$ which is not true.

$b$
The same thing applies to $b$ as to $a$.

Am I right?

 

[SammyS]

Yep, the sum of voltages around a loop is 0.

$a$
Starting from 1V source - voltage across $R_A$ is 1 V according to KVL.

Starting from 2V source - voltage across $R_B$ is 2 V according to KVL.

Wich means that the inner loop containing $R_A$ and $R_B$ is not satisfied because equation (according to KVL) in that loop is $1+(-2)=0$ which is not true.

$b$
The same thing applies to $b$ as to $a$.

Am I right?

Yes.

You will find a number of such inconsistencies in these circuits.

In particular, can you find any loop(s) with only voltage sources and without any resistors?

 

[Rectifier]

In particular, can you find any loop(s) with only voltage sources and without any resistors?

Sorry, but I don't think I understand your question. Should I remove the resistors from a and b in the problem? :)

If yes, then the hole circuit can be summarised in a circuit where the sources are put sieries but one battery has a reversed polarity.

- + + -
--l|---|l--

Thank you for your help!

 

[NascentOxygen]

I can't see what the problem is in $b$.

You can connect one terminal of a battery to one terminal of another battery. There are no problems there.

What you must never do is connect both terminals of a battery^⁕ directly to those of another battery.

Why must you not do that? (Answer in at least 25 words please.)


^⁕ I'm referring to ideal voltage sources

 

[Rectifier]

What you must never do is connect both terminals of a battery directly to those of another battery.

Do you mean like that?

 

[SammyS]

Do you mean like that?

Actually, the polarities or opposite, but voltages are different so, yes, it's a big problem.

 

[NascentOxygen]

That's close to what I had in mind. Whether you connect + to + (and - to -), or + to - (and then - to +), it is still something you must never do. http://physicsforums.bernhardtmediall.netdna-cdn.com/images/icons/icon9.gif

Though were your sketch to have only one voltage source it would still be a forbidden arrangement (shorting out the voltage source).

 

[Rectifier]

When I connect sources like that:

I get infinite current flowing through the wire and batteries. I think that the wire would have burned if I would have done that in real life.

I can't really picture the problem when it comes to the following setup since I don't know what happens when I connect the sources like that:

@SammyS
Yeah, that's what I stated in #7 by
- + + -
--l|---|l-- :)

 

[SammyS]

When I connect sources like that:

[ IMG]http://i.imgur.com/tMf3ZNf.jpg[/PLAIN]

I get infinite current flowing through the wire and batteries. I think that the wire would have burned if I would have done that in real life.

I can't really picture the problem when it comes to the following setup since I don't know what happens when I connect the sources like that:

[ IMG]http://i.imgur.com/pzKrlYf.jpg[/PLAIN]

@SammyS
Yeah, that's what I stated in #7 by
- + + -
--l|---|l-- :)

In this case:

If the voltage is exactly the same for both batteries/cells then the current is zero, so there's no problem.

 

[Rectifier]

If the voltage is exactly the same for both batteries/cells then the current is zero, so there's no problem.

What happens when one V is bigger that the other?

 

[SammyS]

What happens when one V is bigger that the other?

Haven't you answered that already ?

 

[Rectifier]

Oh, have I :O? I still don't get that case in paticular though.

 

[SammyS]

Oh, have I :O? I still don't get that case in paticular though.

It's a short circuit.

Using Kirchhoff gives an invalid equation.

 

[Rectifier]

Using Kirchhoff gives an invalid equation.

Oh!
Thank you SammyS, NascentOxygen and gneill for helping me with this problem!

 

[NascentOxygen]

What happens when one V is bigger that the other?

That arrangement has the EMFs opposing, so if one source is slightly different from the other, then the EMFs won't exactly cancel.

Let's say one EMF is 4.512 volts, and the other is 4.511 volts.
You can predict the loop current as (4.512 - 4.511) ÷ 0 = _□

We are considering ideal elements.

 

[Rectifier]

You can predict the loop current as (4.512 - 4.511) ÷ 0 = _□

We are considering ideal elements.

Shouldnt we take a batteries inner-resistance into consideration?

 

[SammyS]

Shouldnt we take a batteries inner-resistance into consideration?

I think these are considered to be ideal voltage sourses.

Otherwise, yes, internal resistance should be considered.