# Executing circuit

1. Apr 27, 2014

### Rectifier

Hey there!
I dont understand why you cant execute following circuits:

It looks like $R_c$ is short-circuited in $a$. But I cant see what the problem is in $b$.

Last edited: Apr 27, 2014
2. Apr 27, 2014

### Staff: Mentor

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

3. Apr 27, 2014

### Rectifier

I have been sitting with this problem an hour now and I am pretty sure that it is something obvious, but I cant really see what the problem is with the sources :(

4. Apr 27, 2014

### SammyS

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

Do you know Kirchhoff's Voltage Law ?

5. Apr 27, 2014

### Rectifier

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?

6. Apr 27, 2014

### SammyS

Staff Emeritus
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?

7. Apr 27, 2014

### Rectifier

Sorry, but I dont 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--

8. Apr 27, 2014

### Staff: Mentor

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

9. Apr 27, 2014

### Rectifier

Do you mean like that?

10. Apr 27, 2014

### SammyS

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

11. Apr 27, 2014

### Staff: Mentor

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 [Broken]

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

Last edited by a moderator: May 6, 2017
12. Apr 27, 2014

### 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 cant really picture the problem when it comes to the following setup since I dont know what happens when I connect the sources like that:

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

Last edited: Apr 27, 2014
13. Apr 27, 2014

### SammyS

Staff Emeritus
In this case:

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

#### Attached Files:

• ###### pzKrlYf.jpg
File size:
3.8 KB
Views:
88
14. Apr 27, 2014

### Rectifier

What happens when one V is bigger that the other?

15. Apr 27, 2014

### SammyS

Staff Emeritus

16. Apr 27, 2014

### Rectifier

Oh, have I :O? I still dont get that case in paticular though.

17. Apr 27, 2014

### SammyS

Staff Emeritus
It's a short circuit.

Using Kirchhoff gives an invalid equation.

18. Apr 27, 2014

### Rectifier

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

19. Apr 28, 2014

### Staff: Mentor

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.

20. Apr 29, 2014

### Rectifier

Shouldnt we take a batteries inner-resistance into consideration?