Homework help : The difference between these 2 circuits

[Maghraby]

Homework Statement

Electrophysics

Relevant Equations

the source voltage

I want to know the difference between the two circuits below , and Why don't I bundle both batteries into one 24V battery?
pls help me in this .

This is the original circuit:

And to merge the two batteries , What is the resulting value of a battery resulting from the merger?
I thought that the value would be 24 volts, but after experimenting on the application, I found that it must be 12 volts, how is that?

 

[BvU]

Homework Statement: Electrophysics
Relevant Equations: the source voltage

I want to know the difference between the two circuits below , and Why don't I bundle both batteries into one 24V battery?
pls help me in this ,,
View attachment 336082

Hello @Maghraby ,

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Please read the PF guidelines : we need you to post your attempt at solution before we are allowed to help.

In the mean time you could redraw the circuits in such a way that it becomes clearer ...

$\$

 

[Maghraby]

Hello @Maghraby ,

​

Please read the PF guidelines : we need you to post your attempt at solution before we are allowed to help.

In the mean time you could redraw the circuits in such a way that it becomes clearer ...

$\$

done !

 

[BvU]

I agree with the 12 V.
So: is there a difference between the two circuits, or are they completely identical ?

$\$

 

[Maghraby]

I agree with the 12 V.
So: is there a difference between the two circuits, or are they completely identical ?

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OK , there isn't difference ,but I want to know why 12 V and not 24 V ?

 

[phinds]

OK , there isn't difference ,but I want to know why 12 V and not 24 V ?

So, they are different (12 vs 24) but they are not different? Seriously?

 

[Maghraby]

So, they are different (12 vs 24) but they are not different? Seriously?

You misunderstood. I mean the difference between the two circuits in the first picture.When I merge the 2 batteries , Why will the battery after merging be divided into 12 volts and not 24 volts?

 

[DaveE]

Imagine that you are an electron traveling around that circuit. When you get to the junction between the two batteries you have a choice about which battery you go through. This is fundamentally different than a circuit with one battery.

Can you show us a schematic with two 12V batteries that is equivalent to one 24V battery?
Can you show us a schematic with two 2Ω resistors that is equivalent to one 4Ω resistor?
Can you show us a schematic with two 2Ω resistors that is equivalent to one 1Ω resistor?

 

[BvU]

I claim there IS a difference between the two circuits. I redraw them first:

D and D' are both at 0 V thanks to the fact that both voltages are the same.
So effectively both voltage sources are in parallel, hence the 12 V and not 24 V.

That's no longer the case if one of the two voltages changes.
As an example you could recalculate the first circuit when the voltage on the right is 24 V.

$\$

 

[DaveE]

both voltage sources are in parallel

Nope. There's some resistors in the way.

 

[Maghraby]

I claim there IS a difference between the two circuits. I redraw them first:

View attachment 336088 View attachment 336089​

D and D' are both at 0 V thanks to the fact that both voltages are the same.
So effectively both voltage sources are in parallel, hence the 12 V and not 24 V.

That's no longer the case if one of the two voltages changes.
As an example you could recalculate the first circuit when the voltage on the right is 24 V.

I claim there IS a difference between the two circuits. I redraw them first:

View attachment 336088 View attachment 336089​

D and D' are both at 0 V thanks to the fact that both voltages are the same.
So effectively both voltage sources are in parallel, hence the 12 V and not 24 V.

That's no longer the case if one of the two voltages changes.
As an example you could recalculate the first circuit when the voltage on the right is 24 V.

$\$

OK , thanks for your help !

 

[phinds]

So effectively both voltage sources are in parallel, hence the 12 V and not 24 V.

I understand that you covered yourself by saying "effectively", but that is VERY misleading since they are NOT in parallel and telling a beginner that they are is not a good idea, Due to circuit symmetry, they ACT as though they were in parallel which, clearly, is what you meant by "effectively" but again ... it's misleading to a newbie to circuit analysis who likely doesn't understand serial vs parallel.

@Maghraby, I'd like to see you answer the questions in post #8.

 

[Maghraby]

I understand that you covered yourself by saying "effectively", but that is VERY misleading since they are NOT in parallel and telling a beginner that they are is not a good idea, Due to circuit symmetry, they ACT as though they were in parallel which, clearly, is what you meant by "effectively" but again ... it's misleading to a newbie to circuit analysis who likely doesn't understand serial vs parallel.

@Maghraby, I'd like to see you answer the questions in post #8.

I barely understood the problem previously and I considered that the two batteries were connected in parallel as mentioned previously, so if you have any other clarification, please clarify. I am sorry if there is any spelling error. My primary language is Arabic and I try as much as possible to write in English.

 

[Maghraby]

I understand that you covered yourself by saying "effectively", but that is VERY misleading since they are NOT in parallel and telling a beginner that they are is not a good idea, Due to circuit symmetry, they ACT as though they were in parallel which, clearly, is what you meant by "effectively" but again ... it's misleading to a newbie to circuit analysis who likely doesn't understand serial vs parallel.

@Maghraby, I'd like to see you answer the questions in post #8.

My answer for the question in post #8 is :

Yes, I can display a schematic with two resistors of 2 ohms, which is equivalent to 4 ohms by connecting them in series, and it can also be equal to 1 ohm by connecting them in parallel.

 

[phinds]

I barely understood the problem previously and I considered that the two batteries were connected in parallel as mentioned previously, so if you have any other clarification, please clarify. I am sorry if there is any spelling error. My primary language is Arabic and I try as much as possible to write in English.

The voltage sources are NOT in parallel and BvU's having said that they were was a mistake, which he admits. They happen to ACT as though they were in parallel, in the sense of not doubling to 24V in a final circuit, but they are not actually in parallel. He did not mean to mislead you, but he did. If you do not understand this, then you need to review the basic definition of series and parallel until you understand WHY they are not in parallel.

 

[Maghraby]

The voltage sources are NOT in parallel and BvU's having said that they were was a mistake, which he admits. They happen to ACT as though they were in parallel, in the sense of not doubling to 24V in a final circuit, but they are not actually in parallel. He did not mean to mislead you, but he did. If you do not understand this, then you need to review the basic definition of series and parallel until you understand WHY they are not in parallel.

Well, thanks for your help.

 

[haruspex]

What is the resulting value of a battery resulting from the merger?
I thought that the value would be 24 volts,

Nobody, except maybe @DaveE, seems to have addressed your key error, the belief that voltages in parallel add. They don't. Voltages in series add.
Equal voltages in parallel double the current, but the voltage is the same as for each individual battery.
Unequal voltages in parallel gets more complicated as you would have to consider internal resistance to avoid infinite currents.