Question about voltage in parallel

[05med05]

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Hi,

I'm having trouble understanding the explanation to a voltage question on one of my practice exams

To find total or effective voltage, if 2 voltage sources are connected in series, eg) a 3V and 6V source, then the effective voltage is 3 + 6 = 9V, but if they are connected in parallel, I read that they produce the same voltage as a single source. What does this mean? Is it 1/3 + 1/6 = 1/9V, as if I was adding R in parallel or am I mistaken? Thanks!

 

[OrthoFixation]

Voltage and resistance are opposite equations for parallel vs. series. I always have to remember one to get the other. If what you said is accurate then:

1/V1 + 1/V2 = 1/Vtotal


Check your fraction math:

1/3 + 1/6 = 2/6 + 1/6 = 3/6 = 1/2

common denominator 😱

 

[05med05]

Oops, sorry about the math,

Are you sure that adding Voltage in parallel is the same as adding resistance in series?? according to Q # 41 of PS in 3R, the question asks:

2 identical photoelectric devices are connected in parallel with each other. Compare to the voltage generated by a single device. The total voltage supplied to the circuit by the parallel devices will be a) 4 times as large, b) two times as large, c) 1/2 as large, d) the same.

The answer is D and the explanation is as follows: identical voltages in parallel produce the same voltage as a single source, but if connected in series, the voltages would be added.

According to this explanation,

Veff = Vi + V2 (series), and
Veff = V1 = V2 (parallel), but

But if Veff are determined in the opposite way as Reff, then the explanation does not make sense, because:

V eff = V1 + V2 (parallel) vs 1/Reff = 1/R1 + 1/R2
1/Veff = 1/v1 + 1/V2 (series) vs Reff = R1 + R2

Which is correct??

 

[Mister Pie]

Hi,

I'm having trouble understanding the explanation to a voltage question on one of my practice exams

To find total or effective voltage, if 2 voltage sources are connected in series, eg) a 3V and 6V source, then the effective voltage is 3 + 6 = 9V, but if they are connected in parallel, I read that they produce the same voltage as a single source. What does this mean? Is it 1/3 + 1/6 = 1/9V, as if I was adding R in parallel or am I mistaken? Thanks!

I have never seen this before. Wouldn't this break the battery? If they are connected in parallel and do not impose the same potential difference, I would think that one, if not both, of the batteries would break.

 

[05med05]

I have never seen this before. Wouldn't this break the battery? If they are connected in parallel and do not impose the same potential difference, I would think that one, if not both, of the batteries would break.

I'm not sure. I used the above numbers just as an example. I've just never seen a q asking to compare voltages in parallel or series before.

In addition to my previous reply, I know Reff and Ceff are computed in the opposite way in series vs parallel, but I've never seen a q ask for the voltage comparison, and the explanation appears suspect.

 

[speranza]

I'm not sure if this answers your question, but voltage in a parallel circuit is the same across each capacitor/resistor. For example, if you have a circuit with three resistors in parallel and the voltage source is 9V, then the voltage across each resistor will also be 9V.

 

[vmp200]

I have never seen this before. Wouldn't this break the battery? If they are connected in parallel and do not impose the same potential difference, I would think that one, if not both, of the batteries would break.

They would be damaged unless both batteries provided same voltage.

 

[05med05]

They would be damaged unless both batteries provided same voltage.

So if both batteries provided the same voltage, (eg) 9 V each, then would the total voltage supplied to the circuit simply be 9 volts (batteries in parallel), as opposed to 18V (batteries in series)??

 

[vmp200]

So if both batteries provided the same voltage, (eg) 9 V each, then would the total voltage supplied to the circuit simply be 9 volts (batteries in parallel), as opposed to 18V (batteries in series)??

Yes, and each would supply half of total current in the circuit.

 

[vmp200]

If the batteries didnt have the same voltages in parallel. it would falsify KVL(Kirchoff Voltage Law = voltage rises and drops around any loop in a circuit must equal ZERO). Since both batteries have the same voltage in parallel, and you applied KVL to that loop that the batteries made it would equal zero, therefore batteries with same voltage can exist. But if they were different voltages then KVL would not equal zero, which cannot occur.

VP

 

[Mister Pie]

If the batteries didnt have the same voltages in parallel. it would falsify KVL(Kirchoff Voltage Law = voltage rises and drops around any loop in a circuit must equal ZERO). Since both batteries have the same voltage in parallel, and you applied KVL to that loop that the batteries made it would equal zero, therefore batteries with same voltage can exist. But if they were different voltages then KVL would not equal zero, which cannot occur.

VP

Yup, that's exactly what I was thinking, hence the confusion.

 

[NilamPatel]

I'm not sure if this answers your question, but voltage in a parallel circuit is the same across each capacitor/resistor. For example, if you have a circuit with three resistors in parallel and the voltage source is 9V, then the voltage across each resistor will also be 9V.

thats what i was thinking maybe he's talking about resistors/capacitors

 

[gotgame83]

I was wondering if anyone could clarify this. I read this thread a few times and i seem to be confused. Lets say you had a 10v battery in parallel with a 5v battery. what would be the voltage of the circuit.

 

[Mister Pie]

I was wondering if anyone could clarify this. I read this thread a few times and i seem to be confused. Lets say you had a 10v battery in parallel with a 5v battery. what would be the voltage of the circuit.

The bottom line is that you probably won't see this on the test. Such a set up would damage your batteries. Why? Because the 10V battery would be trying to set up a potential difference of 10 volts between its + and - ends but the 5 V battery, which is in parallel, would be trying to set up a potential difference of 5 V across it's + and - terminals. Since they are in parallel, by definition, they're supposed to have the same potential difference, but this can't happen if the two batteries are connected in parallel.

 

[KifsterDDS]

This is very basic stuff and you're just confused because you're looking for something tougher that it really is.

Voltages in series are added. Remember in elementary school when you learned how to connect lightbulbs to batteries? The bulbs glowed brighter when the batteries were connected end to end (i.e. in series). That means the voltages are added and more current is produced.

Batteries connected in parallel produce less current, and thus last longer.