The Student Room Group

Series vs Parallel

series.PNG

Can someone explain why R1 and R2 are both in series and not parallel. I thought that if they were in series, the current going through both of them would have to be equal? But is it not true that there is a branch (into R1 and R3) and so the current entering R1 can not be equal to R2?
Reply 1
Why don't you try reshape the circuit for a clearer picture, for example, put the resistor on the end, R3, parallel to R1. Then ask yourself the same question. Remember to consider R1 and R3 as one resistor with some resistance R say.

Edited this, I put R2 instead of R1 for the two in parallel, didn't read the diagram properly.
(edited 9 years ago)
Original post by Phichi
Why don't you try reshape the circuit for a clearer picture, for example, put the resistor on the end, R3, parallel to R2. Then ask yourself the same question. Remember to consider R2 and R3 as one resistor with some resistance R say.


I'm still confused, how does putting R2 in parallel to R3 explain why R1 is in series with R2? The way I see it, all three resistors are in parallel with each other
Original post by scientific222
I'm still confused, how does putting R2 in parallel to R3 explain why R1 is in series with R2? The way I see it, all three resistors are in parallel with each other
Look at the nodes (junctions) for the components.

Think of it this way:

Fact 1. R2 is connected to the +ve end of the supply. No other component is.

Fact 2. all of the current flowing out of the supply must therefore flow via R2

Fact 3. R3 and R1 share a common point in the circuit at both ends. This is the definition of parallel and means that the voltage potential across both components is identical.

Fact 4. If the voltage across both R3 and R1 is the same, then the current through each resistor (R3 and R1) will only be the same if both resistors are also the same value.

Fact 5. Both R3 and R1 are connected to the -ve end of the supply.




Because R3 and R1 share the same nodes in the circuit, the individual values of R3 and R1 can be replaced with a single 'equivalent' resistor Rparallel i.e. the combined parallel value.

It's this single equivalent resistor which is in series with R2 (and NOT the individual R3 or R1 components).
(edited 9 years ago)
Original post by scientific222
series.PNG

Can someone explain why R1 and R2 are both in series and not parallel. I thought that if they were in series, the current going through both of them would have to be equal? But is it not true that there is a branch (into R1 and R3) and so the current entering R1 can not be equal to R2?


Who says they are in series?
The definition I have of series is that "components are in series such that one current flows in turn through each of them."

More importantly here, what is the question you are trying to answer and does it actually matter how you describe the circuit, can you find the currents?

BTW. They are not in parallel as that requires them to share the same pd. R1 and R3 are in parallel.
(edited 9 years ago)
Original post by uberteknik

Fact 3. R2 and R3 share a common point in the circuit at both ends. This is the definition of parallel and means that the voltage potential across both components is identical.
.


I think I am completely misunderstanding this point (I've seen it before as well).What common points is it that R2 and R3 are sharing? One is the node which is directly next to R2 on the diagram, what is the other one? Whichever points they are, surely R2 and R1 also share the exact same common points at both ends too?
Original post by Stonebridge
Who says they are in series?
The definition I have of series is that "components are in series such that one current flows in turn through each of them."

More importantly here, what is the question you are trying to answer and does it actually matter how you describe the circuit, can you find the currents?

BTW. They are not in parallel as that requires them to share the same pd. R1 and R3 are in parallel.


There is no real question, this is an image from a slide in my lecture where the lecturer wanted us to identify whether or not R1 and R2 were in parallel and the answer was no
Original post by scientific222
I think I am completely misunderstanding this point (I've seen it before as well).What common points is it that R2 and R3 are sharing? One is the node which is directly next to R2 on the diagram, what is the other one? Whichever points they are, surely R2 and R1 also share the exact same common points at both ends too?
Oooooops. I messed up the positions R1, R2 and R3.

I have corrected my original post. Have a look again.

It should make a lot more sense now. Apologies. :colondollar:
Original post by uberteknik
Oooooops. I messed up the positions R1, R2 and R3.

I have corrected my original post. Have a look again.

It should make a lot more sense now. Apologies. :colondollar:


Thanks a lot, it makes sense now

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