resistor in parallel/series

devcat

How can i determine resistors are connected in parallel or in series?
By current or potential.difference?
The following are some complicated example..

danago

Resistors are in parallel when they share the same nodes. They are in series when the same current flows through each.

devcat

Hey thanks! What's the meaning of nodes?

th3plan

its a branching point

danago

A node is where two or more circuit elements meet, so the voltage at every point in a node will be the same.

Ive taken this image from wikipedia, but its a pretty good image for describing what nodes are:



Basically, each of the three colours form a node each.

In your first image, can you see how there are three nodes? Each resistor is connected to two nodes, one at each terminal. If two resistors are in parallel, then they are both connected to the same two nodes.

devcat

In the picture above
R3 & R4 are in parallel
R1 & R2 are in series
When resistor are not in parallel, Are they must be in series?
Thanks so much!

danago

Yep R3 and R4 are in parallel.

If resistors are not in parallel, it does not necessarily mean they must be in series. R1 and R2 are not in series. If there was a current coming out of the left of R2, it would split up at that junction, so not all of the current through R2 would go through R1, some would go through the voltage source.

devcat

R3 and R4 are in parallel , do their Voltage equals to the cell?
Or the voltage of RED and BLUE nodes are the same?

Secondly,In picture above, it seem current would split up in Junction X and Y
however the answer is two resistor in series.Why?

danago

The red and blue nodes are at different voltages.

Have you drawn the diagram correctly? I wouldnt have thought there were any in series.

Defennder

He meant to say there the problem may be reduced to 2 equivalent resistors in series, which is correct. For the second one, I suggest you denote one node by V_A and the other by V_B. Then trace out all the possible paths between V_B and V_A. You'll find that it reduces to a single configuration.

EDIT: danago, I think you're looking at the wrong circuit. He was referring to his, not the Wikipedia one.