Homework Help: Resistors in series/parallel

1. Mar 3, 2006

donjt81

I have a picture attached. Hopefully you guys are able to see it. The question is asking the following:

Find the equivalent resistance Req between points A and B of the resistor network.

I know you have to use the resistors in series/parallel approach. but which of these are in parallel and which are in series. I am having a hard time with these type of question where you have to figure out which resistors are in series/parallel. is there a general rule that i need to follow?

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2. Mar 3, 2006

Astronuc

Staff Emeritus
Series means that there is one common node - i.e. components are end to end, e.g.
*----R1---*---R2---*---R3---*. . . . The current through each resistor is the same.

Parallel means two common nodes.

*---R1---*
| . . . . . .|
*---R2---*
| . . . . . .|
*---R3---*

In the second example the common nodes are at the 'same' potential, so the voltage across each resistor is the same, and currents may be different if R1, R2 and R3 are different.

'Series' implies same current, and 'Parallel' implies same voltage.

3. Mar 4, 2006

Andrew Mason

To work this out, start at the right end and work toward the left.

The last 3 resistors are in series and equivalent to one resistor of 40 ohms. It is in parallel with the 24 ohm resistor. So you can determine the resistance of the right four resistors. (15 ohms). That is in series with the 34 and 21 ohm resistor so those 3 are equal to 70 ohms. So you have a 70 ohm resistor in parallel with a 30 ohm resistor which works out to 21 ohms.

AM

4. Mar 4, 2006

Astronuc

Staff Emeritus