Effective Resistance Between Points in Circuit

[nolachrymose]

Homework Statement

Four resistors are connected as shown (figure attached). Find the resistance between points a and b.

Homework Equations

$$R_{eq} = \sum{R_{n}}$$ for series resistors

The Attempt at a Solution

The four resistors in the figure are in series. Therefore, to determine the resistance between points a and b, we must find the effective resistance of these resistors. Summing them, we find that $$R_{eq} = 201\Omega$$. However, that answer is incorrect.

Any help in understanding this problem would be really appreciated!

 

[gneill]

The four resistors in the figure are in series. Therefore, to determine the resistance between points a and b, we must find the effective resistance of these resistors. Summing them, we find that $$R_{eq} = 201\Omega$$. However, that answer is incorrect.

That's because the resistors are not all in series. Only the 39 and 53 Ohm resistors are in series.

You can tell that the others are not in series with these because there's other "stuff" connected at the nodes where they join each other.

Now, what you do have is some resistors in parallel... Try redrawing the circuit, shortening the lines ad and bc down to insignificance. Don't let artistic license confuse the underlying circuit topology!