Explanation

(b) [image:3_image189.emf]
When switch S is open, the corresponding equivalent circuit diagram is as shown in the figure
[image:3_image190.emf]
The equivalent resistance between $A$ and $B$ is
$R_{eq} = \frac{12 \times 6}{12 + 6} + \frac{6 \times 12}{6 + 12} = 4 + 4 = 8\Omega$
When switch S is closed, the corresponding equivalent circuit diagram is as shown in the figure below
[image:3_image191.emf]
The equivalent resistance $A$ and $B$ is $R_{eq}^{''''''''''''''''''''''''''''''''} = \frac{8 \times 8}{8 + 8} = 4\Omega$