A Question about Series-Parallel Circuits

[bibo_dvd]

Hello Guys !

In This photos i found that
in (a) : RT= 4 Ω+ 10 Ω+ 4 Ω= 18 Ω
and
in (d) : RT= 10 Ω

but i can't understand why in (a) the total resistance equals to 18 and how will the current would pass ??

and in (d) i can't understand why the total resistance equals to 10 and why we didn't take the two 4 ohm resistances in the count ?!

Thank you Guys !

 

[Jony130]

Current can flow only in close loops so for example A

In circuit D you have a short wire. And all current will flow through shorted wire.

 

[zamalek fc]

Current can flow only in close loops so for example A

In circuit D you have a short wire. And all current will flow through shorted wire.

why not R=4+4=8Ω ??

 

[Integral]

What is in parallel with those 2 resistors? What is the resistance of a wire? How do you find the resistance of a parallel network?.