Equivalent resistance when there is only one terminal?
This is your circuit, I just put names and values for resistors. The triangle at the bottom of the circuit represents ground.

R1 &amp; R2 are connected in parallel because the upper terminals are wired together and the other terminals are connected to ground (Which means they are connected to each other in the same time). If you connect any thing else to ground, it is also connected to the terminals of R1 and R2.

When you connect resistors in parallel, you get an equivalent resistor according to the equation:

1/Req = 1/R1 + 1/R2 + 1/R3 + 1/R4 + ....

In our case, 

1/Req = 1/6 + 1/3 

Req = 2 ohms  ( this is the equivalent resistor of R1 and R2 only )

The above calculations means that you can remove both R1 and R2 from the circuit and put one resistor of 2 ohms instead of them. 

So, let's imagine that there is only single resistor rather than R1 and R2. that single resistor has two terminals, one of them is connected to ground and the other one is connected to R1. 

Therefore, R1 is connected in series with the combination of (R2 and R3) because one terminal only of R1 is connected to the combination and the other terminal of R1 is not connected to the combination.

If you have some resistors in series, The equivalent resistance is calculated according to the equation:

Req = R1 + R2 + R3 + ...

In our case,

Req = 2 + 10 = 12 ohms ( this is the total or the equivalent resistance of the circuit)







simulate this circuit &ndash; Schematic created using CircuitLab