I understand that when you put Resistors in parallel the resistance decreases, my question is why does this actually happen? What causes the Resistance to decrease? When Resistors are in serial, their Resistances are added, why when we put them in parallel do the Resistors resist less current? I don't understand how that's possible?
Answer
Instead of thinking of these things as "resistors", try thinking of them as "conductors". After all, that's what they do: conduct.
A resistor with resistance \$R\$ is a conductor with conductance \$S=\dfrac{1}{R}\$.
When you provide multiple conductors connecting one point to another, the conductances simply add. What could be more intuitive? When you provide an additional path for current to flow, more total current flows.
Conductors \$S_1\$ and \$S_2\$ in parallel have a total conductance of:
\$S = S_1 + S_2\$
If you want to express \$S_1\$ and \$S_2\$ as resistances \$R_1\$ and \$R_2\$, you get:
\$S = \dfrac{1}{R_1} + \dfrac{1}{R_2}\$
And, if you want to express the total conductance S as a resistance R:
\$\dfrac{1}{R} = \dfrac{1}{R_1} + \dfrac{1}{R_2}\$
\$R = \dfrac{1}{\dfrac{1}{R_1} + \dfrac{1}{R_2}}\$
Which is the usual expression for the total resistance of two resistors in parallel.
Trivia: the unit of conductance (i.e. inverse ohms) is sometimes called the "mho" ('Ohm' backwards), and is written with an upside-down Omega symbol: ℧. The official SI name for this unit is siemens ("S").
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