Parallel Circuit Resistance
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FIGURE 36-32
Calculating parallel resistance with uneven resistances.

Resistance in a parallel circuit is not as easily calculated as it is in a series circuit, because as branches are added, another path for current to flow to ground is added. Adding extra paths reduces the circuit’s total resistance to current flow. For example, if you have a 12-volt parallel circuit with three branches, each branch having a 12-ohm resistor that allows 1 amp of current flow, and you then add another parallel branch with a 12-ohm resistor to the circuit, the result will be the opposite of what might be expected. Current increases from 3 amps to 4 amps. This is because in a parallel circuit adding more branches provides more pathways, but decreases the overall circuit resistance; thus, current flow increases. This is like having a freeway with three lanes and bumper-to-bumper traffic. If you add a fourth lane, the resistance to flow decreases and cars can move more easily. Another visual example would be if you had a bucket full of water and the bucket had three holes in the bottom. If another hole were added, the total flow out of the bucket would speed up, which means the resistance to flow decreases. To calculate resistance, use the formula in Figure 36-32 and substitute the value of each resistor in the place of R1, R2, etc.