When to combine resistors?

[MedPR]

---

Members do not see this ad.

How do you know if you are supposed to combine the resistors before calculations or not?

Here are two examples.

R1 = 4Ohms
R2=6Ohms
R3=3Ohms
V=36

Answer is B

I=2Amps
R1=4Ohm
R2=2Ohms
R3=2Ohms

Answer is C

 

[chiddler]

when they are in series they are added. when they are parallel then its that one annoying formula

1/R = 1/R1 + 1/R2...1/Rx.

 

[MedPR]

when they are in series they are added. when they are parallel then its that one annoying formula

1/R = 1/R1 + 1/R2...1/Rx.

I know how to do it, I just don't know when to do it.

In the two questions above, you combine in 1 and don't combine in the other. Yet they are both asking about a specific, single resistor.

 

[chiddler]

I know how to do it, I just don't know when to do it.

In the two questions above, you combine in 1 and don't combine in the other. Yet they are both asking about a specific, single resistor.

oh sorry i just went off title. i haven't reviewed circuits yet.

 

[milski]

First question was about current, second was about voltage drop. You'll notice that the current in the first question is the same as the current out of the source. That allows you just calculate the total resistance of the three resistors and use Ohm's law after that. If they were asking about current through R2 or R3, you would have to calculate the potential drops across them and go from there.

In a similar way in the second question the voltage drop across R1-R2 is the same as the potential of the source. So you can do the math only for these two and ignore R3.

The trick in both cases is that you can use what you know about circuits to decide not to calculate currents or voltage drops for parts of the circuit. Brute force with Kirchoff's for all resistors will be a bit longer but ultimately will yield exactly the same results.

 

[SaintJude]

Treat each "fork of the road" as the beginning of one single equivalent resistor. So for first scenario, directly after point B, you'll come across a fork of the road that ends right after point C. So you add those two resistors together. Each "loop" of resistors can be thought of as an entity.

It's hard to explain not in person/without video, so you may want to consult Khan Academy if it doesn't make sense.