Homework Help: Electrical circuit general question

1. Apr 27, 2012

chrisy2012

1. The problem statement, all variables and given/known data
see thumbnail
second part

2. Relevant equations
V=IR

3. The attempt at a solution
I remember the professor saying that current throughout a circuit is constant no matter where it is. So adding all of the resistance together it should be 12/10 or 6/5 Ampere's. Is this correct?

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Last edited: Apr 27, 2012
2. Apr 27, 2012

I cannot see a thumbnail

3. Apr 27, 2012

chrisy2012

oops i swear i had it when i posted it. anyways it should be there now

4. Apr 27, 2012

Yeah, I'd say so...

5. Apr 27, 2012

RoshanBBQ

No. He probably said the current through series elements is the same whereas the voltage across parallel elements is the same.
No.

Last edited: Apr 27, 2012
6. Apr 27, 2012

ahaha... how could I be so silly.

Sorry for the false infomation chrisy
but consider the reason why a lightening strike would rather move through nice conductive human liquids rather than a solid block of concretre. Then apply it to this circuit...

7. Apr 27, 2012

RoshanBBQ

Don't do that consideration. Instead, use ohm's law. What is the voltage across the equivalent series resistance of 7 ohms? Simply take the voltage at one terminal and subtract it by the voltage at another terminal. Once you have this voltage,
I = v/R_eq

8. Apr 27, 2012

Staff: Mentor

Note: this is a trick question.

Just thought I'd better post that in case someone hadn't noticed it underneath the diagram.

9. Apr 27, 2012

RoshanBBQ

It's not a trick question. It's just a really easy question, considering what the voltage across that series resistor is.

10. Apr 27, 2012

chrisy2012

Oh I got it, the professor explained that the 3 ohm resistor is on the side of a short, so the current would not reach there. Therefore it's 0.

11. Apr 27, 2012

RoshanBBQ

Yeah, that's right. The equivalent series resistor of 7 ohms has the same voltage applied on both its ends.
I = 0/r = 0.

12. Apr 28, 2012

Ohh what :rofl:... why would my consideration be wrong then?

Can't you just say that electricity takes the path of least resistance which is in this case is the short circuit in the middle :tongue2:

therefore there would be no current running through the 3 ohm resistor. Or is that scientifically incorrect (sorry for my ignorance, I'm only a laid back tween that goes to a public school)

13. Apr 28, 2012

Staff: Mentor

Which is wrong. Electricity generally goes through all parallel paths, not just one of the many.

Only in the special case of a short circuit does no current go through other parallel paths.

14. Apr 28, 2012

So if there was a parallel path with no added resistance, can it be called a short circuit?

15. Apr 28, 2012

RoshanBBQ

It's important to think about voltages since it tells the entire story. In ideal circuit theory, we say the little wires we draw have zero resistance. We also say everywhere on that wire has the same voltage since there is no resistance for there to be a drop in voltage as the voltage traverses the wire. So if you hook a resistor up at two points on this wire, known as a short circuit, you have some voltage V_a applied on both ends of it.

So the voltage across the resistor is the voltage at the first terminal minus the voltage at the second terminal:
V_a - V_a = 0.

If there is zero voltage, there must be zero current.