Engineering Electrical circuit general question

[chrisy2012]

Homework Statement

see thumbnail
second part

Homework Equations

V=IR

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?

 

[miniradman]

I cannot see a thumbnail

 

[chrisy2012]

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

 

[miniradman]

Yeah, I'd say so...

 

[RoshanBBQ]

Homework Statement

see thumbnail
second part

Homework Equations

V=IR

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?

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

Yeah, I'd say so...

No.

 

[miniradman]

ahaha... how could I be so silly.

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

 

[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

 

[NascentOxygen]

Note: this is a trick question.[/size]

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

 

[RoshanBBQ]

Note: this is a trick question.[/size]

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

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

 

[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.

 

[RoshanBBQ]

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.

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

 

[miniradman]

Ohh what ... 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

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)

 

[NascentOxygen]

Can't you just say that electricity takes the path of least resistance which is

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.

 

[miniradman]

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

 

[RoshanBBQ]

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

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.