Voltage in Parallel Circuits: Calculating Current through a Point

[nfcfox]

Homework Statement

{image inserted in-line by mentor}
http://imgur.com/lnURBGX
Calculate the magnitude of the current through point Y.

Homework Equations

Ohm's Law: V=IR
Kirchoff's Junction: 1/Req=1/R+1/R2+1/R3+...

The Attempt at a Solution

I have been told that in parallel circuits the voltage remains the same therefore I should have 24= IRB
Rb= 3 ohms so 24/3= 8 A. However this is wrong because this problem is from AP Physics B 2012 FR. They actually had to find the total current by finding the voltage of the AB component... But why are we finding this voltage? Like I said I have been told voltage is constant throughout the entire circuit...

 

[gneill]

Homework Statement

View attachment 98797{image inserted in-line by mentor}
http://imgur.com/lnURBGX
Calculate the magnitude of the current through point Y.

Homework Equations

Ohm's Law: V=IR
Kirchoff's Junction: 1/Req=1/R+1/R2+1/R3+...

The Attempt at a Solution

I have been told that in parallel circuits the voltage remains the same therefore I should have 24= IRB
Rb= 3 ohms so 24/3= 8 A. However this is wrong because this problem is from AP Physics B 2012 FR. They actually had to find the total current by finding the voltage of the AB component... But why are we finding this voltage? Like I said I have been told voltage is constant throughout the entire circuit...

No, your understanding of the relationship for voltage and parallel components is not correct. The rule is that components that are in parallel share the same potential difference across them.

In your circuit if you were to take a voltmeter and measure the potential difference across the 6 Ω lightbulb and then the 3 Ω lightbulb, you would read the same value for each. Similarly, you would read identical values for voltage across the 12 Ω and 24 Ω bulbs. But this would be a different voltage than for the other pair.

 

[phinds]

This circuit is NOT a parallel circuit, it is a series circuit with two sub-circuits that have parallel elements. For the voltage to be the same "throughout", you have to have all parallel elements. I suggest you go back and study what parallel and series mean.

EDIT: Ah, beat to it again

 

[nfcfox]

This circuit is NOT a parallel circuit, it is a series circuit with two sub-circuits that have parallel elements. For the voltage to be the same "throughout", you have to have all parallel elements. I suggest you go back and study what parallel and series mean.

EDIT: Ah, beat to it again

Our class didn't teach us this dang. When I look it up online it tells me that the voltage is the same across a parallel circuit...

 

[phinds]

Our class didn't teach us this ****. When I look it up online it tells me that the voltage is the same across a parallel circuit...

Yes, and the voltage IS the same across a parallel circuit. It just helps to know what a parallel circuit IS.

 

[nfcfox]

Yes, and the voltage IS the same across a parallel circuit. It just helps to know what a parallel circuit IS.

So then the voltage should be 24 V across the whole damn circuit??!??! That's how they did it in the videos they showed us.

 

[nfcfox]

No, your understanding of the relationship for voltage and parallel components is not correct. The rule is that components that are in parallel share the same potential difference across them.

In your circuit if you were to take a voltmeter and measure the potential difference across the 6 Ω lightbulb and then the 3 Ω lightbulb, you would read the same value for each. Similarly, you would read identical values for voltage across the 12 Ω and 24 Ω bulbs. But this would be a different voltage than for the other pair.

But they're saying voltage is the same across the circuit...

 

[Hesch]

But why are we finding this voltage? Like I said I have been told voltage is constant throughout the entire circuit...

Voltage is constant across parallel resistors, but AB are serially connected to CD.

Calculate R_AB = R_A || R_B and R_CD = R_C || R_D.

The total current through the circuit will be I_tot = 24V / ( R_AB + R_CD ), hence the voltage across R_AB, V_AB = I_tot * R_AB.

The current I_B = V_AB / R_B.

Sketch the circuit step by step, following the above.

 

[phinds]

The voltage from X to Z is the voltage of the power supply. Between X and Z are two sub-circuits in series, each of which has to parallel elements.

AGAIN, I suggest that you go back to basics and learn about series and parallel circuits. I don't care if your class taught it or not --- look it up on the internet. You are not going to get very far without this basic understanding.

 

[nfcfox]

The voltage from X to Z is the voltage of the power supply. Between X and Z are two sub-circuits in series, each of which has to parallel elements.

AGAIN, I suggest that you go back to basics and learn about series and parallel circuits. I don't care if your class taught it or not --- look it up on the internet. You are not going to get very far without this basic understanding.

So then what is a parallel circuit?

 

[gneill]