Electric Circuit: Finding Unknown Resistance and EMF

[PhilCam]

Homework Statement

Three resistors and two batteries are connected as shown in the figure. The magnitude and direction of the currents in the unknown resistor "R" at the right and the 6.0 ohm resistor on the left are shown in the figure. The current in the 4.0 ohm resistance flows from B to C. The emf of the batter on the left is unknown and is labeled. For parts (a) through (d) fill in the blanks and for part (e) circle the correct choice.

I drew a picture of the diagram, hopefully you guys can see it and understand:

http://img683.imageshack.us/i/imgie.jpg/

a) The current flow in the 4.0 ohm resistor is? I put the answer is 11amps because Kirchoff's rules for a joint.

b) The unknown resistance R is? I know that resistance is equal to V/I, but I have tried to answer this question and it does not match up with the given answer.

C) The unknown emf is? I am not even sure where to begin with this one.

d) The absolute value of the voltage difference between A and B is? I put 50V because 1Ax6ohms + 11Ax4ohms = 50 Volts

Any insight would be helpful. Thanks!

 

[skeptic2]

First, there is disagreement between the battery symbol for E and the direction of the 1 amp current. Which is correct?

 

[PhilCam]

Weird.

I drew it exactly as the problem displays. Maybe there is an issue with the question?

I would assume that the battery is supposed to be the other way, but I cannot be certain.

 

[skeptic2]

Assuming the 1 A arrow is correct, what is the total current through the 125 V source. It's obvious from the diagram.

 

[PhilCam]

Well the answer for c should be 75 volts.

So maybe take the voltage from the battery given and subtract the absolute voltage difference (answer of part d) to get 75 volts?

 

[skeptic2]

a) is correct.

b) If you have 11.0 A through the 4.0 ohm resistor, what is the voltage across it and its polarity?

 

[PhilCam]

the voltage across is 44? I'm not sure what's mean by its polarity. If that is direction, then it is flowing towards C.

 

[skeptic2]

Good. For resistors and other passive components the node that current is flowing into is considered positive. For sources it's the node that current is flowing out of that is considered positive. So adding the voltage across the resistor to the 125 V source, what is the total voltage from the top of the 4.0 ohm resistor to C?

 

[ideasrule]

a) The current flow in the 4.0 ohm resistor is? I put the answer is 11amps because Kirchoff's rules for a joint.

Right.

b) The unknown resistance R is? I know that resistance is equal to V/I, but I have tried to answer this question and it does not match up with the given answer.

How did you calculate V? It should be equal to 125V minus the voltage drop across the 4-ohm resistor.

C) The unknown emf is? I am not even sure where to begin with this one.

The voltage across the left branch (with the emf and the 60-ohm resistor) has to be equal to the V you used in part b), so by calculating the voltage drop across the 60-ohm resistor, you can calculate the emf.

d) The absolute value of the voltage difference between A and B is? I put 50V because 1Ax6ohms + 11Ax4ohms = 50 Volts

Any insight would be helpful. Thanks!

Correct.

 

[ideasrule]

oh wow, i didn't notice so many replies have already been posted...

 

[PhilCam]

So it is 125V - 44V = 81V/11Amps = Current of 7.4?

Thanks both of you for the help.

 

[skeptic2]

No.

The 81V is correct but that 81V is also across the unknown resistor. Now you should be able to calculate the value of the unknown resistor.

 

[skeptic2]

Also since you also have 81V across the 6 ohm resistor and the unknown source, you should be able to calculate the voltage of that source, but be careful about the polarities.

 

[PhilCam]

Ahh so the resistor is 8.1?

81V/10Amps = 8.1?

 

[skeptic2]

Yes.