Did my teacher make a mistake? (Kirchoff's Rules)

[carpelumen]

Consider the electric circuit shown in the figure. Using Kirchhoff's rules, select True or False for the following equations.

a.) I2 + I3 = I1
b.) ε1 - I2R2 - I3R3 - I1R1 = 0
c.) ε2 - I1R5 - I3R3 - I2R4 = 0

http://tinypic.com/r/2zyws55/8

My answers were: True, False, False.

However, the homework systems answers are False, False, False...

How is 'a' false? Anyone??

 

[berkeman]

Consider the electric circuit shown in the figure. Using Kirchhoff's rules, select True or False for the following equations.

a.) I2 + I3 = I1
b.) ε1 - I2R2 - I3R3 - I1R1 = 0
c.) ε2 - I1R5 - I3R3 - I2R4 = 0

http://tinypic.com/r/2zyws55/8

My answers were: True, False, False.

However, the homework systems answers are False, False, False...

How is 'a' false? Anyone??

You could save us a *whole* lot of work if you would post the KCL equations you wrote for this circuit. That way we don't have to spend the time to write them out for ourselves and try to interpret why you answered the way you did. The more work you show us, the better and faster help you will get here.

 

[willem2]

a.) I2 + I3 = I1

How is 'a' false? Anyone??

you should be able to tell us why you think a.) is true.

According to Kirchhoff's current law, the current entering a junction, is equal to the current leaving a junction.
Look at the top junction. What's the current entering it and leaving it? Does setting those currents equal produce a.) ?

 

[carpelumen]

I got it! Dumb mistake on my part

 

[HalfLostAndSearching]

I cannot definitively say whether your teacher made a mistake or not without more information. However, I can provide an explanation for why the homework system may have marked your answer for part a as false.

Kirchhoff's Current Law states that the sum of all the currents entering a node (or junction) in a circuit must equal the sum of all the currents leaving that node. In the circuit shown in the figure, the node at the top has three currents entering (I1, I2, and I3) and only one current leaving (I1). Therefore, according to Kirchhoff's Current Law, the equation should be I1 = I2 + I3, not I2 + I3 = I1.

It is possible that your teacher or the homework system made a mistake in labeling the currents or nodes in the circuit, which could have led to the incorrect equation being given. Without more information, it is difficult to determine the exact reason for the discrepancy. I would recommend discussing this with your teacher to clarify any confusion and better understand the correct application of Kirchhoff's Rules.