# Electric eqs q

For the following question, the aim is to find the value of I_3 through the 15ohm resistor, but apparently I have a mistake in the equations I set up, using loops (kirchoffs second law) - could someone please help me find the mistake?
Working and diagram of question
I colour coded to make it clear to myself that i3=i1+i2
(edited 2 months ago)
Original post by mosaurlodon
Working and diagram of question
I colour coded to make it clear to myself that i3=i1+i2

I am unsure what your convention is for writing positive and negative potential differences.
Based on your right mesh/loop (yellow coloured and green coloured lines), you move clockwise through the cell, which is a potential drop, it should be -V2 and based on your assigned current direction, I have
-V2 - I2(1000) - I3(15) = 0
Ohhhh thank you!
So in my diagram If I flipped the right cell vertically ie

Would my original eqs be correct or would it be -V2 = I2(1000) + I3(15)
Original post by mosaurlodon
Ohhhh thank you!
So in my diagram If I flipped the right cell vertically ie

Would my original eqs be correct or would it be -V2 = I2(1000) + I3(15)

What is the actual drawing of the circuit?
The original q comes from part b; https://isaacphysics.org/questions/two_cs_three_rs?board=07259f72-2a30-45f9-8a89-35627f34d9a5&stage=a_level
I just worked out the voltages of each capacitor and made a simplified diagram.
Original post by mosaurlodon
The original q comes from part b; https://isaacphysics.org/questions/two_cs_three_rs?board=07259f72-2a30-45f9-8a89-35627f34d9a5&stage=a_level
I just worked out the voltages of each capacitor and made a simplified diagram.

Original post by mosaurlodon
Ohhhh thank you!
So in my diagram If I flipped the right cell vertically ie

Would my original eqs be correct or would it be -V2 = I2(1000) + I3(15)

If you want to flip the polarity of V2, you are changing the question to suit your written equation and you would still get wrong I3.
Then I have a further q - for part b which polarity for both the capacitors do you take, since you have to assume they act like cells for kirchoffs laws?
Original post by mosaurlodon
Then I have a further q - for part b which polarity for both the capacitors do you take, since you have to assume they act like cells for kirchoffs laws?

Original post by mosaurlodon
Working and diagram of question
I colour coded to make it clear to myself that i3=i1+i2

These assigned polarities are good.

PS: I think you mean Kirchhoff's Laws not kirchoffs.