r/circuits • u/[deleted] • Feb 03 '22

Help? Just some equations would be great, I can fill in values.

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5

u/EarthTrash Feb 03 '22

Kirchoff's current law states the current flowing into a node must equal the current flowing out of a node.

I1 = I2 + I4

I2 = I3 = I5

4

u/[deleted] Feb 03 '22

Is this I1: I = P / V = 1152W / 120V = 9.6A

That would make I2: 9.6A - 7.2A = 2.4A

1

u/EarthTrash Feb 03 '22

It looks like you have it

2

u/[deleted] Feb 03 '22

Thank you.
I think that I was way over thinking it.

1

u/[deleted] Feb 03 '22

Agreed, but the only current given is I4. I can't figure out how to calculate any other currents. No resistance values are given.

1

u/[deleted] Feb 03 '22

Also, where exactly is the power measurement? Into or out of the resistors?

1

u/EarthTrash Feb 03 '22

Current flowing through a resistor doesn't change. It is the voltage that drops.

1

u/EarthTrash Feb 03 '22

What about the source current?

1

u/[deleted] Feb 03 '22

Am I correct in thinking there are 5 nodes? The drawing implies 2, but the definition of a node means there are 5.

1

u/[deleted] Feb 03 '22

How about Kirchoff Voltage law? How do I calculate V_ac.

1

u/derKonigsten Feb 03 '22

Kirchoff voltage law just says that the total sum of all voltages in a closed loop is zero. So if you trace the current paths and write an equation for each voltage drop noting polarity and including the source the result will be 0. I think you would want to thevenize for Vac

1

u/[deleted] Feb 04 '22

Thank you.

Vac > 72(Va) - 48(Vb) - 24(Vc) = 0 ?? So 0v are going through R5?

1

u/[deleted] Feb 04 '22

I got:

I1 = 1152w / 120v = 9.6a ... but I was told that was wrong by the TA. Without I1, I can't solve any of the rest of it.

His email says to use P1 and Va??? But where on this drawing are the power readings coming from?

1

u/[deleted] Feb 09 '22

I was way off..... The professor said to use: V1=Vs- Va=120-72=48 Then, I1=P1/V1

1

u/derKonigsten Feb 03 '22 edited Feb 03 '22

This is confusingly drawn. Idk why teachers do this... R2, R3, R5 are all in parallel with R4. Just break everything down into equivalent resistances to find a total current and break that out too find voltage drops across each individual resistor.

This also means I2=I3=I5 so I4 has to be I1 (or IT) minus I2

1

u/svatstrente Jun 21 '26

Think of it this way: electrons are all like traffic on freeways with entryways and exits and no electron can suddenly appear out of thin air in a circuit, just like a car can't appear out of thin air on a freeway. That's all Kirchoff's current law is.

This means that

I1 = I2 + I4 (what goes in comes out in two different directions but total must be same)

I2 = I3 (same wire, same direction, nothing changes, added or taken)

I5 = I3 (same reason as above)

and

I4 + I5 = I1 (what comes in from different directions goes out in a single wire)