 # Kirchoff's Law

### Kirchoff's Law

Hi, I was just going through Kirchoff's Law and there was one sentence that confused me on PHY-142:
In a multiloop circuit, voltage changes around a clockwise direction was positive and counterclockwise was negative. However, in the next paragraph they say that by moving in a clockwise direction from battery's negative terminal to positive terminal , the emf is negative. Not sure if I am missing something or if this is contradictory.
Thank you!
kumara71113828

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Joined: Thu Jan 02, 2020 9:14 pm

### Re: Kirchoff's Law

kumara71113828 wrote:Hi, I was just going through Kirchoff's Law and there was one sentence that confused me on PHY-142:
In a multiloop circuit, voltage changes around a clockwise direction was positive and counterclockwise was negative. However, in the next paragraph they say that by moving in a clockwise direction from battery's negative terminal to positive terminal , the emf is negative. Not sure if I am missing something or if this is contradictory.
Thank you!

Every time that current goes through a resistor, there is a voltage drop. For each resistor, you can calculate that voltage drop as: V = IR.

But if the voltage keeps dropping, what is driving the voltage? It is the emf (we will use the symbol E) which is the voltage of the battery. Using this logic:

E = iR1 + iR2 ....

A standard way to look at the equation above is to allow the voltage changes to sum to zero (Kirchoff's 2nd Law), and so:

0 = -E + iR1 + iR2 ....

Notice that the preceding necessarily means that the voltage of the battery must have the opposite sign as the voltage drops due to resistors along the circuit. Why? Because the battery drives the voltage changes.

Note that none of the above needs to be memorised. Although, it is possible that the exam will present rules in a similar fashion that need to be adhered to in order to correctly answer the question. Of course, at the end of the chapter, you will get questions formatted in a way consistent with the real exam so that you have a clearer idea as to what is required. Keep up the good work!
goldstanda3269

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Joined: Wed Aug 25, 2010 10:59 pm 