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I am looking at the problem in page 5 of this document. It is an example of KVL (Kirchoffs voltage law).
The circuit is as shown below -
.
I have 2 questions -
1) Why is the voltage -5V ? Is it because the polarities reversed ? 2) The sign convention in the 8 Ohms resistor is wrong as I assumed the current loop as shown below and the sign convention as stated in the diagram. The voltage across 8 Ohms is -12V. Is the minus sign because of wrong convention ?
1) Yes, you could also reverse the polarity of the voltage source and write +5V instead. It makes the schematic easier to read. Both variants are equivalent.
2) If the polarity of a voltage across a resistor is not known, it is required to make an assumption. A minus sign indicates that the actual polarity of the voltage is against the assumed direction. But this is not "wrong" because the result is just with respect to an assumed polarity.
Voltage value is actually the potential difference between the positive and negative terminal of an element. A negative voltage say -5V indicates that the positive terminal has a 5V lower potential than negative terminal and is equivalent of changing sign of voltage and reversing polarities. So don't get worried about a negative sign in answer as its only indicates that you have chosen the direction opposite to the actual polarity and nothing else.
