Kirchhof's voltage law

[Fawad]

Using kirchhof's voltage law we say that there can be potential drop while going through a battery.But what about resistor?Will there always be a potential drop at resistor if there are two batteries of different voltages.I've seen it in a pakistani book that there will aways be a potential drop.But i don't think so.

 

[Boron]

I'm not sure I understand the question entirely but I'll try and answer.

There will always be a voltage drop accross a resistor if it has a current flowing through it. If by two batteries you mean that each side of the resistor is at a different voltage, then there must be a voltage drop accross it equal to the difference between either end. This voltage difference or drop will cause current to flow through the resistor.

So a resitor will only drop zero volt if the voltage on one side is equal to the voltage on the other side and thus no current is flowing through it.

 

[Fawad]

I know that all what u describe and thnx.But in some books it is given that during solving a circuit sometimes a voltage rise is taken at resistors.
E.G basic electronics by B.L Theraja.(indian)

 

[Fawad]

Thnx Boron.U really did ur best it is my question which i can't really express.In fact i m posting from mobile so i can't attach files with it.i need paper etc to draw a ckt diagram.In our country books always make u mislead.

 

[rahul.sadtm]

there will surely be a voltage drop across the resistor if the potential difference is developed...

to make sure you once see this simulation pic....

 

[Boron]

Sometimes the voltage accross a resistor will be considered as a rise when performing Kirchoff's voltage law around a circuit loop.

This is dependant on the direction in which you are passing through the resistor. If you are going around a loop, but in your loop enter the negative terminal of the resistor (which is at a lower potential) it means that the other side is at a higher potential. So while performing this loop the resistor had a gain.

5 V 5 V
(+) -----/\/\/\/------ (-) (-) -----/\/\/\/------ (+)

If I'm doing a loop and go through both of these from left to right, the resistor on the left is dropping 5 volts whereas the one on the right is gaining 5.

This voltage gain only exists when doing loops and analysis in KVL. Resistors never have a voltage gain in circuits because they are passive elements and cannot supply power.

 

[rahul.sadtm]

right as boron said... current has the tendency to flow from higher potential to the lower potential and it flows only in the close loop...

so in a close loop, if potential difference is develop than and only than the voltage drop across the resistor is develop.

 

[mbarazeen]

little more about boron told,

say in a network once you have found all values of current, then there will be some questions to find Vab, etc, ie: voltage of (a ) relative to (b), in such cases the way to find out Vab is start from the reference point (b) and go via any path on the network towords (a) add all voltages which is on the same direction and subtract voltages that are oposite. so when you pass some resistors depends on the direction of current flow the voltage is added or substracted.

normally these voltages are maked with arrows indicating high side by head and low side by tail near every component to eleminate any confusion during this. so once the arrow is in the same direction its given +ve sign (add) and if its oposite to the path then its given -ve sign (substract)