 # how can we apply kirchoff's law on capacitor and inductor

13 years ago

I can understand the doubt that had aroused in your mind!! See, when we apply kirchhoff's laws to capacitors and inductors we dont actually appy it to them but to certain junction points we decide!! I mean kirchhoff's rule hold true everywhere in this universe and its one of the most fundamental laws of physics but we dont have that applicability and usefulnessto apply it on a pont somewhere inside a capacitor or in middle of a inductive coil and even if we have that mathematics we avoid that for unneccessary analysis involved.So just appy these laws at junction point where some wires meet.the laws are restated below for ur convenience.

### Kirchhoff's Current Law

Kirchhoff's Current Law, also known as Kirchhoff's Junction Law and Kirchhoff's First Law, defines the way that electrical current is distributed when it crosses through a junction - a point where three or more conductors meet. Specifically, the law states that:

The algebraic sum of current into any junction is zero.
Since current is the flow of electrons through a conductor, it cannot build up at a junction, meaning that current is conserved: what comes in must come out. When performing calculations, current flowing into and out of the junction typically have opposite signs. This allows Kirchhoff's Current Law to be restated as:

The sum of current into a junction equals the sum of current out of the junction.

### Kirchhoff's Voltage Law

Kirchhoff's Voltage Law describes the distribution of voltage within a loop, or closed conducting path, of an electrical circuit. Specifically, Kirchhoff's Voltage Law states that:

The algebraic sum of the voltage (potential) differences in any loop must equal zero.

The voltage differences include those associated with electromagnetic fields (emfs) and resistive elements, such as resistors, power sources (i.e. batteries) or devices (i.e. lamps, televisions, blenders, etc.) plugged into the circuit.

Kirchhoff's Voltage Law comes about because the electrostatic field within an electric circuit is a conservative force field. As you go around a loop, when you arrive at the starting point has the same potential as it did when you began, so any increases and decreases along the loop have to cancel out for a total change of 0. If it didn't, then the potential at the start/end point would have two different values. AskIITians Expert Hari Shankar IITD
17 Points
13 years ago

I think you have problem with Kirchoff's second law.

Capacitor

A capacitor is just like a cell/power source. It has a negative and positive side.

When you use Kirchoff's second law,

- if you are going from negatively charged  to positively charged  side, Voltage dropped is +Q/C.

- If you are going from +vely charged side to negatively charged side, it is -Q/C.

Inductor

An inductor is also similar. Remember that an inductor tried to reduce any change in current. Thhe voltage dropped across an inductor is equal to L (dI/dt).

- If the current is increasing in the direction you are going with the loop, then the inductor tries to reduce this increase in current. So it produces a voltage drop of -L(dI/dt). This way it acts like a cell with positive direction opposite to direction of current flow, to decrease current.

- If the current is decreasing in the direction you are going with the loop, then the inductor tries to reduce this decrease in current. So it produces a voltage drop of +L(dI/dt). This way it acts like a cell with positive direction in the direction of current flow, to increase current.

Both inductor and capacitor are like cells when applying Kirchoff's law. Capacitor develops a voltage of Q/C and inductor develops a voltage of L(dI/dt). You only need to know the direction in which this voltage is created.

10 years ago

Just need to say that as opposed to a previous poster, Kirchhoff''s law is very far from a law of nature.

To quote: "I mean kirchhoff''s rule hold true everywhere in this universe and its one of the most fundamental laws of physics"

The presence of a magnetic field will effect the voltage around the loop. That is Faraday''s law and there is a magnetic everywhere. That takes considerable accuracy away from Kirchoff''s Law.

Also, less likely but soon to be more relevant, superconductors in a ciruit deft Kirchoff''s law.