# Theory of Kirchhoff's law help!Watch

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#1
So It is about this conductors in a2 physics where I got to sketch a P.D. Across a charging conductor against time. With a little help from Google and Wikipedia I've found and proved the equation for voltage variation in a charging circuit(which's V=emf(1-e^(-t/Rc)).

But in the process of proofing I had to agree to Kirchhoff's voltage law(the sum of potential rises and falls in a loop is 0). But for this law to be valid but in which I do not have a deep understanding, it is to be a closed circuit. The charging circuit is not looped or connected across terminals of battery, so it doesn't even gain the courtesy of calling it a circuit eh? So how does this Kirchhoff's law hold true for this situation? Somebody know the answer to this?... Thank you!
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4 years ago
#2
(Original post by Hsakbo)
So It is about this conductors in a2 physics where I got to sketch a P.D. Across a charging conductor against time. With a little help from Google and Wikipedia I've found and proved the equation for voltage variation in a charging circuit(which's V=emf(1-e^(-t/Rc)).

But in the process of proofing I had to agree to Kirchhoff's voltage law(the sum of potential rises and falls in a loop is 0). But for this law to be valid but in which I do not have a deep understanding, it is to be a closed circuit. The charging circuit is not looped or connected across terminals of battery, so it doesn't even gain the courtesy of calling it a circuit eh? So how does this Kirchhoff's law hold true for this situation? Somebody know the answer to this?... Thank you!
Kirchoff's law holds true.

There is a resistance in the path of the charging current formed by the internal battery resistance and conductor resistance together with any physical resistor in series with the supply and the capacitor.

As the capacitor charges, a p.d. will be developed across this series resistance which sums with the building p.d. across the capacitor so that:

Vbatt = Vseries + Vcap

Although no direct current path between the supply terminals exists because the capacitor plates break the circuit, the electron charge building on one plate will repel electrons on the other plate across the dielectric gap.

This means the electric force is maintained across the capacitor plates and hence Kirchoff's law is valid.
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#3
(Original post by uberteknik)
Kirchoff's law holds true.

There is a resistance in the path of the charging current formed by the internal battery resistance and conductor resistance together with any physical resistor in series with the supply and the capacitor.

As the capacitor charges, a p.d. will be developed across this series resistance which sums with the building p.d. across the capacitor so that:

Vbatt = Vseries + Vcap

Although no direct current path between the supply terminals exists because the capacitor plates break the circuit, the electron charge building on one plate will repel electrons on the other plate across the dielectric gap.

This means the electric force is maintained across the capacitor plates and hence Kirchoff's law is valid.

So it doesn't necessarily have to be a "closed" circuit huh. As long as there's movement in charges the law holds true eh? Ok Thanks uberteknik! I'll jot that down.
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4 years ago
#4
(Original post by Hsakbo)
So it doesn't necessarily have to be a "closed" circuit huh. As long as there's movement in charges the law holds true eh? Ok Thanks uberteknik! I'll jot that down.
Yes, as long as there is movement of charge.

This is the reason why capacitors block d.c., because when the capacitor has completed charging, the electric field between the capacitor plates reaches an equilibrium maximum and no further charge movement is possible.

With a.c. however, the charge/discharge of the plates is continually varying and as a result, charge movement will be continuous on both sides of the plates.
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