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Kirchoff's 2nd Law

A conservation of energy law

Voltage is to do with the energy transferred by each coulomb of charge.

Kirchoff's 2nd (or 'voltage') Law essentially says that all the energy put into a circuit from the battery has to go somewhere.  And this balance must be exact.  You can't have even a small amount of energy appearing from nowhere or disappearing without trace.

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Kirchoff's 2nd Law in simple series circuits

In it's simplest form the law says that if you have a 6 V battery then whatever you connect across it must have 6 V across it too (provided you ignore the internal resistance of the battery).

The current depends on the actual component but the voltage is always 6 V.

If you have two bulbs in series then the two voltages must add up to the battery voltage.  If the bulbs are identical they'll have 3 V each across them.

If they're not identical then if one bulb has 4 V across it the other must have 2 V.

Kirchoff's 2nd Law in parallel circuits

In parallel circuits there's more than one conducting path back to the battery.

But whichever path you choose, a given charge must lose all its energy as it makes its way from one terminal of the battery to the other.  This means you can take any complete loop and apply Kirchoff's 2nd Law to it.

back to Lesson 5: Voltage and Current