Year+13+DC

__**Direct Current**__

Kirchhoff's Laws give rules for voltage in a loop and current at a junction:

**Kirchhoff's Current Law:** The current any **junction** is equal to the current leaving that junction. The law is based on the conservation of charge whereby the charge (measured in coulombs) is the product of the current (in amperes) and the time (in seconds).

**Kirchhoff's V ****oltage Law: ** The directed sum of the voltages around any **closed** **circuit** (loop) is zero. Essentially, the voltage used (in components) is the same as the voltage supplied (by cells). This law is based on the principle of the conservation of energy, in that any charge that starts and ends up at the same point with the same velocity must have gained as much energy as it lost.

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**__Internal Resistance and EMF__**

 Inside a dry cell there is a mix of gungy chemicals which convert chemical energy to electrical energy. The electric current that is produced still has to flow around all the circuit, including through the cell itself. These chemicals have a significant resistance called the “internal resistance”.

 The EMF is the voltage when the battery is supplying an insignificant/zero current.

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This video represents emf of a battery differently to how we do, we show emf like this: As a dotted rectangle around the cell and it's internal resistance as a resistor, r.

<span style="display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 180%; text-align: center;">**V = emf - Ir**