Electrical Current: Current, Charge and Kirchoff’s 1St Law

Revision on Electrical current, charge and Kirchoff’s 1st law. From As OCR physics A, but suitable for most other courses too, enjoy :)

Electric Current: Current and charge & Kirchoff’s 1st Law

Electric current is the flow of charged particles around a circuit. Normally these particles are negatively charged electrons, often notated as ‘e-’. The size of the charge on an electron is 1.6×10^-19 C. 

Conventional current flows from positive to negative, however electrons move in the opposite direction; negative to positive, as they are attracted to the positive terminal. Conventional current is different to electron flow as it was decided before the electron was discovered. 

Electrical current in a circuit is measured in Amperes (A) which are often shortened to amps, and charge is measured in Coulombs (C). Electrical current can also be defined as the rate of flow of charge from the equation.

Current=charge flow/time.

This is also sometimes written as Q=It where Q=Charge I=current & t=time. 

When writing out the equation remember Q&C are capitals, but t is lower case, or you may not get the formulae mark if one is available. 

From the equation you can also deduce that 

1A = 1C/s … 1Ampere = 1 Coulomb per second. 

Kirchoff’s 1st law states that:

“The sum of current’s entering a point, or junction in a circuit is equal to the sum of the currents leaving the same point”

This is called the CONSERVATION OF CHARGE. This applies because if 300 electrons enter a given point, they must also leave it in order for more electrons to enter after, and thus for current to flow. If the same amount of electrons enter and leave a point, then the current entering and leaving the point must also be the same. 

For example if 7A enter a point, and there are 2 entry paths for the current to take, 3A leaves through one path, 4A must leave through the other. 

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