# A-level Physics (Advancing Physics)/Current

Current is the amount of charge (on particles such as electrons) flowing through part of an electric circuit per second. Current is measured in amperes (usually abbreviated A), where 1 ampere is 1 coulomb of charge per second. The formula for current is:

([The triangle (Greek letter delta) means change in the quantity])

where I is current (in A), Q is charge (in C) and t is the time it took for the charge to flow (in seconds).

In a series circuit, the current is the same everywhere in the circuit, as the rate of flow of charged particles is constant throughout the circuit. In a parallel circuit, however, the current is split between the branches of the circuit, as the number of charged particles flowing cannot change. This is Kirchoff's First Law, stating that:

“ | At any point in an electrical circuit where charge density is not changing in time [ie. there is no buildup of charge, as in a capacitor], the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point. | ” |

In mathematical form:

(The character that resembles a sideways M is the Greek letter sigma, meaning 'sum of'.)

## Questions[edit]

1. 10 coulombs flow past a point in a wire in 1 minute. How much current is flowing through the point?

2. How long does it take for a 2A current to carry 5C?

3. In the diagram on the left, I = 9A, and I_{1} = 4.5A. What is the current at I_{2}?

4. What would I equal if I_{1} = 10A and I_{2} = 15A?

5. In the diagram on the left, in 5 seconds, 5C of charged particles flow past I_{1}, and 6.7C flow past I_{2}. How long does it take for 10C to flow past I?