A-level Physics (Advancing Physics)/Current

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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:

I = \frac{\Delta Q}{\Delta t} ([The triangle (Greek letter delta) means change in the quantity])

i1 + i4 = i2 + i3

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:

\sum I_{in} = \sum I_{out} (The character that resembles a sideways M is the Greek letter sigma, meaning 'sum of'.)

Questions[edit]

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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 I1 = 4.5A. What is the current at I2?

4. What would I equal if I1 = 10A and I2 = 15A?

5. In the diagram on the left, in 5 seconds, 5C of charged particles flow past I1, and 6.7C flow past I2. How long does it take for 10C to flow past I?

Worked Solutions