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:

$I={\frac {\Delta Q}{\Delta t}}$ ([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

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