Current

Current

1. An electric current I is a measure of the rate of flow of electric charge Q through a given cross-section of a conductor.
2. In other words, current is the measure of how fast the charge flow through a cross section of a conductor.

Equation
$$Current=\frac{\text{The Amount of Charge Flow}}{\text{Time Taken}}$$ or
$$I=\frac{Q}{t}$$

Direction of Current

1. Conventionally, the direction of the electric current is taken to be the flow of positive charge.
2. The electron flow is in the opposite direction to that of the conventional current.
3. In a circuit, current flow from the positive terminal to the negative terminal.
4. In a circuit, electrons flow from the negative terminal to the positive terminal.

Unit of Current

1. The SI unit for current is the ampere (A).
2. The current at a point is 1 ampere if 1 Coulomb of electric charge flows through that point in 1 second. Therefore, 1 A = 1C/s.

Example 1:
If 30 C of electric charge flows past a point in a wire in 2 minutes, what is the current in the wire?

Answer:
Charge flow, Q = 30C
Time taken, t = 2 minutes = 120s

Current,
$$\begin{array}{l}I=\frac{Q}{t}\\ I=\frac{30}{120}=0.25A\end{array}$$

Example 2:
Current of 0.5A flowed through a bulb. How many electrons had flowed through the bulb in 5 minute? (The charge of 1 electron is equal to -1.6×10^-19 C)

Answer:
Current, I = 0.5A
Time taken, t = 5 minutes = 300s
$$\begin{array}{l}I=\frac{Q}{t}\\ Q=It\\ Q=(0.5)(300)=150C\end{array}$$ Charge of 1 electron, e = -1.6×10^-19 C
Number of electrons, n = ?
$$\begin{array}{l}Q=ne\\ n=\frac{Q}{e}\\ n=\frac{150}{1.6\times {10}^{-19}}=9.375\times {10}^{20}\end{array}$$