1. What Electric Current Really Means
Electric current simply refers to how fast electric charges are moving through a conductor. When charges start drifting in an organised manner under the influence of a potential difference, we say that an electric current is flowing.
This idea is similar to water flowing through a pipe. Water moves because of pressure difference, and charges move because of potential difference.
1.1. The Movement of Charges
Inside a metal wire, the charges that actually move are electrons. They are free to drift from one end to another. As soon as a potential difference is applied, these electrons start moving in a fixed direction.
The direction of conventional current is taken opposite to the direction of electron flow, i.e., from higher potential to lower potential.
2. Definition of Electric Current
Electric current is defined as the rate at which charge flows through a conductor.
If a small amount of charge \( dq \) passes through a cross-section of the conductor in a small time \( dt \), the current is:
\( I = \dfrac{dq}{dt} \)
2.1. Understanding the Formula
The formula tells us how quickly charge is moving. A higher current means more charge passes through every second. A smaller current means charge is flowing at a slower rate.
2.1.1. A Quick Example
If \(4\,\text{C}\) of charge passes through a wire in \(2\,\text{s}\), the current is:
\( I = \dfrac{4}{2} = 2\,\text{A} \)
3. How Current Flows in a Conductor
Even though electrons are always moving randomly inside a conductor, they have no net flow without a potential difference. When a battery is connected, an electric field is established almost instantly inside the conductor.
This field causes electrons to drift slowly in a particular direction. This slow motion is called drift, and it results in a steady current.
3.1. Drift Speed vs Effect of Current
The drift speed of electrons is extremely small, often just a few millimetres per second. But the effect of current, like a bulb glowing, appears almost immediately because the electric field propagates through the wire at a very high speed.
4. Types of Current
Electric current can flow in different ways depending on how the potential difference is applied. Two common types are direct current (DC) and alternating current (AC).
4.1. Direct Current (DC)
In DC, charges flow in one fixed direction. A battery is a common source of DC. The direction of current remains constant with time.
4.2. Alternating Current (AC)
In AC, the direction of current continuously changes back and forth. The voltage supporting the current also changes with time. The current in household supply is AC.
5. Current and Charge Flow
There is a simple relationship between total charge \( Q \), current \( I \) and time \( t \). If a constant current flows, the total charge moved is:
\( Q = I t \)
This relation is useful whenever we know how long current flows and want to find how much charge has passed.
5.1. Small Example
If a current of \(3\,\text{A}\) flows for \(5\,\text{s}\), the charge moved is:
\( Q = 3 \times 5 = 15\,\text{C} \)
6. What Determines the Value of Current?
The current in a wire depends on two main factors:
- The potential difference applied across its ends
- The resistance offered by the conductor to the motion of electrons
A higher potential difference or lower resistance gives a larger current.