Electricity is an essential phenomenon in physics that plays a vital role in science and technology. Among its fundamental concepts, electric current holds a special place. It not only explains how charges move in a conductor but also provides the foundation for electrical circuits, machines, and modern electronic devices.
Definition of Electric Current
Electric current is the rate of flow of electric charge through a conductor. In simple words, when a potential difference (voltage) is applied across the ends of a conductor, free electrons present in it start traveling in a definite direction.
This orderly motion of charges constitutes an electric current.
Mathematically, I = dq / dt, Where,
* I = electric current,
* dq = amount of charge flowing,
* dt = time interval.
The SI unit of electric current is ampere (A). One ampere is defined as the flow of one coulomb of charge per second through any cross-section of a conductor.
Nature of Electric Current
Conventional Current vs Electron Flow:
On convention, current is taken to flow from the positive terminal of a cell or battery to the negative terminal. However, in metals, actual charge carriers are electrons, which move in the opposite direction. This distinction is important in theoretical analysis.
Steady and Time-varying Current:
Direct Current (DC) remains constant with time. For example, current supplied by a dry cell or battery and even by rectifier.
Alternating Current (AC) changes its magnitude and direction periodically. For example, household electricity supply and supplied by inverter.
Microscopic View of Current:
In a conductor, electrons move randomly without any applied potential difference. When voltage is applied, these electrons acquire a “drift velocity” in the opposite direction of the electric field, creating current.
Although the drift velocity is very small e.g mm/s, the electric field propagates almost at the speed of light, which is why current appears instantly when a circuit is completed.
Ohm’s Law and Relation with Current
Ohm’s law states that the current through a conductor is directly proportional to the potential difference across its ends, provided the temperature and physical conditions remain constant.
V α I → V = IR
Where R is the resistance of the conductor. This relation shows that for a given resistance, larger potential difference produces a higher current.
Current Density
The concept of “current density” helps in understanding current flow in terms of area. Current density (J) is defined as the current flowing per unit area of cross-section of a conductor: J = I / A.
Where A is the area normal to current flow. Current density is a vector quantity, directed along the flow of positive charges.
Drift Velocity and Mobility
The microscopic relation between current and the motion of electrons is expressed using drift velocity (vd). I = nAevd
Where,
* n = number of electrons per unit volume,
* A = cross-sectional area of conductor,
* e = charge of electron,
* vd = drift velocity.
Drift velocity itself is related to the applied electric field (E) and mobility (µ) as:
Vd = µE
Mobility is defined as the drift velocity acquired per unit electric field, and it depends on the nature of the conductor.
Types of Electric Current
Direct Current (DC): Flows in one direction only. Examples: cells, batteries, solar panels.
Alternating Current (AC): Changes direction periodically. Examples: power stations, commercial supply.
Transient Current: Exists for a short time during switching operations in circuits.
Heating Effect of Current
When current flows through a conductor, electrical energy is converted into heat energy due to collisions of electrons with atoms. This is called the “Joule’s heating effect”. The heat produced is given by: H = I2Rt
This principle is used in electric heaters, electric bulbs, and fuses.
Magnetic Effect of Current
Hans Christian Orsted discovered that an electric current produces a magnetic field around the conductor. This effect forms the basis of electromagnets, electric motors, transformers, and generators. The relationship between electricity and magnetism gave rise to the field of “electromagnetism”.
Chemical Effect of Current
When an electric current passes through a conducting liquid (electrolyte), it causes chemical reactions such as decomposition or deposition. This is known as the chemical effect of current, explained by “Faraday’s laws of electrolysis”. Applications include electroplating and extraction of metals.
Applications of Electric Current
1.Domestic and Industrial Power Supply: Used for lighting, heating, and running machines.
2.Transportation: Electric trains, trams, and electric vehicles depend on current.
3. Communication: Current is the backbone of telecommunication, computers, and the internet.
4. Medical Field: Devices like X-ray machines, MRI scanners, ECGs, and defibrillators rely on current.
5. Research and Technology: Particle accelerators, superconductors, and quantum computing all utilise electric current principles.
