Electric Current and Theory of Electricity

Electric current is the ratio of charge transferred in a given period. In other words, the rate of transfer of electric charge is the electric current. If the one-coulomb charge is transferred in one second from one point to another, the circuit carries the one-unit current.

What is an Electric Current?

Electric current is the flow of electric charge through a conductor or circuit. For electric current to flow, there must be a potential difference (voltage) between two points in the circuit.

This potential difference acts like pressure, pushing the charged particles—usually electrons—to move from one point to another. The movement of these charges constitutes the electric current.

Formula for Electric Current

Let q charge flow in t time,

Formula for Constant Electric Current

The electric current formula is:

I= q/t

where:

• I= electric current (in amperes, A)
• q = total electric charge (in coulombs, C)
• t= time interval over which the charge flows (in seconds, s)

Example Calculation of Current: If a 500 Coulomb charge passes through a conductor in 100 seconds, then the current is:

I = 500 / 100 = 5 Ampere

Differential Form of Electric Current Formula

However, when the electric charge varies continuously with time, it is more accurate to express the current in its differential form:

I= d_q / d_t

Where:

• I = instantaneous current (in amperes, A)
• q = electric charge (in coulombs, C)
• t = time (in seconds, s)
• dQ/dt = derivative of charge with respect to time, representing how fast the charge changes at that instant

This differential form is particularly useful in AC circuits, transient analysis, and physics problems, where current is not constant but changes over time.

Example:
If q(t)=5t² coulombs, then:

i(t)=dq/dt=10t A

At t=2 seconds:

i(2)=10×2=20 A

Thus, the differential form allows precise calculation of current at any instant.

SI Unit of Electric Current

The unit of electric current is coulomb per second. This is also known as ampere after the name of physicist André-Marie Ampère. The SI unit of electric current is Ampere.

How Does Electric Current Flow in a Circuit?

The electric current flows from the higher potential to the lower potential. If the two points in the circuit are at the same potential, the flow of electric charge is zero; hence, the current is zero.

The magnitude of the current depends on the rate of flow of charge from one point to another in a given circuit. The higher the rate of flow of charge, the higher the current.

In metal, free electrons, which move randomly within the metal, are abundantly available.

When we connect metal to a supply source, the source electron repels the electrons of the metal towards the higher potential of the supply source, and thus, the positive point of the supply source at the higher potential attracts the drifted electrons.

This way, the flow of electrons causes the current to flow.

Initially, people assumed that electric current flows due to positive charges. Experiments have shown that the main reason for current flow is the drift of electrons from lower to higher potential. The flow of current caused by the external field is the drift current.

The direction of electric current is opposite to that of electrons. The conventional direction of the current is from the higher to the lower potential point. This is why circuit diagrams often show arrows pointing in the opposite direction of actual electron movement.

In simple terms, electrons (negative charges) move from the negative terminal to the positive terminal of a power source, while conventional current flows from positive to negative.

Example- Flow of Electric Current in a Circuit

In a torch with a battery, when you switch it on, electrons flow through the filament of the bulb from the battery’s negative terminal to the positive terminal. However, Conventional current flows from the positive terminal of the battery, through the bulb, and returns to the negative terminal

History of Electric Current Theory

The understanding of electricity developed over centuries:

• Benjamin Franklin (1706–1790): Introduced the concept of positive and negative charges and proposed the idea of electric “fluid.”
• André-Marie Ampère (1775–1836): Defined electric current and studied its magnetic effects.
• Michael Faraday (1791–1867): Discovered electromagnetic induction, showing how electricity can be generated.
• Georg Ohm (1789–1854): Formulated Ohm’s Law, relating voltage, current, and resistance.
• 1897 – J.J. Thomson: Discovered the electron, clarifying that electric current in metals is due to moving electrons.
• Conventional Current Definition: Before the discovery of the electron, scientists assumed that current flows from positive to negative—a convention that circuit diagrams still follow today.”

Basic Laws Governing Electricity

Several fundamental laws form the backbone of electric current theory:

• Ohm’s Law: States that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: I=V/R​ (Read detailed article: Ohm’s Law — Definition, Formula, Examples)
• Kirchhoff’s Laws:
  1. Current Law (KCL): The sum of currents entering a junction equals the sum leaving it.
  2. Voltage Law (KVL): The sum of voltage drops around any closed loop is zero.
     (Read detailed article: Kirchhoff’s Laws — Explanation and Applications)

Charge, Voltage, and Resistance Basics

Before understanding current types, it’s essential to know three core concepts:

• Electric Charge (Q): The physical property of matter that causes it to experience a force in an electric field. Measured in coulombs (C). (Read detailed article: Electric Charge– Definition, Types, Unit and Properties)
• Voltage (V): Also called potential difference, it represents the work required to move a unit charge from one point to another. Measured in volts (V).
• Resistance (R): The opposition offered by a material to the flow of current. Measured in ohms (Ω). (Read detailed article: Electrical Resistance-Definition, Formula, and its Unit)

These quantities are related through Ohm’s law and determine how electricity flows in a circuit.

Types of Electric Current

There are two types of electric current.

• Direct Current (DC)
• Alternating Current(AC)

Direct Current (DC) 

Direct current (DC) refers to the flow of electric charge in a single, constant direction — making it unidirectional. In a DC circuit, electrons move from the negative terminal to the positive terminal of the voltage source.

However, by the conventional current definition, DC is said to flow from the positive terminal toward the negative terminal, since the current’s direction is taken opposite to the actual movement of electrons.

DC is commonly used in batteries, solar panels, and electronic devices because it provides a steady and constant voltage. The symbol for direct current is usually denoted by a straight line (—) above a dashed line (···).

Alternating Current (AC)

When the current flows in positive and negative directions alternatively in a cycle, it is called alternating or AC.

This type of current changes its direction and magnitude periodically, and its frequency is measured in hertz (Hz).

In most countries, household AC supply has a frequency of 50 Hz, meaning it changes direction 50 times per second. AC is used for transmitting electricity over long distances because it can be easily transformed to different voltages using transformers.

The symbol for alternating current is a wavy line (~).

Heating Effect of Electric Current (Joule’s Law)

Electric current flows in a circuit when a voltage is applied across it. The magnitude of the current depends on the voltage V and circuit resistance R.  As the current passes through a conductor, electrical energy is converted into heat energy due to the opposition offered by the conductor’s resistance.

The rate of heat produced in the circuit depends on the resistance and the square of the current. 

H = I² Rt

Where,

I = Current in ampere
R = Resistance in Ohms
t   = Time in Seconds

This is known as Joule’s Law of heating. It states that the heat produced in a conductor is directly proportional to the square of the current, the resistance of the conductor, and the time for which the current flows.

Common applications of the heating effect of electric current include electric heaters, toasters, electric irons, water geysers, and filament bulbs. In these devices, materials with high resistance (such as nichrome) are used to produce heat efficiently.

Read detailed article on: Heating Effect of Electric Current- Definition, Examples and Applications

Magnetic Effect of Electric Current

A magnetic field is set up around the current-carrying conductor. The strength of this magnetic field increases with the magnitude of the current and decreases with distance from the conductor.

We can find the direction of the magnetic field using the right-hand rule. In this rule, if you hold the conductor with your right hand so that your thumb points in the direction of current flow, the curl of your fingers will indicate the circular path of the magnetic field lines.

If a current-carrying conductor is placed in a magnetic field, the conductor experiences a force.

The magnitude of the force depends on the magnitude of the current through the conductor and on the strength of the magnetic field. The direction of the force can be found using Fleming’s left-hand rule.

This rule helps to determine the direction of the force acting on the conductor. Here, the forefinger represents the magnetic field, the middle finger represents the current, and the thumb represents the direction of the force.

Fleming’s Left Hand Rule. This interaction between current and magnetic field is the principle behind devices like electric motors, loudspeakers, and electromagnetic relays.

Applications of the Magnetic Effect of Electric Current include:

• Electric motors – convert electrical energy into mechanical rotation.
• Loudspeakers – use electromagnetic force to produce sound.
• Electromagnetic relays – control high-power circuits with low-power signals.
• Magnetic cranes – lift and move heavy metallic objects.
• Galvanometers – detect and measure small currents.

Formula for Force on a Current-Carrying Conductor

The magnitude of the force can be expressed as;

F= BILSinθ

Where:

• F = Force on the conductor (in Newtons)
• B = Magnetic flux density (in Tesla)
• I = Current flowing through the conductor (in Amperes)
• L = Length of the conductor within the magnetic field (in meters)
• θ = Angle between the conductor and the magnetic field lines

This equation shows that the force is directly proportional to the strength of the magnetic field, the current through the conductor, and the length of the conductor in the field. The term sin θ indicates that the force is maximum when the conductor is perpendicular to the magnetic field (θ = 90°) and zero when it is parallel (θ = 0°).

This principle forms the basis for the operation of electric motors, where current-carrying conductors are intentionally placed in magnetic fields to produce rotational motion.

Example:
If a conductor of length 0.5 m carries a current of 3 A through a magnetic field of 0.2 T, placed at a right angle (θ = 90°) to the field:

F= BILSinθ
=0.2×3×0.5×sin90^∘
F=0.3 N

So, the conductor experiences a force of 0.3 Newtons.

Applications of Electric Current

Electric current plays a vital role in almost every aspect of modern life. Some common applications include:

• Electronics: Powers devices like computers, smartphones, and home appliances.
• Lighting: Supplies energy to incandescent bulbs, LED lights, and street lamps.
• Transportation: Runs electric trains, trams, and electric vehicles (EVs).
• Power Grids: Distributes electricity from power plants to homes and industries.
• Electroplating: Used in industries to coat metals for corrosion resistance or decorative purposes.
• Medical Equipment: Powers X-ray machines, ECG monitors, and other diagnostic devices.

Safety Precautions with Electric Current

While electric current is useful, improper handling can be dangerous. Essential safety measures include:

• Grounding: Ensures stray current flows safely into the ground.
• Fuses: Break the circuit in case of excess current flow to prevent overheating.
• Circuit Breakers: Automatically stop the current in case of faults or overloads.
• Insulation: Prevents accidental contact with live wires.

⚠️ Always handle electrical devices with care and follow safety guidelines, especially in school or college laboratories.

Electric Current Theory – FAQs

Related Articles:

1. What are volts and amperes?
2. Difference between Voltage and Current
3. Direction of Electric Current
4. Difference between AC and DC
5. An ammeter is used to Measure?