What Is Electric Current?

What Is Electric Current?

Electric Current

Electric current is defined as the rate of flow of charge or charge flowing per unit time interval. Its direction is the direction of flow of positive charge. The SI unit of electric current is the ampere (A), and it is a scalar quantity.

In a metal wire, an electric current is set up due to the flow of electrons.

A current of one ampere flowing through a conductor means 6.25 x 10¹⁸ electrons are entering at one end or leaving the other end of the conductor in one second.

Resistance

• Resistance is the opposition offered by a conductor to the flow of current through it. It arises due to collisions of drifting electrons with the core ions. The SI unit of resistance is the ohm (Ω).

Ohm's Law

Ohm’s law states that if physical conditions like temperature and intensity of light remain unchanged, the electric current flowing through a conductor is directly proportional to the potential difference across its ends.

If V is the potential difference across the ends of a conductor and I is the current through it, then according to Ohm's law:

V∝I or V=RIV \propto I \quad \text{or} \quad V = RI V∝I or V=RI

Where R is a constant called the resistance of the conductor.

Types of Resistance

• Ohmic Resistance: The resistances of conductors that obey Ohm's law, such as the resistance of manganin wire.

• Non-Ohmic Resistance: The resistances of materials that do not obey Ohm's law, such as the resistance of diode valves and triode valves.

Conductance

• The reciprocal of the resistance of a conductor is called its conductance. It is denoted by G.

• The SI unit of conductance is mho, or siemen (S).

Relation Between Resistance and Conductor Properties

The resistance of a conductor is directly proportional to its length and inversely proportional to its cross-sectional area. If l and A are the length and cross-sectional area of a conductor, and R is its resistance, then:

R=ρlAR = \rho \frac{l}{A} R=ρAl​

Where ρ is a constant called specific resistance or resistivity. The SI unit of resistivity is ohm meter (Ω·m).

Specific Conductance or Conductivity

The reciprocal of resistivity is called conductivity (σ). The SI unit of conductivity is mho/meter (S/m).

• Metals are good conductors of electricity, with silver having maximum conductivity.

• Graphite, though a non-metal, is also a good conductor.

• The conductivity of superconductors is infinite, while diamond is a poor conductor of electricity.

Combination of Resistances

Resistances can be combined in two main ways:

1. Series Combination: The equivalent resistance is equal to the sum of the resistances of individual conductors.

Req=R1+R2+⋯+RnR_{\text{eq}} = R_1 + R_2 + \dots + R_nReq​=R1​+R2​+⋯+Rn​

2. Parallel Combination: The reciprocal of equivalent resistance is equal to the sum of the reciprocal of individual resistances.

1Req=1R1+1R2+⋯+1Rn\ frac{1}{R_{\text{eq}}} = \frac{1}{R_1} + \frac{1}{R_2} + \dots + \frac{1}{R_n} Req​1​=R1​1​+R2​1​+⋯+Rn​1​

Related Information and Devices

Electric Power

• The rate at which electrical energy is consumed in a circuit is called electric power. Its SI unit is watt (W).

Kilowatt Hour (kWh)

• A kilowatt-hour is the unit of energy and is equal to the energy consumed in the circuit at the rate of 1 kilowatt (1000 J/s) for 1 hour.

1 kWh=3.6×106 joules1 \, \text{kWh} = 3.6 \times 10^6 \, \text{joules}1kWh=3.6×106joules

1 kWh is also called a Board of Trade unit.

Ammeter

• An ammeter is a device used to measure electric current in a circuit. It is connected in series in the circuit. The resistance of an ideal ammeter is zero.

Voltmeter

• A voltmeter is a device used to measure the potential difference between two points in a circuit. It is connected in parallel to the circuit.

• The resistance of an ideal voltmeter is infinite.

Electric Fuse

• An electric fuse is a protective device used in series with an electric appliance to prevent damage due to high current.

• It is usually made of an alloy of copper, tin, and lead. Pure fuses are made from tin.

• The fuse material should have a low melting point and high resistance.

Galvanometer

• A galvanometer is a device used to detect and measure small electric currents in a circuit.

Shunt

• A shunt is a wire with very small resistance.

• A galvanometer can be converted into an ammeter by connecting a shunt in parallel to it and into a voltmeter by connecting a very high resistance in series.

Electromagnetic Induction

• When there is a change in magnetic flux linked with a circuit, an electromotive force (emf) is induced in the circuit. This phenomenon is called electromagnetic induction.

• The direction of the induced current is determined using Fleming’s right-hand rule or Lenz's law.

Transformer

• A transformer is a device that converts low-voltage AC into high-voltage AC and vice versa. It works on the principle of electromagnetic induction and can only be used with alternating current (AC).

AC Dynamo (Generator)

• An AC dynamo is a device used to convert mechanical energy into electrical energy. It operates on the principle of electromagnetic induction.

Electric Motor

• An electric motor converts electrical energy into mechanical energy.

Microphone

• A microphone converts sound energy into electrical energy and works on the principle of electromagnetic induction.

Power Generation and Transmission

• The current generated in power stations is alternating current (AC) with a voltage of 22,000 volts or more. In grid substations, transformers increase the voltage to 132,000 volts to minimize energy loss during long-distance transmission.

Street Lamps

• Sodium and mercury street lamps light up due to atomic emission. The electrons from the filaments hit sodium/mercury atoms, exciting their valence electrons to higher energy levels, which then relax by emitting characteristic light.

Fluorescent Tube

• The purpose of a choke coil in a fluorescent tube is to produce high voltage to ionize the gas in the tube, which is required for a high current to flow through the filament.

Final Thoughts

Electric current is the flow of charge through a conductor, measured in amperes (A), and is caused by the movement of electrons in a metal wire. Resistance is the opposition to this flow, and it arises due to the collisions of electrons with ions in the conductor.

According to Ohm’s Law, the current is directly proportional to the potential difference across the conductor and inversely proportional to the resistance. Materials like metals are good conductors, while materials like wood and glass are insulators.

The conductance is the reciprocal of resistance, and it measures how easily current can flow through a material. Electrical devices like ammeters and voltmeters help measure current and potential difference, while fuses protect circuits from high currents. Understanding these concepts is essential for grasping how electrical circuits and devices operate in everyday life.