- Introduction to Electric Current
- Unit of Electric Current
- Current Density
- Effects of Electric Current
- Types of Electric Current

## Introduction to Electric Current

**Electricity**, phenomenon associated with stationary or moving electric charges. Electric charge is a fundamental property of matter and is borne by elementary particles. In electricity the particle involved is the electron, which carries a charge designated, by convention, as negative. Thus, the various manifestations of electricity are the result of the accumulation or motion of numbers of electrons. [source] Electric Current is defined as rate of flow of charge through any cross-section. If $\Delta q$ charge passes through a cross-section in time $\Delta {\rm{t}}$ then average current ${{\rm{I}}_{{\rm{av}}}} = {{\Delta {\rm{q}}} \over {\Delta {\rm{t}}}}\quad $and $\quad $ instantaneous current ${\rm{I}} = \mathop {\lim }\limits_{\Delta {\rm{t}} \to 0} {{\Delta {\rm{q}}} \over {\Delta {\rm{t}}}} = {{{\rm{dq}}} \over {{\rm{dt}}}}$

## Unit of Electric Current

Current is a fundamental quantity with dimension $\mathrm{M}^{\circ} \mathrm{L}^{\circ} \mathrm{T}^{\circ} \mathrm{A}^{1}$ Current is a scalar quantity with its SI unit**Ampere.**

**Ampere:**The current through a conductor is said to be one ampere if one coulomb of charge is flowing per second through a cross-section of wire. CGS unit of current is

**Biot**(Bi) 1 ampere $ = {{1{\rm{ coulomb }}} \over {{\rm{ sec ond }}}} = {{(1/10){\rm{ emu of charge }}} \over {{\rm{ sec ond }}}} = {1 \over {10}}$ Biot

### Important Points

- The conventional direction of current is the direction of flow of positive charge or applied field. It is opposite to direction of flow of negatively charged electrons.
- The current may be constituted by motion of different type of charge carries in different situations.
- If n electrons pass through a point in a conductor in time $t$ then current through the conductor is ${\rm{I}} = {{\rm{q}} \over {\rm{t}}} = {{{\rm{ne}}} \over {\rm{t}}}$. Number of electrons flowing through conductor in ${\rm{t}}$ second is${\rm{n}} = {{{\rm{I}} \times {\rm{t}}} \over {\rm{e}}}$. For 1 ampere of current ${\rm{n}} = {1 \over {1.6 \times {{10}^{ – 19}}}} = 6.25 \times {10^{18}}$ electrons/second
- The conductor remains uncharged when current flows through it because the charge entering at one end per second is equal to charge leaving the other end per second.
- For a given conductor current does not change with change in its cross-section because current is simply rate of flow of charge.
- If n particles each having a charge q pass per second per unit area then current associated with cross-sectional area A is ${\rm{I}} = {{\Delta {\rm{q}}} \over {\Delta {\rm{t}}}} = {\rm{nqA}}$
- If there are n particles per unit volume each having a charge q and moving with velocity v then current through cross-sectional area A is ${\rm{I}} = {{\Delta {\rm{q}}} \over {\Delta {\rm{t}}}} = {\rm{nqvA}}$
- If a charge q is moving in a circle of radius r with speed v then its time period is T = 2πr/v. The equivalent current ${\rm{I}} = {{\rm{q}} \over {\rm{T}}} = {{{\rm{qv}}} \over {2\pi {\rm{r}}}}$

## Current Density

This is defined as current flowing per unit area held normal to direction of flow of current. Current density $J = {I \over A} = {{nq} \over {At}}$ and current $I = \int {\vec J} \cdot \overrightarrow {da} $ The direction of current density $\vec \jmath $ is same as that of electric field $\vec E.$ J is a vector with unit ${\rm{amp}}/{{\rm{m}}^2}$and dimension $\mathrm{M}^{\circ} \mathrm{L}^{\circ} \mathrm{T}^{\circ} \mathrm{A}^{1}$## Effects of Electric Current

### (a) Heating Effects of Electric Current

The phenomenon of heating of a conductor by the flow of an electric current through it is called**heating effect of electric current**.

### (b) Magnetic Effects of Electric Current

The phenomenon of production of a magnetic field in space around a current carrying conductor is called**magnetic effect of electric current**.

### (c) Chemical Effects of Electric Current

The phenomenon of decomposition of an electrolyte into ions on passing an electric current through it is called**chemical effect of electric current**.

## Types of Electric Current

**
** (a) Direct Current

The current whose magnitude and direction does not vary with time is called **direct current**(dc). The various sources are cells, battery, dc dynamo etc.

### (b) Alternating Current

The current whose magnitude continuously changes with time and periodically changes its direction is called**alternating current**. It has constant amplitude and has alternate positive and negative halves. It is produced by ac dynamo.

**Also Read:**

- Ohm’s Law and Resistance
- Combination of Resistances
- EMF and Internal Resistances of a Cell
- Cells Connected in Series, parallel and Mixed
- Kirchhoff’s Circuit Law
- Electric Currents in Conductors
- Wheatstone Bridge
- Post office Box
- Wheatstone Meter Bridge
- Moving Coil galvanometer
- Ammeter and Voltmeter
- Potentiometer Working Principle

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