# Electric Current ,Resistance and Resistivity

## (1) Introduction

• In our previous few chapters of electrostatics,we have discussed various terms and characterstics related to charge at rest
• Now in this chapter we will study about the moving charges,phenomenom related to them and various effects related to charge in motion
• Consider two metallic conducting balls charged at different potential are hanged using a non conducting insulating wires .Since air is an insulator ,no charge transfer takes place
• Now if we join both the metallic wire using a conducting metallic wire then charge will flow from metallic ball at higher potential to the one at lower potential.
• This flow of charge will stop when the two balls would be at the same potentials.
• If somehow we could maintian the potential between the metallic balls,we will get constant flow of the charge in metallic wire,connecting the two conducting balls
• This flow of charge in metallic wire due to the potential dicference between two conducters used is called electric current about which we would be dicussing in this chapter.

## (2)Electric current and Current density

Electric Current

• We already had a brief idea about the electric current which wed defined as the state of motion of the electric charge .Now we are going to study about the electric current in details
• Quantitatively electric current is defined as the time rate of flow of the net charge of the area of crosssection of the conducter i.e
Electric current = Total charge flowing / time taken
• if q is the amount of charge flowing through the conducter in t sec,The current through the conducter is given by
I=q/t                          (1)
• SI unit of the current is Ampere(A) named so in the honour of french scientist Andee marie Ampere(1775-1836).Now,
1 Ampere= 1 Coulumb/ 1 sec=1 Cs-1
• Thus current through any conducter is said to be 1 ampere,if 1 C of charge is flowing through the conducter in 1 sec
• Small amount of currents are accordingly expressed in milliamperes (1mA=10-3 A) or in micro ampere (1 mA=10-6 A)
• Direction of electric current is in the direction of the flow of positive charged carriers and this current is known as conventional current.
• Direction of the flow of electron in conductor gives the direction of electronic current. Direction of conventional current is opposite to that of electronic current
• Electric current is a scalar quantity .Although electric current represent the direction of the flow of positive charged carrier in the conductor,still current is treated as scalar quantity as current in wires in a circuit does not follows the laws of vector addition
Current density
• The current density at a point in the conductor is defined as the current per unit cross-section area.Thus if the charge is flowing per unit time uniformaly over the area of crosss-section A of the conductor,then current density J at any point on that area is defined as
J=I/A -(2)
• It is the characterstic property of point inside the conductor nor of the conductor as a whole
• Direction of current density is same as the direction of conventional current
• Note that current density is a vector quantity unlike electric current
• Unit of current density is Ampere/meter2 (Am-2)