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)

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