Introduction

Introduction

• It is known that the flow of charges constitutes an electric current. The current can be measured by measuring how many charges are passing through a specified surface or a point in a material per second. The flow of charge per unit time i.e. rate of flow of charge at a specified point or across a specified surface is called an electric current. It is measured in the unit Ampere, which is coulombs/sec (C/s). Thus mathematically the electric current can be expressed as,

I = dQ / dt C/s i.e. A

• The Ohm's law relates the applied voltage, an electric current and a resistance. The relation is simple and straight forward for simple d.c. circuits. But in electromagnetic engineering, motion of charges in various media such as liquid, gas, dielectrics etc is considered. In such media, both positive and negative charges are present with different characteristics. The basic Ohm's law is not sufficient to find current through such media. Hence in electromagnetic engineering instead of current, the current density plays an important role. This chapter explains the current density, continuity equation and the properties of conductors and dielectrics. Then it explains the boundary conditions, the concept of capacitance and calculation of capacitance under various conditions.