Dielectric And Dielectric Constant

Card # 57 Dielectric and Dielectric constant Dielectric is a substance which contains no free charge carrier’s example wood, glass etc. If two charges placed in some other medium in place of vacuum or air then force between them

Fmedium =

1 4πε

q1q2 r2

ε=

absolute permittivity of medium permittivity of vacuum or free space ∴ Dielectric constant K or εr = ε/ε0 For vacuum K = 1 for air K = 1.0006 ε 0=

F medium F vacuum

=

1 q1q2 4π ε r2 = 1 q1q2 4πε0 r2

1/ε = ε0 = 1 1/ε0 ε K

Electric field Electric filed: The region which a charge experiences a force is called the electric field. Electric line of force – Electric line of force in an electric field is an imaginary smooth curve along which a unit free positive test charge tends to move. Test charge – The test charge is assumed so small that it doesn’t cause any change in initial electric field.

ur Electric field strength E ur E at any point in an electric field is a vector quantity whose magnitude is equal to the force acting on per unit positive test charge and whose direction is along the direction of force.

ur ur F E= qo

ur F is the force of positive test charge q0 ur

Unit of E

= Newton/ coulomb = N/C = Volt/Metre = V/m

Properties of electric line of force

1. The electric lines of forces appear to start from positive charge and to terminate at negative charge. 2. The tangent drawn at any point on the line of force gives the direction of electric field strength. 3. No two electric lines of force can intersect each other. 4. The electric lines of force may be in the form of open and closed curve. 5. The equidistant electric lines of force represent uniform electric field Fig (a). Electric lines of forces at different separations represent non-uniform electric field Fig (b)