electrostatic

electrostatic

1. Charge

   • Static electricity is associated with materials on which electrical charge moves only slowly (insulating materials) and with electrically isolated conductors.

2. Force between charges

   • The force between two charges, Q1 and Q2, is proportional to the quantities of charge, and inversely proportional to the square of the separation distance, d:

3. electric field

   • Around a charged body there is a force of attraction or repulsion for any other charges.

4. potential

   • The 'potential' at a point is defined as the amount of work needed to bring a unit charge from infinity to that point.

5. Electric flux density

   • The electric field from an isolated point charge is radial. The field may conveniently be represented diagrammatically by lines of force.εo

6. Gauss' law

   • Gauss' Law states that the nett flux through any surface equals the nett charge enclosed within that surface. ∇. E = q / εo

7. induced charge

   • Thus a conducting body placed in an electric field will have charge 'induced' on it with the density of charge proportional to the electric field.

8. capacitance

   • The proportionality between the quantity of charge Q and the voltage V is called the 'capacitance' C of the system. The unit of capacitance is the 'farad' with units of coulomb/volt. For the pair of parallel plates: C = Q/V = εo A / d

9. Dielectrics

   • If an insulator is placed between the plates of a parallel plate capacitor the capacitance is increased. If a block of conductor were placed between the capacitor plates the field in the remaining gap would be increased.

10. Charge migration and dissipation

    • insulating materials have very few free electrical charges so only very small currents flow under the influence of an electric field. In more usual terms the volume 'resistivity' may be from 1012 to 1016 ohm m and the surface resistivity over 1010 ohms per square.

11. Charge separation mechanisms

    • Contact between surfaces results in transfer of charge as electrons to match up energy levels of neighboring atoms around the points of contact.

12. Electrical breakdown

    • If an electric field, in air at normal temperature and pressure, increases above about 3 MV m-1 then ‘electrical breakdown’ occurs