Why static cling and exploding grain silos obey the same ancient amber rule
Rub a piece of amber and it picks up scraps of fluff, a trick known since antiquity. The Greek word for amber, elektron, gave us electricity. The forces behind it explain everything from plastic wrap clinging to your fingers to photocopiers, laser printers and the sudden, dangerous explosion of grain silos.
Electrostatics is the physics of electric charges that sit still or move only slowly, on objects large enough that quantum effects can be ignored. Under those conditions the relationships between charge, the electric field it creates and the electric potential are clean, free from the tangle of magnetic effects. Its everyday reach is surprisingly wide: the same forces damage delicate electronic parts on factory lines and make office printers work.
At its heart sits Coulomb's law. Two point charges push or pull along the straight line between them, with a force that grows with the product of their charges and weakens with the square of their separation. Like signs repel; opposite signs attract. A constant called the vacuum permittivity, about 8.85 × 10−12 farads per metre, sets the scale. Because contributions from many charges simply add together, an example of the superposition principle, the field around any arrangement can be built up piece by piece.
To picture the result, physicists draw field lines. They start on positive charges and end on negative ones, point along the field everywhere, and crowd together where it is strongest. Gauss's law offers a shortcut: the total electric flux through any closed surface depends only on the charge trapped inside, so many problems yield to an imaginary enclosing surface. Combined with the idea of potential, this leads to Poisson's equation, which becomes Laplace's equation wherever no free charge sits.
The discipline does not demand a world without magnets or currents. What matters is that any magnetic fields or currents stay steady, or change only very slowly. When that holds, engineers can apply the electrostatic approximation and ignore the coupling to magnetism altogether. Electrostatics and its magnetic twin, magnetostatics, can both be seen as slow-motion limits of full electromagnetism.
Source: Electrostatics
