Summary
Highlights
There are two types of charge: positive and negative. The smallest unit of positive charge is a proton, and for negative charge, it's an electron. Charges are quantized, meaning they come in discrete packages, and they are conserved, meaning the total charge before and after an event remains the same.
Protons and electrons have the same magnitude of charge but opposite polarities. However, their masses differ significantly, with a proton being 1836 times more massive than an electron. Like charges repel, and opposite charges attract.
The flow of charge is called current (I), defined as the rate of change of charge over time (dQ/dt). The separation of charges creates a potential difference (V), also known as voltage, defined as the rate of change of energy per charge (dU/dQ). These are fundamental to how electrical energy works.
Conductors, typically metals like copper, allow charges to flow easily due to free valence electrons. Insulators, such as glass or plastic, are non-conductors that resist charge flow.
Semiconductors, like silicon, have electrical properties that can be modified by 'doping,' allowing for the creation of transistors. Superconductors offer no resistance to current flow at very low temperatures, a property that could revolutionize technology if achieved at room temperature.
Resistance is the opposition to the flow of electrons, caused by collisions between electrons and atomic nuclei within a conductor. These collisions release energy, usually in the form of heat.
In electrostatic situations, charge on a conductor resides on its outer surface. If the conductor is irregularly shaped, charge concentrates at sharp corners. This distribution ensures the net electric field inside the conductor is zero.
The electronic charge (e) is 1.6 x 10^-19 coulombs. Coulomb's Law describes the force between two point charges (F = k * |q1 * q2| / r^2), where k is Coulomb's constant (approximately 9 x 10^9 N·m²/C²).
An example calculation demonstrates Coulomb's Law: for two charges, 3 microcoulombs and -2 microcoulombs, separated by 1 cm, the attractive force is 540 Newtons. The direction is determined by the polarities of the charges.
Beyond point charges, charge can be distributed along a line (linear charge density, lambda, in C/m), over a surface (surface charge density, sigma, in C/m²), or throughout a volume (volume charge density, rho, in C/m³).
Two shell theorems simplify calculations for charged shells: 1. A shell of uniform charge acts on an external charged particle as if all its charge were concentrated at its center. 2. A charged particle inside a shell of uniform charge experiences no net electrostatic force from the shell.
Objects can be charged by contact (e.g., friction, where charges are redistributed upon touching) or by induction (bringing a charged object near a neutral conductor, separating charges, and then grounding to remove unwanted charges, leaving the conductor with an opposite net charge).
Comparing the electrostatic force and gravitational force between a proton and an electron separated by one meter reveals that the electrostatic force is about 10^39 times stronger than the gravitational force. This immense difference explains why gravity can often be ignored in electrostatic calculations.