Electric Charge and Static Electricity

Outcomes of Learning

• Define electric charge, electric attraction, and repulsion
• State and explain the law of conservation of charge
• Differentiate between conductors and insulators
• Describe the methods of charging an object
• Relate Coulomb's law to the universal law of gravitation
• State Coulomb's law and use it to calculate the electric force between charges
• Relate the concept of electric charge to real-life applications or phenomena

Electric Charge

• An object becomes "charged" when rubbed, resulting in a net electric charge
• Static electricity is the buildup of electric charge on an object's surface
• Static electricity can be observed by rubbing objects together
• Charge is the fundamental quantity of electricity
• Electric charge determines the electric interaction between subatomic particles or objects at the atomic level
• Benjamin Franklin named the two types of electric charge positive and negative

Electric Charge in the Atom

• A neutral atom can gain or lose electrons, which is called ionization
• Anion: An atom with a net negative charge
• Cation: An atom with a positive net charge
• Proton: A stable subatomic particle with a positive charge equal in magnitude to a unit of electron charge, stays inside the atom, with a mass (mp) of 1.67 x 10^-27 kg and a charge (e) of +1.60 x 10^-19 C
• Electron: The lightest stable subatomic particle with a negative charge, considered the basic unit of electric charge, capable of moving out from its orbit, with a mass (me) of 9.11 x 10^-31 kg and a charge (e) of -1.60 x 10^-19 C
• The term "neutral" refers to the presence of positive and negative charges in equal amounts
• The net charge is zero when the sum of positive and negative quantities is equal, which means the object is electrically neutral
• Electric attraction and repulsion state that "like charges repel and opposite charges attract"
• When a balloon and a sweater are rubbed together, the balloon acquires a net negative charge, and the sweater acquires a net positive charge of the same magnitude

Law of Conservation of Charge

• No net electric charge can be created or destroyed; it is merely transferred from one body to another
• The total electric charge on the objects does not change

Electrical Properties of Materials

• Conductors allow the easy movement of charge, most metals are generally good conductors like electrolytes (ionized liquid) and plasmas (ionized gases), with copper being a common conductor
• Insulators have no or very few free electrons and resist the flow of charges, most nonmetals are insulators, including rubber, plastic, paper, glass, and air
• Semiconductors fall into an intermediate property between conductors and insulators and have paved the way for developing miniaturized electronic devices such as transistors and integrated circuits

Methods of Charging

• Charging by Friction: When an object is rubbed over another object, electrons are transferred from one object to another
• The object that transfers electrons loses negative charge, and the object that accepts electrons gains negative charge

Electron Affinity

• Electron affinity measures an atom's attraction to an electron or tendency to become negatively charged
• Greater Affinity: Holds electrons stronger and often gains electrons
• Less Affinity: Holds electrons weaker and often loses electrons
• Triboelectric Series: Arranged in order of increasing electron affinity from top to bottom, generally, when two different materials are rubbed together, the one higher on the list will become positively charged
• Charging by Conduction: Involves charging an uncharged object by bringing it into contact with another charged object
• Charging by Induction: Occurs when an insulator experiences an induced-charged effect, causing polarization when a charged object is brought close

Polarization

• Separates opposite charges within an object

Coulomb's Law

• Electrostatic Force: The degree of repulsion or attraction between charged objects, an action at a distance originally studied and quantified by French engineer and physicist Charles Coulomb (1736 - 1806)

Coulomb's Law Explained

• The force between charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them
• The magnitude F of the force between two point charges q1 and q2 a distance r apart can be expressed as: Fe = k|q1q2|/r^2
  • k = Coulomb's law constant 9.0 x 10^9 N ⋅ m²/C²
  • q1 and q2 = magnitude of the charges (SI unit: coulomb, C)
  • r = distance between charges (SI unit: meter, m)
  • Fe = magnitude of the electrostatic force (SI unit: Newton, N)
• In SI units, Coulomb's proportionality constant is k = 9.0 x 10^9 N ⋅ m²/C² expressed in two significant figures
• The most fundamental unit of charge is the magnitude of the charge of an electron or a proton, referred to as the elementary charge, e = 1.60 x 10^-19 C

Direction of the Electrostatic Force

• The direction of the electric force is always along the line joining the two objects
• If the two charges have the same sign, the force on either object is directed away from the other (repel)
• If the two charges have opposite signs, the force on one is directed toward the other (attract)

Coulomb's Law vs. Newton's Universal Law of Gravitation

• Coulomb's Law describes the electrostatic force Fe = k|q1q2|/r^2, while Newton's Universal Law of Gravitation describes gravitational force, Fg = G(m1m2)/r^2
• Electrostatic force can be either attractive or repulsive, while gravitational force is always attractive
• Electrostatic force is stronger than gravitational force in interactions of atomic and subatomic particles
• Gravitational force is usually much greater than electrostatic force for objects of a person or a planet
• Coulomb's Law explains why the electron revolves around the nucleus, while Universal Law of Gravitation explains why Earth revolves around the sun
• Coulomb's law is observable in the Micro-world, gravitation is observable in the Macro-world
• Charge of proton= 1.60 x 10^-19 C, charge of electron = -1.60 x 10^-19
• k = 9.0 × 10^9 Nm²/c² and distance is = 5.30 x 10^-11m
• Mass of Proton = 1.67 x 10^-27 kg, mass of Electron = 9.11 × 10^-31kg G = 6.67 x 10^-11 Nm²/kg² and distance is = 5.30 x 10^-11m

Determining Magnitude

• Electric and gravitational forces on the electron of a hydrogen atom exerted by the single proton, with an average distance of 5.30 x 10^-11 m, demonstrate that electric force is much stronger
• Electrical forms are the same. Electrical forces can be either attractive or repulsive while gravitational forces are always attractive
• The electrical force between the electron and proton of a hydrogen atom is 8.20 x 10^-8 N, the gravitational force is 3.61 x 10^-47 N Electrostatic force is stronger than the gravitational force by a large magnitude

Sample Problems

• The magnitude of the electric force of attraction between an iron nucleus (q = +26e) and its innermost electron at a distance of 1.5 x 10^-12 m can be calculated using Coulomb's law, q₁ = +26e = + 4.16 x 10−18 C and q₂ = − 1. 60 x 10−19 C
• In a scenario with two charges in a line, finding the magnitude and direction of the electric force exerted between the charges
• Three charges arranged in a line can be used to determine the magnitude of the electrostatic force and the net electrostatic force, using superposition F3 = √(Fx)² +(Fy)² and direction θ=tan-1(Fy/Fx)