Electric Charge

Summary

Charge is the fundamental quantity of electricity. (Electricity is all about charge.)
- No one can tell you what charge is. They can only tell you how charges interact.
The classical study of electricity is generally divided into three general areas.
- electrostatics: the study of the forces acting between charges
- electric current: the study of the forms of energy associated with the flow of charge
- electromagnetism: the study of the forces acting between charges in motion
The connection between the types of charge and the mathematical symbols is intentional and ingenious.
- Electric charge (often just called charge) comes in two and only two types.
  - positive (+) and
  - negative (−)
- The term neutral does not refer to a third type of charge, but to the presence in a region of positive and negative charges in equal amount.
  - The sum of identical positive and negative quantities is zero (0). This is what it means to be electrically neutral.
  - The assumed charge of all macroscopic objects is neutral unless otherwise indicated.
  - Although regions of space might be described as being "positive" or "negative" the universe as a whole is electrically neutral.
- The choice of assignment of positive to one type of charge and negative to the other was completely arbitrary.
  - There is no objective test that can be used to distinguish positive charge from negative charge.
  - The sign of a charge can only be determined by comparison to a charge with a charge whose sign is already known.
Rule of Action (the way to tell one type of charge from another)
- like charges repel
- opposite charges attract
Methods of Charging
- triboelectricity: different materials originally in contact that are then separated; one becomes positive, the other negative
  - the means by which electric charge was first discovered
  - often mistakenly called "charging by friction"
- conduction: transfer by contact with an already charged object
  - dielectric breakdown (sparks): given enough stress in an insulator can be made to conduct electricity
- induction: separation of charge when near an already charged object
  - Charged objects can attract electrically neutral objects through induction.
    Here's the logic …
    - Like charges move away from the charged object.
    - Opposite charges move toward the charged object.
    - Attraction predominates since the like charges are closer together than the opposite charges.
- Some methods of charging are best left to the chemists of this world to explain.
  - electrochemical: as found in batteries and electric fishes
  - polarity: charge separation on the molecular scale
- Some methods of charging are best left to the materials scientists of this world to explain.
  - piezoelectricity: charge separation in materials under mechanical stress
  - pyroelectricity: charge separation brought about by heating
Origin of Charge (Atomic Structure)
- Atoms as a whole are …
  - eternal
  - electrically neutral
  - mostly empty space
  - small (~ 10⁻¹⁰ m)
- The nucleus is …
  - the center of the atom
  - electrically positive
  - relatively massive (the source of nearly all the mass of the atom)
  - fixed (effectively unmovable)
  - very small (~ 10⁻¹⁵ m)
- The electron is …
  - spread out over the entire volume of the atom (~ 10⁻¹⁰ m)
  - electrically negative
  - relatively lightweight
  - mobile (comparatively easy to move around)
  - infinitesimally small when isolated (< 10⁻¹⁸ m)
- Most electrical phenomena on earth are due to the transfer of electrons.
Electrical Properties of Materials
- conductors: charge moves easily
  - metals
  - electrolytes (ionized liquids)
  - plasmas (ionized gases)
- insulators: charge does not move easily
  - nonmetals (pure water, organics, gases, …)
- semiconductors: sometimes a conductor, sometimes an insulator
  - metalloids (silicon, germanium, doped materials, …)
- superconductors: the perfect conductor; offers no resistance below critical temperature
  - many substances are superconductors below some critical temperature
The SI unit of charge is the coulomb [C]
- the amount of charge transferred by one ampère of current in one second [As].
  - The reasoning behind this definition is best left to later chapters in this book.
- an unusually large unit for day-to-day applications

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