Electric Charge

Summary

• Charge (or more formally, electric 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 comes in two and only two types.
    • positive (+)
    • 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 with a charge whose sign is already known.
• Rule of action
  • Like charges repel.
  • Opposite charges attract.
• Methods of charging
  • triboelectricity…
    • is the separation of charge that occurs when different materials are in contact and then separated such that one material becomes positive, the other negative
  • is the method by which electric charge was first discovered
  • is often mistakenly called "charging by friction"
  • conduction…
    • is the transfer of charge by contact with an already charged object
  • can also occur by dielectric breakdown
    • Given enough stress, an insulator can be made to conduct electricity.
    • A spark occurs when air experiences dielectric breakdown.
  • induction…
    • is the separation of charge that occurs when a neutral object is brought near a charged object
  • is the means by which an uncharged object can be attracted to a charged object
    • 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 uniform mechanical stress (compression or extension)
    • flexoelectricity: charge separation in materials under non-uniform mechanical stress (bending)
    • pyroelectricity: charge separation brought about by heating
• Charge has its origins in 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 electric phenomena on Earth are due to the transfer of electrons.
• Electric properties of materials
  • Charge can flow easily through a conductor.
    • metals
    • electrolytes (ionized liquids)
    • plasmas (ionized gases)
  • Charge does not flow easily through an insulator.
    • nonmetals (pure water, organics, gases, …)
  • A material that behaves sometimes like a conductor and sometimes like an insulator is called a semiconductor.
    • metalloids (silicon, germanium, doped materials, …)
  • Materials where charges flow with absolutely no resistance are called superconductors.
    • Superconductors are perfect conductors in a sense.
    • Many substances are superconductors below some critical temperature.
• The SI unit of charge is the coulomb [C].
  • One coulomb is the amount of charge transferred by one ampère of current in one second of time [C = A s].
    • The reasoning behind this definition is best left to later chapters in this book.
  • One coulomb is a unit that is too large for day-to-day applications.
    • The net charge on human-sized objects with a noticeable charge is best measured in nanocoulombs [nC] or picocoulombs [pC].
• Elementary charge
  • Charge is quantized in multiples of the elementary charge [e].
  • The charge on a proton is +1 e.
  • The charge on an electron is −1 e.
  • 1 e = 1.602176634 × 10⁻¹⁹ C exactly, by definition.
• Conservation of charge
  • The total charge of a closed system is constant.
  • The universe is a closed system.
  • When subatomic particles are created, they do not add or subtract charge from the universe as a whole.