The Physics
Hypertextbook
Opus in profectus

# 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 of time [A s].
• The reasoning behind this definition is best left to later chapters in this book.
• an unusually large unit for day-to-day applications.
• Elementary Charge
• 1.60 × 10−19 coulombs
• the magnitude of the charge on an electron or proton
• Charge is quantized in multiples of the elementary charge.
• 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.