The Physics
Hypertextbook
Opus in profectus

# 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.