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Electric charge is a fundamental property of some particles of matter, such as electrons and quarks.  The polarity of electric charge can be either positive or negative.

The electron has one unit of negative charge and the proton, composed of three charged quarks, has one net unit of positive charge.

Charged matter and its movement are coupled to an electro-magnetic force field wherein opposite charges attract and like charges repel.



Electrons are lightweight and attracted into orbitals around clumps of positive protons and uncharged neutrons.  The central clump, called a nucleus, is held together by the nuclear force, not by electromagnetism.


A positive nucleus and its negative electron cloud is called an atom.  Atoms, themselves, bond electromagnetically (chemically) with other atoms, creating even larger structures.

Tennis balls bounce, buildings stand, and aspirin thins the blood, all thanks to electromagnetic forces.

The mathematical symbol for charge is Q and its unit of measurement is the coulomb, abbreviated C (capital C because Coulomb was a person).  1C is the combined charge of about 6¼ billion billion electrons.


AA Battery


An alkaline AA battery supplies about 5,000C of electric charge over its useful life whereas an average bolt of lightning supplies only around 15C of charge!






In the periodic table of the elements, the metals have electrons that are delocalized.  That is, they're not associated with a single atom or chemical bond.

The metallic structure consists of a lattice of positive ions (atoms with an extra proton in the nucleus) sitting in a "sea" of mobile electrons.

These electrons can flow en masse through a metal, much like water flows through a pipe.  Flowing charge is called electric current and materials that support current, like metal, are called conductors.

The unit of measurement for electric current is the ampere, or amp, abbreviated A (capital A because Ampère was a person).  The math symbol for current is I from the French phrase "Intensité de courant".

1A is the passage of one coulomb of charge in one second (t)


I = Q/t


Well-known metals include copper, tin, nickel, silver and gold.

Materials that don't allow electric charge to flow are called insulators.  Insulators include wood, rubber, ceramics, plastic and glass.





The force behind electrical attraction and repulsion is often called the electromotive force (emf) because, like all forces, it has the potential to change the motion of an object by transferring energy to it.

The unit of measurement for energy (math symbol E) is the joule, abbreviated capital J because Joule was a person.

Potential energy goes up when oppositely charged particles are separated, just as it does when an apple is lifted off the earth.  The potential energy is a result of the energy spent in separating the particles.

Voltage (math symbol V) represents the potential energy of separated electrical charges, per charge.


V = E/Q


The unit of measurement for voltage is the volt, abbreviated V (capital V because Volta was a person).  1V equals one joule of potential energy per coulomb of charge.





Energy exchange (i.e. work) can be done quickly or slowly but doing it quickly requires more power.  For example, more power is needed to run up a hill than to walk up, even though both ways add equally to your gravitational potential energy.

Power (math symbol P) is defined as the amount of energy exchanged per second.


P = E/t


The unit of measurement for power is the watt, abbreviated W (capital W because Watt was a person).  One watt is the transfer of one joule of energy in one second.

Electrical power can be calculated from the values of voltage and current.  Since


(E/t) = (E/Q) x (Q/t)


we see that

P = VI


So 1W is the power delivered by a 1A current flowing between two points separated by 1V.


Alkaline AA


An alkaline AA battery can transfer 9 kilojoules of energy over its useful life while an average bolt of lighting can transfer 1,000,000 kilojoules in 30 microseconds!






In the ordinary world, perfect electrical conductors don't exist.  Even in metals, mobile electrons bump into positive ions, losing energy in the form of heat.

This type of impedance to current is called resistance.  In electronic circuits, resistance is employed to control current and establish voltage levels.

Experiments show that the ratio of the voltage across a resistance to the current through it is a constant.  This number is defined to be value of the resistance (math symbol R).


R = V/I


Electronic components engineered to have a specific resistance are called resistors.  The standard unit of resistance is the ohm, abbreviated Ω (capital Omega because Ohm was a person).  1Ω is the resistance between two points separated by 1V and conducting a current of 1A.

The above equation is called Ohm's Law.  Multiply both sides of Ohm's Law by I to find that V = IR.  Then divide both sides of this result by R to find that I = V/R.

Ohm's Law lets us calculate the power dissipated by a resistor using either the value of the voltage across the resistor or the value of the current through it:


P = VI = V(V/R) = V2/R

P = VI = (IR)I = I2R





Like garden hoses, electrical conductors that are long and thin have more resistance than those that are short and fat.


Wirewound cement resistor


In fact, one way to make a resistor is to coil up a long, thin piece of wire.

Wirewound resistors can be precise and also handle large currents.



Another way to construct a resistor is to use materials that fall between a conductor and an insulator, such as carbon.  Carbon has relatively few delocalized electrons.


Carbon comp resistor



These vintage, carbon composition resistors are composed of tiny carbon particles bound with clay.



Many modern resistors are made from laser-cut, helical tracks of carbon or metal film.


Metal film resistor


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