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

The electron has one unit of negative charge.  The proton, made up of three charged quarks, has one net positive charge.

Charges come with a surrounding electric field that pulls on opposite charges and pushes like charges away.



Electrons are very lightweight and energetic and the electric force pulls them into orbits around the more massive protons.

Protons, meanwhile, clump together with other protons, and also with neutrons (three quarks whose combined charge is zero).  The protons and neutrons form a nucleus held together by the nuclear not the electromagnetic force.

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 math symbol for charge is q (from the 17th - 18th century term "Quantity of electricity").  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 charge over its useful life while 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.

Metals are a 3-D lattice of positive ions (atoms with an extra proton in the nucleus) sitting in a "sea" of mobile electrons.

Electrons can flow en masse through a metal much like water flows through a sieve.  Flowing charge is called current and materials that support current, like metals, are 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 1C in 1 second (t) :


I = q/t


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

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





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

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

Potential energy rises when oppositely charged particles are separated just as it does when an apple is lifted from the earth.  The potential energy arises from the energy spent in separating the charges or lifting the apple.

Voltage (math symbol V) is the potential stored by separated charges per unit charge.


V = E/q


The unit of measurement for voltage is the volt, abbreviated V (capital V because Volta was a person).  1V equals 1J of stored energy per 1C of electric 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 even though both ways add equally to your gravitational potential.

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).  1W is the transfer of 1J in 1 second.

Electrical power can be calculated from voltage and current:



(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, free electrons bump into positive ions, losing energy in the form of heat.

This type of impedance is called resistance.  Resistance is used to control currents and establish voltage levels in electronic circuits.  Components called resistors are engineered to provide the resistance.

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


R = V/I


The unit of resistance is the ohm, abbreviated Ω (capital Omega because Ohm was a person).  1Ω is the resistance between two points that are 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  (Voltage equals current times resistance).

Divide both sides of this result by R to find that:

I = V/R  (Current equals voltage divided by resistance).


Ohm's Law lets us calculate the power consumed by a resistance using either the voltage across the resistance or the amperage passing through it:


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

P = VI = (IR)I = I2R





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


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 make a resistor is to use materials that fall in between a conductor and an insulator, such as carbon.  That's because 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





Current, voltage, power and resistance are the four pillars of direct current (DC) circuits.  Direct current is the steady flow of electrons in one direction.

Alternating current (AC) faces two additional types of impedance, capacitance and inductance, whose values depend on the frequency of the alternating flow.


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