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   What's Electricity?



Electric charge is a fundamental property of some elementary particles, like electrons and quarks, and can be either negative or positive in polarity.  Charge, as well as its motion, are linked to an electromagnetic force field wherein opposite charges attract and like charges repel.

The electron has one negative charge and the proton, which is made up of charged quarks, has one net positive charge.  Electrons, which are tiny, fast moving and abundant, settle into orbitals around heavy, proton laden nuclei.  A positive nucleus and its surrounding negative charge is called an atom.

Atoms, themselves, bond electromagnetically with other atoms to form even grander force fields.  Tennis balls bounce, buildings stand, and aspirin thins the blood, all thanks to electromagnetic forces.

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

The amount of charge that passes through an alkaline 'AA' battery over its useful life is about 5,000 C (5 kC).  The amount of charge that passes through an average bolt of lightning is just 15 C.




In the periodic table, the metals have electrons that are delocalized (not associated with a single atom or chemical bond).  The metallic structure consists of aligned positive ions (atoms with a surplus proton in the nucleus) amid a "sea" of electrons.

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

The standard unit of current is the ampere or amp which is abbreviated A (capital A because Ampère was a person).  The symbol for current is I from the French phrase "Intensité de courant".  One ampere is the flow of one coulomb of charge per second (i.e., per unit of time, symbol 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 that causes electric charge to attract and repel is called an electromotive force or emf  because, like all forces, it has the potential to change an object's motion.  It does this by transferring energy to it.

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

A 'AA' battery can deliver 9 kJ over its useful lifetime.  A bolt of lightning can deliver 1,000,000 kJ in just 30 microseconds!  Energy is clearly more significant than the number of charges (electrons) involved.  [see Section I]

Potential energy results from separating electric charge much as it does from lifting an apple.  Voltage is a measure of the energy between two points per charge.   It's symbol is V and its standard unit is the volt , abbreviated V (Volta was a person).

So, volts is the number of joules per coulomb.

V = E/Q




Power ( symbol P) is the rate of energy transfer (i.e., joules per second).

P = E/t

Notice that

E/t = E/Q x Q/t.

In other words,

P = VI.

So, power equals voltage times current.

The unit of measurement for power is the watt, abbreviated W (Watt was a person).  One watt is the power consumed by a 1A current flowing between two points separated by 1V.




Resistance is an opposition to the flow of current, analogous to friction.  Even a good conductor has some resistance and so a long thin wire has more resistance than a short fat one.  The same is true for a water pipe.

A resistor is an electronic component that's engineered to possess a certain amount of resistance.  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 between a conductor and an insulator—carbon, for example, has relatively few delocalized electrons to conduct charge.

Carbon composition (carbon-comp) resistors are composed of tiny carbon particles bound with clay.  Other resistors, called carbon-film, contain helical tracks of carbon film.

Electrical energy is dissipated (i.e., given up) in a resistor, converted mostly into thermal energy (heat).  Experiments show that the energy loss (voltage drop) across a particular resistor divided by the current through it is a constant.  This constant ( symbol R) is the resistance of that resistor.

R = V/I

This relationship is called Ohm's Law.  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.

With the help of Ohm's Law we can calculate the power dissipated by a resistor using either the voltage across it or the current through it:

P = VI = V x (V/R) = V2/R
P = VI = (IR) x I = I2 R


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