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   What is Electricity?

CHARGE IS FUNDAMENTAL

 

Electric charge is a fundamental property of some particles of matter, for example, electrons and quarks.  Charge can have a polarity of either negative or positive.

The electron has one negative elementary charge (-e).  A proton, made up of three charged quarks, has one positive elementary charge (+e).

The mathematical symbol for charge is q, from the 18th century phrase "quantity of electricity".  The unit of charge is the coulomb, abbreviated C (capital C because Coulomb was a person).

Around 1910, Robert Millikan became the first person to measure the electron's charge, a fundamental physical constant.  His value was extremely close to today's value:

e = 1.602 x 10-19C

(Inversely, 1C = about 6╝ billion billion electrons.) 

 

AA Battery





An alkaline AA battery supplies about 5,000C during its useful life.  An average bolt of lightning delivers just 15C.

 

 

 

The Atom

 

An electric charge is surrounded by a force field that attracts oppositely charged particles and repels similar ones.

Electrons, which are lightweight and energetic, are attracted into the space around protons, which are heavier.

Meanwhile, protons clump together with other protons, and also with neutrons (three quarks with a combined charge of zero), forming a nucleus that's held together by the strong nuclear force, not the electric force.

A nucleus and its associated electrons is called an atom.  The diameter of a nucleus is between 1.6 and 15 femtometers, abbreviated fm.

1 fm (often called a fermi) = 1 quadrillionth (10-15) of a meter

 

Atom
An atom is a nucleus surrounded by a "cloud" of electrons

 

The wavelength of an electron is longer than the nuclear diameter.  Hence, an electron can't exist closer to a nucleus than a couple of electron wavelengths—the electric force isn't powerful enough to squash an electron into a proton to make a neutron.

Repelling one another, the electrons crowd around the atomic nucleus as close as they can get, forming what's called a cloud.

Hydrogen, the lightest atom, has one proton and one electron but large atoms can have over a hundred of each.  Each atomic size is one element in the periodic table of the chemical elements.

Chemical elements can bond together, sharing electrons to form compounds, molecular elements, and fancy materials.

Tennis balls bounce, buildings stand, and aspirin thins the blood, all thanks to electric charge.

 

 

CURRENT — CHARGE IN MOTION

 

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

The metallic structure is a lattice of positive ions (atoms lacking one electron) sitting in a "sea" of mobile electrons.

The mobile electrons can flow en masse through the lattice much like water flows through a sieve.  The flowing charge is called an electric current.  Materials that support current (like metals) are called conductors.

The unit of 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 "IntensitÚ de courant".

1A of current (I) is the passage of 1C of charge (q) in 1 second of time (t) :

 


I = q/t [1]

 

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.

 

 

VOLTAGE — SEPARATED CHARGE

 

The electric force is sometimes called the electromotive force or emf because, like all forces, it has the potential to change an object's motion by transferring energy to it.

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

Potential energy rises when opposite charges are separated, just as it does when an apple is lifted from the earth.  The energy gain comes from that spent separating the charge or lifting the mass.

Voltage (symbol V) is simply defined as the energy stored by separated charge, per unit of charge (q):

 


V = E/q [2]

 

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

 

 

POWER

 

Energy exchange (that is, work) can be done quickly or slowly but doing it quickly takes 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.

So power (symbol P) is defined as the energy transferred per second.  In other words, it's the rate of energy exchange:

 


P = E/t [3]

 

The unit of measurement for power is the watt, abbreviated W (capital W because Watt was a person).  1W is the power needed to transfer 1 joule of energy in 1 second.

Equation 3 says that E = Pt.  Your energy bill, in fact, is based on kilowatt hours (power x time).

1 kilowatt hour (kWh) is 1000 watts of power for 1 hour, which translates to 3.6 megajoules of energy:

1 kWh = 1000 W x 3600 s = 3,600,000 J = 3.6 MJ of energy

 

Alkaline AA

 



An alkaline AA battery delivers 9 kilojoules (kJ) of energy over its useful life while an average bolt of lighting delivers 1,000,000 kJ in just 30 microseconds!

 

 

 

POWER = VOLTS x AMPS

 

Now take a look at the next algebraic identity, where the two q's on the right side of the equation cancel out:

 


Algebraic identity [4]

 

Since we've already defined the three terms in this equation as P, V, and I (see Eq. 3, 2, and 1), we know that the power of a given voltage and current is:

 


P = VI [5]

 

1 watt is the power of a 1 amp current flowing between two points having a potential difference of 1 volt.

 

 

RESISTANCE

 

In the ordinary world, there are no perfect electrical conductors.  Even in metals, electrons collide with positive ions, losing energy in the form of heat.  This type of impedance to electric current is called resistance.

Resistance is used productively in electronic circuits to control currents and establish voltage levels.  Components called resistors are engineered to provide this resistive impedance.

Experiments show that the ratio of the voltage across a given resistance to the current through it is a constant.  If V is tripled, for example, I will also triple.

Resistance (symbol R) is defined as this constant ratio:

 


R = V/I [6]

 

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.

 

 

OHM'S LAW

 

Equation 6 is called Ohm's Law.  Multiply both sides of Ohm's Law by I to see that the voltage across a resistance equals the current times the resistance:

 


V = IR [7]

 

Divide both sides of Eq. 7 by R to see that the current through a resistance equals the applied voltage divided by the resistance:

 


I = V/R [8]

 


 

In equation 5, we can replace V with IR to find the power that a resistor must dissipate as heat when a current flows through it:



P = (I^2) x R
[9]

 

Or we can replace I with V/R to find the power a resistor must dissipate when a voltage is placed across it:



P = (V^2)/R
[10]

 

A resistor that can't dissipate enough power will overheat and eventually burn out.

 

 

RESISTOR CONSTRUCTION

 

Like garden hoses, conductors that are long and thin offer more resistance to current than do ones 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 make a resistor is to use materials that fall in between a conductor and an insulator, like 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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