ELECTRIC CHARGE In ancient times, amber was noticed to attract light bits of matter Greek word for amber is “elektron” Later it was assumed other material transferred “amber stuff” Became “charge” This charge creates another, independent force – the electromagnetic force

Two types of charge given arbitrary names by Ben Franklin Positive (+) & negative (-) The math signs turn out to be useful – objects with equal amounts of positive & negative are electrically neutral Atoms are neutral, a small dense positive nucleus surrounded by a “cloud” of negative charge  composed of electrons

All electrons have same charge q e = -1.6 x C Unit of electric charge = coulomb (C) All charged objects are due to excess (negative) or absence (positive) of electrons Because of this, charges must be multiples of this fundamental charge  q = n q e total charge Electric charge is quantized

The carrier of positive charge = proton q p = -q e exactly equal Masses of each m e = 9.11 x kg m p = 1.67 x kg m p = 1860 m e, but still the same charge Also within nucleus is the neutron, no electric charge, m n ~ m p 1 C is a huge charge N (for 1 C) = q /q e = 6.25 x electrons

Objects can become charge due to friction This causes a transfer of e The total charge is constant Charge can never be created or destroyed – conservation of electric charge Total electric charge in universe is constant

Objects loosing e become positive charged – a positive ion Gaining e creates a negative ion Protons are locked in nucleus and are not mobile

Charging by Friction Triboelectric Charging Depends on material Some acquire e easily – rubber, plastic Some easily give up e (to become positive) – fur, glass

This type of charge separation can occur during collisions Crystals of ice colliding = lightning Electrostatic discharges also in the rings of Saturn

Insulators & Conductors Insulators - charges acquired stay in place, are not allowed to move around Usually nonmetallic Usually good thermal insulator Conductor – charges allowed to move more or less freely – a metal Deposited charge spreads out over surface due to mutual repulsion Causes a uniform distribution of charge

At a microscopic level: Conductors have 1 or more outermost electrons – conduction e These are easily detached and move freely throughout conductor “held” weakly to parent nucleus Insulators have very few free e Tightly bound to nucleus

Some materials have intermediate properties – semiconductors Conductivity can be “fine tuned” by changing components (& voltage) Used in electronics & computors Photoconductive materials only conduct electricity when exposed to light Selenium is used in photocopies & laser printers

Coulomb’s Law The forces exerted on charges was described by Charles Coulomb ~ 1780’s

Coulomb’s Law – Cont. Consider two nonmoving electric charges (electrostatics), q 1 & q 2 a distance r apart F α q 1 q 2 / r 2 A constant of proportionality, k gives:  Fe = k q 1 q 2 / r 2 Where k = 8.99 x 10 9 N m 2 / C 2 This determines the magnitude The direction is found by considering the sign of each charge

Comparing electric force to gravitational force Intrinsic qualities  charge & mass Force: Electric  repel & attract, Mass  always attract No charge = no force Why gravity rules & electricity plays no role in astronomy

The opposite in atomic systems Consider the hydrogen atom Radius r = 5.29 x m = distance between proton & electron The ration F e / F g = 2.26 x Why gravity plays no role within atom

1 C charge separated by 1 m exerts a force ~ 1 million tons The charges on your body if separated & placed on opposite sides of room The force needed to keep them apart ~ tons Thus everyday objects are never far from electrical neutrality

Polarization It is also possible for a charged object to attract a neutral object – polarization A charged object causes atoms to become elongated as negative charge tries to get closer/further away

Polarization – Cont. The net result is that charge develops on the surface and is attracted to the object