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Material Type: Notes; Professor: Zhou; Class: GEN PHYS-ELEC, MAGN & MOTION; Subject: Physics; University: Montana Tech of the University of Montana; Term: Unknown 1989;
Typology: Study notes
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Review Questions 1. How many types of charges in nature? 2. A glass rod rubbed with silk will attract orrepel with a rubber rubbed with fur? 3. After a glass rod is rubbed with silk, ithave a total charge of 1000 protons. Howmuch charge does the silk have?
New ConceptInsulators^ ^ Electrical insulators are materials in which allof the electrons are bound to atoms^ ^ These electrons can not move relatively freelythrough the material^ ^ Examples of good insulators include glass, rubberand wood^ ^ When a good insulator is charged in a smallregion, the charge is unable to move to otherregions of the material
New ConceptSemiconductors^ ^ The electrical properties of semiconductorsare somewhere between those of insulatorsand conductors^ ^ Examples of semiconductor materialsinclude silicon and germanium
Charging by Induction^ ^ Charging by inductionrequires no contact withthe object inducing thecharge^ ^ The electrons in theneutral sphere areredistributed as thenegatively chargedrod is approaching
Charging by Induction, 2 ^ Some electrons willleave the sphere throughthe ground wire ^ There will now be morepositive charges after theground wire is removed ^ The positive charge hasbeen^
induced
in the sphere
Charging by Induction, 5^ ^ The electrons remainingon the sphereredistribute themselves^ ^ There is still a netpositive charge on thesphere after rod isremoved
Charge Rearrangement inInsulators ^ In neutral molecules,the center of positivecharge coincides withthat of negativecharge ^ The charges within themolecules of thematerial arerearranged
Æa layer of charge facing thecharged object
Point Charge^ ^ The term
Proton, electron, and atomic mass ^ Proton:^ ^ proton
is a particle smaller than an atom (called subatomicparticle) with an electric charge of one positive fundamental unit−^19 (+1.602 × 10C) and a mass of
1.6726 × 10
−^27 kg
^ Electron:^ ^ electron
is a fundamental particle smaller than an atom (calledsubatomic particle) which carries a negative fundamental unit−^19 (-1.602 × 10C) and a mass of 9.11x
-31^ kg.
^ 1 C needs 6.24 x 10
18 electrons or protons ^ Typical charges can be in the
μ C range: 1
-6^ μ C = C
^ Unit of atomic mass:^ ^ 1u = 1.66x
-27^ kg
+4^. Find the
charge and mass of a quadruply ionized nitrogen atoms. Solution: 1) Charge = 4 electrons x 1.602x10-19C
=6.408x10-19C
kg
kg m kg mm u m
e e NN
26
31 (^2710) (^3248). 2
(^1011). 9 4 (^1066). 1 (^444007). 14 007. 14 −
−
×× − ×× =
×− =+
Coulomb’s Law ^ Charles Coulomb measured the electricforces between two small chargedspheres ^ He found that:^ ^ The force is inversely proportional tothe square of the separation
r
between the particles and directedalong the line joining them.
Æ^ F^^ ∝ e (^2) 1/r. The force is proportional to theproduct of the charges,
q and^ q^1
, on
the two particles.
Æ^ F^^ ∝^ qe
q. 12 F^^ ∝^ (q^ qe^12
(^2) )/r ^ The force is a conservative force(work done just depends onpositions not on path).
Torsion balance
Coulomb’s Law, Equation ^ Mathematically, ^ The SI unit of charge is the
^ k^ = 8.9875 x 10 e^
9.^2 Nm^ /C
permittivity of free space
.^2 m (^1) = 2 2 e^ e
q^ q F^ k^
r
