Download LECTURE NOTE ELECTRIC POTENTIAL

LECTURE NOTE ELECTRIC POTENTIAL
ELECTRIC POTENTIAL – POTENTIAL ENERGY PER
UNIT CHARGE
ELECTRIC POTENTIAL IS CALLED VOLTAGE
ALESSANDOR VOLTA – INVENTED ELECTRIC
BATTERY
V = -W/q
Vab = Va – Vb = -Wba/q
1 VOLT = 1 JOULE/COULOMBE
A POSTIVIELY CHARGED ITEM - HIGHER POTENTIAL
THAN A NEGATIVELY CHARGED ITEM
NATURAL MOTION IS FROM HIGHT TO LOW
ΔPE = PEb – PEa = qVba
CLIFF TRANSPARENCY # 151
CHANGE IN POTENTIAL ENERGY OR THE WORK DONE DEPENDS ON
POTENTIAL DIFFERERENCE = HEIGHT
AND CHARGE = MASS
ELECTRIC POTENTIAL (V) IS SCALAR SO IT’S EASIER TO USE TO
DISCUSS THE EFFECTS OF CHARGE DISTRIBUTION
WORK DONE BY ELECTRIC FIELD TO MOVE A POSTIVE CHARGE FROM
B TO A
W = qVba
W=FxD
F = qE
E = ELECTRIC FIELD
D = DISTANCE PARALLEL TO TFIELD LINES
W = qED
qVba = qED
Vba = ED
Vba /D= E
E CAN BE MEASURED IN VOLTS/METER OR NEWTON/COULOMBS
ELECTRIC POTENTIAL CAN BE IAGRAMED BY DRAWING
EQUIPOTENTIAL LINES OR 3-D EQUIPOTENTIAL SURFACES
1. POTENTIAL DIFFERENCE BETWEEN ANY TOW POINTS EQUAL
ZERO
2. WORK TO MOVE THE CHARGE EQUALS ZERO
3. SURFACE MUST BE PERPENDICULAR TO THE FIELD
OTHERWISE IT WOULD REQUIRE WORK TO MOVE THE CHARGE
PARALLEL
ELECTRIC FIELD LINES AND EQUIPOTENITILA LINES ARE
PERPENDICULAR
TRANSPARENCY
COMPARE TO A TOPOGRAPHICAL MAP
CONDUCTOR ARE EQUIPONTEIAL SURFACES
17-4
ELECTRON VOLT – UNIT OF ENERGY
JOULE TOO LARGE TO DEAL WITH ENERGIES OF ELECTRONS
eV = electron volt
ENERGY ACQUIRED BY A PARTICLE CARRYING A CHARGE EQUAL TO
THAT ON THE ELECTRON (q= e)
AS A RESULT OF MOVING THROUGH A POTENTIAL DIFFERENCE OF 1
VOLT
q=e= 1.6 X 1-19 C
V = 1 VOLT
ΔPE = qV
1Ev = 1.6 X 1-19 J
1000Ev = 1KeV
Ev IS USEFUL TO STATE THE ENERGIES OF MOLECULES AND
ELEMENTARY PARTICLES BUT IT IS NOT A PROPER SI UNIT.
FPR CALCULATIONS Ev SHOULD BE CONTINUED TO JOULES
EX: 5000 Ev = 8.0 x 10-16 j/1.6 x 10-19 Ev
USING eV TO STATE ENERGY IS FINE BUT TO MAKE FURTHER
CALCULATIONS THE CONVERSION MUST BE MADE.
ELECTRIC POTENTIAL DUE TO POINT CHARGES
V=KQ/r
K-COULOMB’S CONSTANT 9.0 X 109 N.M2/C2
K = 1/4Πεο
POTENTIAL NEAR A POSITIVE CHARGE IS LARGE AND DECREASES
TOWARD ZERO AT GREAT DISTANCES.
NEAR A NEGATIVE
POTENTIAL IS NEGATIVE AND INCREASESES TOWARD ZERO AT GREAT
DISTANCES
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ELECTRIC DIPOLES
TWO EQUAL POINT CHARGES Q SEPAREATED BY l ARE ELECTRIC
DIPOLES
OCCUR OFTEN IN PHYSICS AND MOLECULAR BIOLOGY
EQUATIONS
V= KQ/R + K(-Q)/R + ΔR
V = KQ ΔR/R(R + ΔR
V= SUM OF THE POTENTIAL
R = DISTANCE FROM P TO POSITIVE
R + ΔR = DISTANCE FROM P TO NEGATIVE
V= KQl(COSθ)/R2
THE PRODUCT Ql IS CALLED THE DIPOLE MOMENT.
P= Ql
REWRITE EQUATION
V= Kp (COSθ)/R2
DIPOLE MOMENT HAS UNITS OF COULOMB-METERS (C.M) FOR
MOLECULES BECAUSE THEY SPEND MORE TIME IN THE VICINITY OF
ONE ATOM THAN ANOTHER IN THE MOLECULE.
THIS IS IMPORTANT TO MOLECULAR BIOLOGY.