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Electric Potential
Chapter 17
• The conservation of energy very
useful way to view electrical
activity
• Also a great problem solving tool
17-1 Electric Potential Energy &
Potential Difference
• The electrostatic force, F=kQ1Q2/r2, between
any two charges is conservative
• Depends upon position, just like gravitational
potential
• Therefore, the work done to move a charge is
equal to the change in potential energy
• PE = -W
• The change in PE is when a point charge q
moves from some point a to another point b, as
the negative of the work done by the electric
force to move the charge from a to b
• The work is done by the electric field so
• W = Fd = qEd
• PEb-PEa=-qEd
• In this case the PE is negative and decreases
while the KE increases by an equal amount
• A +q has the greatest PE near the + plate.
Electric Potential & Potential
Difference
• Electric Potential (potential) is the
electric potential energy per unit charge
PEa
Va 
q
• Only the difference in potential is meaningful thus
• Difference in potential or potential difference
between two points, a and b
• When the electric force does +W on a charge the
KE increases and PE decreases
• The difference in PE, PEb-Pea, is equal to the
negative of the work, Wba
PEb  PEa
Wba
Vba  Vb  Va 

q
q
• The unit of potential difference is J/C and
given a special name of volt in honor of
Alessandro Volta who invented the electric
battery
• Potential difference is often referred to as
voltage
• There must be a reference position as 0 V,
just like a reference level of 0 GPE.
• Earth is usually 0 V or at in infinite distance
• PEba=q(Vb-Va)=qVba
• If a charge q moves through a
potential difference of Vab, it
potential energy changes by an
amount of qVba
• Energy is the ability to do work,
the electric potential difference is
also a measure of how much work
a given charge can do.
17-2 Relation between Electric
Potential and Electric Field
•
•
•
•
•
•
W=Fd=qEd
W=-qVba
qEd=-qVba
E=-Vba/d
Units, V/m or N/C
Minus sign tell us that E point in the
direction of decreasing potential
17-3 Equipotential Lines
• All points along the equipotential line will
have the same potential
• Similar to a topographic map showing equal
altitude lines.
• Drawn perpendicular to electric field lines
17-4 The Electron Volt, a Unit of
Energy
• Joule is very large when dealing with
energies on the molecular level so the unit
of electron volt (eV) is used
• 1 eV = 1.6 x 10-19 J
17-5 Electric Potential Due to Point
Charges
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•
•
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E=kQ/r2
V=Ed
Combine together then
V=kQ/r