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

q

PEV aa

• 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

q

W

q

PEPEVVV baababba

• 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

• E=kQ/r2

• V=Ed

• Combine together then

• V=kQ/r