Chapter 18 Summary
Charge behavior:
• Net charge is conserved.
• Electronsmovein solids,not protons.
• Like charges repel; oppositecharges attract.
• Neutral objectsare attractedto both positiveand negative charges because of polarization.
• Net charge mustbe in multiplesof ‘e’.
• Charge by contact;charge by induction.
Insulators& Conductors:
• Charges don’tflow on insulators. Charges do flow on conductors.
• Both polarize but in different ways.
• Understandbehavior when charged insulatorsand conductorscomeinto contactwith each other.
𝑞1 2 • Valid for point charges only.
𝐹𝑒𝑙 = 𝑘 𝑟 2 • Obeys superposition.
• Valid for point charges only.
𝑞 • Obeys superposition.
𝐸 𝑝𝑜𝑖𝑛𝑡 𝑐ℎ𝑔 = 𝑘 𝑟2 • Pointsaway from positivecharges and towardnegative charges.
• The dielectric strengthof air = 3e6 N/C.
• This is a general equation (always true)
𝐹 𝑒𝑙𝑞𝑜𝐸 • q os the objectcharge.
• F and E pointin the samedirectionfor positive qo, and oppositedirectionsfor negativeoq .
• Forcesand electric fields obey superposition.
E-field Lines:
• Run from positiveto negative charges (unless go to infinity).
• The directionof the E-field is tangentto the E-field line.
• The magnitudeof the E-field is proportionalto the densityof the E-field lines.
• The number of lines goinginto or out of an object is proportionalto the magnitudeof the charge.
• They cannotcross.
• For a uniform E-field, the E-field lines are parallel and equally spaced.
• Know how to draw E-field lines for isolatedcharges and for an electric dipole.
Conductorsin ElectrostaticEquilibrium:
• The E-field inside is zero.
• Excesscharges moveto the surface.
• E-fields just outsidea conductorare perpendicular to its surface.
• Charges are more concentratedat bends.
• E-fields just outsidea conductorare greater at bends.
• Electric potentialis the same everywhere in and on a conductorin staticequilibrium.
Uniformly Charged Sphere:
• The E-field inside is zero.
• The E-field outsideis the same as a pointcharge.
• Electric potentialis the same everywhere inside.
• The electric potent
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