Ch 19 Electric Charges, Forces, And Fields

CH 19 Electric Charges, Forces, and Fields Subject

Electric Charge

Relevant Equations Magnitude of Electron's Charge: e e=1.60x10-19 C S.I.= Coulomb, C µC=10-6 C me=9.11 x 10-31 kg mp=1.673 x 10-27 kg mn=1.675 x 10-27kg

Conservation of Electric Charge

Polarization

Insulators/Condu ctors

Insulator s •Charges are not free to move. •Usually nonmetalli c substance s

Relationships

•Electric charge is quantized meaning all objects have a net charge that is an integral of e. •Total electric charge of the universe is constant. •No physical process can increase or decrease the total amount of charge in the universe. •Charge by separation either creates a positive (lose electron) or negative (gain) ion. •Charge separation occurs when objects are rubbed against each other or when they collide. •Attractive charge in one object induces a charge in the neutral object yielding attracting charges. •Can be positive or negative

Conductors

Semiconductor s

•Charges can move freely. •Usually metals. •Conductors need an insulating base to prevent conducted charge from flowing freely to the ground. •Photoconductive: Materials that conduct when exposed to light but insulate in the dark.

•Can be good insulators or conductors at certain temperatures.

•Magnitude of electrostatic force is proportional to the product of the charges and inversely proportional to the square of the distance between them.

Coulomb's Law Superposition

(Force between protons and electrons) F=kq1q2/r2 S.I.= N k=8.99 x 109N•m2/C2 (Force between point charge and sphere) F=kqQ/r2 Total force is equal to the vector sum of all charge because force is a vector quantity.

Coulomb's Law •F=kq1q2/r2 •Intrinsic quantity is charge. •Electric force can be both positive or negative. •Electric force is greater than gravitational force. •Electric force cancels for neutral force objects.

Newton's Law

•F=Gm1m2/r2 •Intrinsic quantity is mass. •Gravitationa l force is always attractive.

•When determining total force acting on charge, always calculate magnitude of each individual force acting on charge first. •Then assign directions. •Surface charge density is the amount of charge

CH 19 Electric Charges, Forces, and Fields per area. Q=σA A=4πR2 S.I σ=C/m2 Q=σ(4πR2) •Spherical charge distribution behaves the same as if call charge were concentrated at a center point. Electric Field of Point Electric Field Charge •E=F/q0 •Where q0=test charge •F=charge at given location S.I.= N/C •Applies to whether force is due to single charge or to a group of charges. •Positive charge experiences a force in the direction of E. •A negative charge experiences a force in the direction opposite of E.

•E=kq/r2 •Positive charge in a field points outward. •A negative point charge points inward. •Vector sum can also calculate the total electric field in point charges.

Electric Fields

Electric Field Lines Parallel Plate Capacitor

1.) Point in the direction of the electric field vector E out every point. 2.) Start at positive or infinity 3.) End at negative or infinity 4) More dense E has greater magnitude Number of lines entering /leaving is proportional to magnitude of charge. Separated by distance d Conduction Situations Excess Charge •Excess charge on a conductive, or negative surface, moves to the exterior surface of the conductor.

Shielding •When electric charges are in equilibrium, the electric field within a conductor is 0, E=0. •This works in one direction only.

Conductor Surfaces •Electric field lines make contact with conductive surfaces at right angles. •Electric fields charges are more densely packed at sharp points, hence they are more intense.

Induction •Charging by contact. •Grounding is used to describe the connection of the sphere with the ground with a conducting wire. •After grounding, positive charge is Induced Inductio Spheres n •Have •Contact opposite between signs the rod compare and the d to the sphere charged produces rod charge of the same signs. trapped.

Conduction

Electric Flux

Φ=EA Φ=EAcosΘ S.I.=N•m2/C

Perpendic ular Φ=EA

Parallel Φ=0

At Angle Θ Φ=EAcosΘ S.I.=N•m2/C

CH 19 Electric Charges, Forces, and Fields

Gauss's Law

Permittivity of Free Space ε0=1/4πk or 8.55 x 10-12 C2/N•m2 Φ=q/εo N•m2/C

Gaussian Surface (an imaginary spherical surface) Gaussian Surface Through Gaussian Encloses Gaussian Surface Spherical Surface with Shell Shell •Φ=Q/ε0 •E=kQ/ri2 •Spherical shell does not affect electric flux •E=0 •Φ=E(4πr32) because they •Φ=0 •E=kQ/r32 are not •Net Q/4πε0r32 contained charge=0 •Φ=E(2A) within a •Induced surface charge =-Q •E=σ/2ε0