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UNIT A: ELECTROSTATICSAP Physics C Review

Electric forces and charges

• The electric force is one of four fundamental forces.

• The electric force is the result of electric charges.

• There are two kinds of electric charges.

• Electric charge is conserved.

• Electric charge is quantized.– The charge on an

electron is -1.60 x 10-19 C.

Methods of Charging an Object

• Conductors and insulators can be charged by contact.

• Conductors can be charged by induction.

• A charge can induce a surface charge in insulators by polarization.

Charging by Induction

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Clarification: The conductor never touches the charged plate.

© 2009 Richard White, LearnAPphysics.com

Coulomb’s Law

• The electric force one charge exerts on another is given by Coulomb’s Law

• A negative force means attraction, positive means repulsion.

• kC = 8.99 x 109 Nm2/C2

• Coulomb’s constant is sometimes written

21electric rF ,221 ˆ

r

qqkC

04

1

Ck

6

Superposition

• Superposition- the force on a charge due to several other charges is the vector sum of the forces from the other charges.

©2008 by W.H. Freeman and Company

7© 2009 Richard White, LearnAPPhysics.com

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Electric Field

• The presence of charges causes electric fields. The electric field at a location is the force / Coulomb of charge felt by a positive test charge at that location.

• The electric field caused by a point charge is

• Electric fields are measure in Newtons/Coulomb (or Volts/meter).

• The principle of superposition applies to fields as well.

q

FE

2r

qkE C

itot EE

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• An electric dipole consists of two equal and opposite charges that are a small distance apart.

• Far away from the dipole, the E field is

iE ˆ43x

kqa

Electric Field of a Dipole

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Electric Field Lines

– Electric fields can be represented using electric field lines.

• Lines begin on positive charges (or infinity) and end at negative charges (or infinity).

• Lines enter or leave an isolated charge symmetrically.

• The magnitude of the field is shown by the number of lines in a given area.

• The number of lines entering or leaving a charge is proportional to the magnitude of the charge.

• At a large distance, every arrangement of charges approximates a point charge.

• Lines do not cross.

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12© 2009 Richard White, LearnAPphysics.com

13© 2009 Richard White, LearnAPphysics.com

©2008 by W.H. Freeman and Company