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Electric Charge and Fields. AP Physics B Chapter 16 Notes. Static Electricity. Charge is a property of subatomic particles Charges can be positive or negative Opposites attract likes repel. Static Electricity. Charge can be transferred by rubbing two objects together

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Electric charge and fields

Electric Charge and Fields

AP Physics B Chapter 16 Notes

Static electricity
Static Electricity

  • Charge is a property of subatomic particles

  • Charges can be positive or negative

  • Opposites attract likes repel

Static electricity1
Static Electricity

  • Charge can be transferred by rubbing two objects together

  • Electrons in the outer shell of one object “jump” to the other object

  • Both objects become charged (oppositely)

Static electricity2
Static Electricity

  • Charge is conserved and is not created nor destroyed

  • Even in interaction it stays the same

    • Initially attracted

    • Touch

    • Repelled, but total charge is the same

Static electricity3
Static Electricity

  • Free electrons are allowed to move—from one object to another or within an objects

  • Some molecules are polar, meaning charge is not evenly distributed

Static electricity4
Static Electricity

  • This leads to different types of materials based on the ability of electrons to flow:

    • Conductors

    • Insulators

    • Semiconductors


  • There are two ways to charge something (in addition to friction)

    • Conduction

      Requires contact


  • Induction

    No contact is required—you bring a charge object near a conductor and polarize it (charge separation) and then ground it.


  • An electroscope is a device that can detect a charge

  • Free electrons can flow down the metal knob to the moveable leaves


  • It can be charge by induction or conduction

  • Only charging by conduction give a net charge to the device

  • It can be discharged by touching the metal knob


  • Only a charged electroscope can be used to determine the type of charge (positive or negative)

Electric force coulomb s law
Electric Force--Coulomb’s Law

  • Recall Cavendish’s experiment to find G and experimentally confirm Newton’s Law of Gravity

    FG∝ mm’

    FG∝ 1/r2

    FG =(Gmm’)/r2

Electric force coulomb s law1
Electric Force--Coulomb’s Law

  • Coulomb conducted a similar experiment with charged objects

    FE∝ qq’

    FE∝ 1/r2

    FE =(kqq’)/r2

    Where k=8.988 x 109Nm2/C2

    (constant of proportionality)

Electric force coulomb s law2
Electric Force--Coulomb’s Law

  • The laws are similar except the direction of force and size of proportionality constant

  • One C is a large charge--the charge of one electron e = 1.602 x 10-19C

Electric forces and newton s laws
Electric Forces and Newton’s Laws

Example: An electron is released above the surface of the Earth. A second electron

directly below it exerts an electrostatic force on the first electron just great enough to cancel out the gravitational force on it. How far below the first electron is the second?

Electric Forces and Fields obey Newton’s Laws.

FNet= F E– F G= 0

F E= F G




5.1 m

r = ?


Electric forces and vectors
Electric Forces and Vectors

  • Electric (or electrostatic) forces are all vectors

Consider three point charges, q1 = 6.00 x10-9 C (located at the origin), q2 = -2.00x10-9 C and q3= 5.00x10-9 C, and, located at the corners of a RIGHT triangle. q2 is located at y= 3 m while q3 is located 4m to the right of q2. Find the resultant force on q3.


Which way does q2 push q3?

Which way does q1 push q3?






Fon 3 due to 1





Fon 3 due to 2

q = 37


q= tan-1(3/4)

Electric forces and vectors1
Electric Forces and Vectors





Fon 3 due to 1





Fon 3 due to 2

q = 37

q= tan-1(3/4)



5.6 x10-9 N

7.34x10-9 N

1.1x10-8 N

64.3 degrees above the +x

Electric fields
Electric Fields

  • FE and FG both act over a distance--permeate through space

  • Think of as a field force, like gravity

  • A second charge at P will interact with the electric field due to Q

Electric fields1
Electric Fields

  • How to determine Electric Field around Q?

  • Convention is to use small positive test charge

  • Place it at various locations around Q (field charge) and measure F

  • The electric field E is

    E = F/q [N/C]

    E is a vector quantity





Electric fields2
Electric Fields

  • E= F/q= (kqQ/r2)/q so

    E= (kQ)/r2

  • Field direction depends on the sign of the field charge—always use positive test charge

  • Use superposition for multiple fields

Electric vs gravity fields

Electric Fields

Gravity Fields

Electric Vs. Gravity Fields

Electric fields examples
Electric Fields--Examples

P. 36 Pg. 466 Two point charges are separated by 12 cm. The electric field at P is zero. How far from Q1 is P?

Electric field lines
Electric Field Lines

  • Electric fields are represented by field lines using a positive test charge

  • Positive field charge, lines radiate outward

  • For a negative field charge lines radiate inward

Electric field lines1
Electric Field Lines

  • For multiple charges you can do sample calculations of E= (kQ)/r2 and add E vectors

  • E vector is tangent to field line

Electric Dipole

Electric field lines more examples
Electric Field Lines—More Examples

  • Two same and equal charges

  • Unequal charges

  • Parallel plates

    (E is constant)

Electric fields examples1
Electric Fields—Examples

P. 41 Pg. 467 An electron is accelerated in an E field (1.45 x 104 N/C2) between two plates 1.1 cm apart. The electron is accelerated from rest and passes through a hole in the positive plate. How fast is it going at the hole?

Electric fields and conductors
Electric Fields and Conductors

E field ⊥ to surface

  • Imagine a positive Q inside a conductor—it will polarize the surface of the conductor

  • The inner side will have charge –Q and the outer +Q

  • The E inside the conductor is zero (or else free e will move)

Electric field of a conductor
Electric Field of a Conductor

Another situation

There must be a negative charge on this side OR this side was induced positive due to the other side being negative.

There must be a positive charge on this side

Electric flux information
Electric Flux--Information

  • Electric flux is a measure of the electric field passing through an area

    Units are Nm2/C