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Electrostatic Forces. Atoms: protons (p+) bound in nucleus electrons (e-) freer to move about e- and p+ have equal amounts but opposite charge When objects rub together, electrons may be exchanged Objects that have more e- than p+ are negative

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electrostatic forces
Electrostatic Forces
  • Atoms:
      • protons (p+) bound in nucleus
      • electrons (e-) freer to move about
  • e- and p+ have equal amounts but opposite charge
  • When objects rub together, electrons may be exchanged
      • Objects that have more e- than p+ are negative
      • Objects with fewer e- than p+ are positive
  • Charge can not be created or destroyed, only transferred.
  • Charge is “quantized” - there is a smallest amount (the amount on one e- or p+)
slide2

Electrostatic Forces

  • Unit of Charge (q): the Coulomb (C)
  • 1 Coulomb = 6.24 x 1018 e-
  • Charge on one qe- = 1.60 x 10-19 C
electrostatic forces1
Electrostatic Forces
  • Unit of Charge: the Coulomb (C)
  • 1 Coulomb = 6.24 x 1018 e-
  • Charge on one e- = 1.60 x 10-19 C

Example Problem: A penny has a charge of -1.00 x 10 -15 C. How many

More electrons than protons does it have?

Answer: #e- = q x e-/q

-1.00 x 10-15 C x 6.24 x 1018 e-/C = 6.24 x 10 3 excess e-

electrostatic forces2
Electrostatic Forces

Conductors and insulators

  • Conductors: e- free to move about
  • Conductors: mostly metals
  • Insulators: e- not free to move about
  • Insulators: generally non-metals
electrostatic forces3
Electrostatic Forces

Charging Objects

  • Three basic methods of charging objects
      • Friction
      • Conduction
      • Induction
electrostatic forces4
Electrostatic Forces

Charging Objects: Friction

  • Objects rubbed together can transfer electrons. Certain materials are more likely to give up e-, other objects tend to grab e-.
  • The object that loses e- becomes positively charged
  • The object that gains e- becomes negatively charged
electrostatic forces5
Electrostatic Forces

Charging Objects: Conduction

  • If a charged object touches another object, sometimes charge will transfer.
  • If the two objects are the same size and shape and they are conductors, they will share the excess charge equally.
electrostatic forces6

e-

+

+

+

+

+

+

Electrostatic Forces

Charging Objects: Induction

  • Charge can be “induced” to flow onto or off of an object by using an oppositely charged object and a ground.
  • A negative object can be used to induce electrons to flow from an object down to a ground - making it positive.
  • A positive object can be used to induce electrons to flow up from a ground onto an object - making it negative.
electrostatic forces7
Electrostatic Forces

Remember the general form for Gravitational force?

G = 6.67 x 10-11 Nm2/kg2

Well electrostatic force is another “force-at-a-distance”, and

Has a very similar form….

k = 8.99 x 109

Nm2/C2

electrostatic forces8
Electrostatic Forces

k = 8.99 x 109 Nm2/C2

Use this equation to find the magnitude of the mutual force on any two point charges. Don’t bother plugging in the signs of the charges.

If the point charges have opposite signs, the force is attraction. If they have the same sign the force is repulsion.

Each object experiences the same amount of force, even if one has more charge or mass – remember Newton’s third law!

electrostatic forces9
Electrostatic Forces

Lets compare the gravitational and electrostatic forces on a proton

and electron located 1.0 m apart….

me- = 9.11 x 10-31 kg qe- = -1.60 x 10-19 C

mp+ = 1.67 x 10-27 kg qp+ = +1.60 x 10-19 C

G = 6.67 x 10-11 Nm2/kg2 k = 8.99 x 109 Nm2/C2

Fg = 1.01 x 10-67 N

(attraction)

Fe = -2.30 x 10-28 N

(also attraction)

electrostatic forces10
Electrostatic Forces

Tougher example:

Given q1, q2, and q3 and the distances between them, find the net force on q1 due to the other two

charges.

3

F3on1

1

+

+

Q

F

F2on1

2

-