Ch. 32 Electrostatics

Electricity at rest

Electric Forces

• Arise from subatomic particles– Positive nucleus surrounded by negative electrons

Charge: physical property that causes particles to experience a force

2 TYPES: PositiveNegative

Conservation of Charge

• Since matter is neither created nor destroyed, subatomic particles are neither created not destroyed.

• Subatomic particles may be transferred from one object to another, but overall, charge is conserved!!!

Standard unit of Charge

• Coulomb (C )• The charge of an electron is 1.6 x 10-19 C

A pair of identical coins are electrically charged. One with a charge of + 15 C and the

other a charge of – 8 C. When they are brought together, what is the charge of each

after contact? • Net Charge = + 15 C – 8 C = 7 C

• The coins are identical, therefore they will share the charge equally. 3.5 C charge for each coin!

Coulomb’s Law• The force between the charges is directly

proportional to the product of their charges and inversely proportional to the distance between

F = k q1q2 k= constant = 9 x109 N m2

d2 C2

q1 = charge 1q2 = charge 2

d = the distance between q1 &

q2

F = k q1q2

d2

q1 q2

dk = 9.0 x 109 N M2/C2

What other equation does this remind you of????

Practice ProblemF = k q1q2

d2

What is the electrostatic force between a pair of electrons 1 m apart?

k = 9.0 x 109 NM2/C2

The charge of an electron is 1.6 x 10-19 C

Practice ProblemF = k q1q2

d2

What is the electrostatic force between a pair of electrons 1 m apart?

k = 9.0 x 109 NM2/C2

The charge of an electron is 1.6 x 10-19 C

F = (9.0 x 109 N M2/C2 ) (1.6 x 10-19 C) (1.6 x 10-19 C) (1 m)2

F = 2.3 x 10-28 N

What is the electrostatic force between a 5 x 10-6 C charge and a 3 x 10-8 C charge

that are 0.25 m apart?

What is the electrostatic force between a 5 x 10-6 C charge and a 3 x 10-8 C charge

that are 0.25 m apart?F = k q1q2

d2

F =

What is the electrostatic force between a 5 x 10-6 C charge and a 3 x 10-8 C charge

that are 0.25 m apart?F = k q1q2

d2

F = (9.0 x 109 N m2/C2 ) (5 x 10-6 C )(3 x 10-8 C )(0.25 m)2

= 0.0216

Conductors & Insulators

Charging

• 3 Methods that result in charging: – Friction – Contact– Induction

Charging

• Friction: When electrons are transferred by friction from one object to another

• Contact• Induction

Charging

• Friction• Contact: when electrons are transferred from

one object to another by direct contact• Induction

Charging

• Friction• Contact• Induction: when electrons are caused to move by the presence of a nearby charge.

Charge Polarization

• Occurs in insulators• Rearrangement of the positions of charges within the atoms and molecules

The End

• In Class: Practice Problem Solving with coulomb’s Law