Electrostatic Force

Coulomb’s Law

ChargesTwo charges of the same type repel one another

+ +

The two charges will experience a FORCE pushing them apartor pulling them together

Two charges of the opposite type attract one another

+-

How much force?The amount of force that two charged objects experience

depends on three factors

q1 q2r

1. The charge on the 1st object (q1)

2. The charge on the 2nd object (q2)

3. The distance between them (r)

We use Coulomb’s Law to calculate this force

221

r

qkqFe

The constant for Coulomb’s Law is “k”“k” is the ELECTROSTATIC CONSTANT

8.99 x 109 N∙m2/C2

Coulomb vs. Newton

221

r

qkqFe

Look carefully at the Coulomb’s Law equation

221

r

mGmFg

This equation looks very much like Newton’s Law of Universal Gravitation!

Both have an INVERSE SQUARE relationshipbetween FORCE and DISTANCE

F

r

Gravitational force concerns MASSCoulomb force is about CHARGE

Which is stronger?

• Consider two electrons 1μm apart

– What is the gravitational force between them?

• Depends on mass

– What is the electrostatic force between them?

• Depends on charge 26

1919229

)101(

)1060.1)(1060.1)(/1099.8(

mx

CxCxCmNx

26

31312211

)101(

)1011.9)(1011.9)(/1067.6(

mx

kgxkgxkgmNx

221

r

mGmFg

5.54 x 10-59 N

221

r

qkqFe

2.30 x 10-16 N

Electrostatic force is MUCH stronger thangravitational force for small, chargedobjects like electrons and protons!!!

Example #1

221

r

qkqFe 2

229

)2(

)2)(2)(/1099.8(

m

CCCmNxFe

What is the electrostatic force between these two objects?

r = 2.0 m

q1 = -2C q2 = +2C

NxFe91099.8

A negative answer shows that the force is ATTRACTIVE

Example #2

221

r

qkqFe 29

66229

)105(

)102)(105)(/1099.8(

mx

CxCxCmNxFe

What is the electrostatic force between these two objects?

r = 5 nm

q1 = -5μC q2 = -2μC

NxFe151060.3

A positive answer shows that the force is REPULSIVE