Electric Charge and Electric Force

What is an Electric Charge?

• Protons have positive (+) electric charge

• Electrons have negative (-) electric charge.

What is an Electric Charge?

• Atoms become charged by gaining or losing electrons.

• Static electricity —the accumulation of excess electric charges on an object.

Calculating Charge• Electric charge was quantified by Robert

Millikan in 1909.

BatteryOn- - - -

+ + ++

Atomizer

- charge

• Millikan discovered that all the oil droplets had a charge of 1.60 x 10-19 Coulombs (C) or multiples of this charge.

Electron = -1.60 x 10-19 C

Proton = +1.60 x 10-19 C

Neutron = 0 C

Calculating Charge

Charged Objects• Electrically charged objects obey the

following rules:

- Law of conservation of charge- charges may be transferred, but they cannot be created or destroyed.

–Opposite charges attract, and like charges repel.

- +-

Charged objects• How are these charges able to move How are these charges able to move

towards or away from each other?towards or away from each other?

Electrical forceElectrical force: force that is exerted by + : force that is exerted by + and – charges on one another.and – charges on one another.

• Like gravity, electrical force is a Like gravity, electrical force is a “field “field force”force” – – force without physical contactforce without physical contact

• Objects placed close together will Objects placed close together will experience a greater force – experience a greater force – Coulomb’s Coulomb’s LawLaw

Charged objects

Coulomb’s Law

Felectric = Kc [(q1q2 )/r²]

Kc = Coulomb constant = 8.99 x 109 N • m²/C²

q = charge

r = distance between the two charges

Transferring Electric Charge• Charging by contact

– The process of transferring charge by touching or rubbing

• Example: static electricity from your feet rubbing the carpet

–Works best on non-conductive materials.

Transferring Electric Charge• Charging by induction

– The rearrangement of electrons on a neutral object caused by a nearby charged object. Occurs in conductors!

+-

+ -

+-

+-

----Charged Object

Conductor

Transferring Electric Charge• Charging by Polarization

– Same concept as induction, but in insulators!

– The rearrangement of electrons on a neutral object caused by a nearby charged object.

----Charged Object

----Charged Object

Insulator

= +

= -

= neutral

Transferring Electric Charge• Static discharge

–A transfer of charge through the air between two objects because of a buildup of static electricity.

Example: lightning

• Grounding—using a conductor to direct an electric charge into the ground.