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