Charge (Q) is measured in coulombs (C) 1C = 6x1018 e Smallest possible amount of charge: 1 extra electron: q = -1.60 x 10-19 C 1 missing electron: q = +1.60 x 10-19 C … For any charge q: q = ne , where n = 1, 2, 3, etc… Charge is quantized Charge is conserved A positively charged object has lost electrons A negatively charged objects has gained electrons
Example A solid sphere has a net charge of –4 nC. How many electrons must be added or removed for the sphere to have a net charge of +5 nC?
Charging by Contact (rubbing…) - - - - - - - - - - + + + + glass (rub) fur plastic glass - - - - (rub) silk (rub) Charging by Contact (rubbing…)
Separating Charge A material on the bottom of the list will gain electrons from a material above it on the list…
Opposites Charges Attract Like Charges Repel
Charging by Conduction
Charging by Induction
- - - - - - Grounding Object is discharged or “grounded” + The earth is a huge reservoir of positive and negative charge + - © Laura Fellman
Coulomb’s constant (k) Coulomb’s law r Charge (Q) Coulombs (C) 1 C = 6.2421 x 1018 e e = 1.602 x 10-19 C Force (N) Distance (m) Coulomb’s constant (k) k = 8.988 x 109 Nm2/C2
0= permittivity of free space = 8.85 x 10-12 C2/Nm2 Notes on Coulomb’s Law 1) It has the same form as the Law of Gravitation: Inverse-Square Force 2) But… (can you spot the most basic difference between these two laws?) 3) The electrostatic constant (k) in this law is derived from a more fundamental constant: 0= permittivity of free space = 8.85 x 10-12 C2/Nm2 Permittivity is the measure of resistance that is encountered when forming an electric field in a medium. 4) Coulomb’s Law obeys the principle of superposition © Laura Fellman
Tricky!
Q 2Q 3Q 4Q L At each corner of a square of side L there are four point charges. Determine the force on the charge 2Q.