Physics 121: Electricity & Magnetism – Lecture 2
Carsten Denker
NJIT Physics DepartmentCenter for Solar–Terrestrial
Research
January 24, 2007 Center for Solar-Terrestrial Research
Introduction
Charges and ForcesElectric FieldElectric Field LinesElectric Field Due to a Point ChargeElectric Field Due to an Electric DipoleElectric Field Due to a Line ChargePoint Charge in an Electric Field Dipole in an Electric Field
January 24, 2007 Center for Solar-Terrestrial Research
Electric Field
Uranium nucleus: 3 1021 N/C
Electric breakdown in air: 3 106 N/C
Photocopier drum: 105 N/C
Charged comb: 103 N/C Copper wire: 102 N/C
Scalar fields: temperature, pressure, …
Vector fields: electric field, flow field, …
Electric field0
FE
q
January 24, 2007 Center for Solar-Terrestrial Research
Electric Field Lines
Electric field lines extend away from positive charge (where they originate) and toward negative charge (where they terminate).
Electric Dipole
January 24, 2007 Center for Solar-Terrestrial Research
Electric Field Due to a Point Charge
Point charge
Superposition principle
02 2
0 0 0
1 1
4 4
q q qFF E
r q r
0 01 02 0
01 02 0
0 0 0
1 2
n
n
n
F F F F
F F FE
q q q
E E E
January 24, 2007 Center for Solar-Terrestrial Research
Electric Field Due to an Electric Dipole and a Line Charge
Point charge q [C] Line charge
density [C m1] Surface charge
density [C m2] Volume charge
density [C m3]
30
1 and
2
pE p qd
z
3/ 22 204
qzE
z R
January 24, 2007 Center for Solar-Terrestrial Research
Point Charge in an Electrical Field
The electrostatic force acting on a charged particle located in an external electric field has the direction of if the charge q of the particle is positive and has the opposite direction if q is negative. Ink–jet printer
Millikan experiment
F
E
E
F qE
with 1, 2, 3,q ne n
January 24, 2007 Center for Solar-Terrestrial Research
Dipole in an Electric Field
Torque
Potential energy
p E
U p E