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