Physics 121: Electricity

1
Physics 121 Electricity Magnetism Lecture 2

• Carsten Denker
• NJIT Physics Department
• Center for SolarTerrestrial Research

2
Introduction

• Charges and Forces
• Electric Field
• Electric Field Lines
• Electric Field Due to a Point Charge
• Electric Field Due to an Electric Dipole
• Electric Field Due to a Line Charge
• Point Charge in an Electric Field
• Dipole in an Electric Field

3
Electric Field

• Scalar fields temperature, pressure,
• Vector fields electric field, flow 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 10?2 N/C

Electric field
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Electric Field Lines
Electric Dipole
Electric field lines extend away from positive
charge (where they originate) and toward negative
charge (where they terminate).
5
Electric Field Due to a Point Charge
Point charge
Superposition principle
6
Electric Field Due to an Electric Dipole and a
Line Charge

• Point charge q C
• Line charge density ? C m?1
• Surface charge density ? C m?2
• Volume charge density ? C m?3

7
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.
Inkjet printer
Millikan experiment
8
Dipole in an Electric Field
Torque
Potential energy