Chapter 21 The Electric Field

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# Chapter 21 The Electric Field - PowerPoint PPT Presentation

Chapter 21 The Electric Field. Topics. Coulomb’s Law Electric Field Motion in an Electric Field Electric Field Lines Electric Dipole Moment. O. Vectors (in 2D): Recap. Magnitude of vector:. Coulomb’s Law. q 1. q 2. Force of charge q 1 on q 2. Net force on charge q 0

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Presentation Transcript

### Chapter 21The Electric Field

Topics
• Coulomb’s Law
• Electric Field
• Motion in an Electric Field
• Electric Field Lines
• Electric Dipole Moment

O

Vectors (in 2D): Recap

Magnitude of

vector:

Coulomb’s Law

q1

q2

Force of charge q1 on q2

Net force on charge q0

is sum of forces due to

charges q1 and q2

force of q1 on q0

q0

+

force of q2 on q0

+

q1

-

q2

Net force on charge q0

is sum of forces due to

charges q1 and q2

force of q1 on q0

q0

+

force of q2 on q0

Net force

The Electric Field

q0

The expression

+

suggests the concept of the electric field

of a point

charge where is the unit

vector from charge

q to charge q0.

The electric field at a given point

is just the sum of the electric fields

due to every point charge

-

+

+

-

-

+

+

Motion in an Electric Field

The total electric force on a charge q is

which, together with Newton’s 2nd Law,

can be used to

calculate the motion

of an electric charge,

of mass m

Motion in an Electric Field

Newton’s 2nd Law for an electric charge

can be written as

If E is constant, both

in direction and

magnitude, so

to is the acceleration

Note that the acceleration depends

on the charge to mass ratio.

An ink-jet printer

uses an electric

field to alter the

path of charged ink

droplets

Electric Field Lines

A field line

shows the

direction of

the electric

force on a

positive

point charge

The Electric Dipole

a

a

P

-q

+q

x

The electric field at point P far away is

The Electric Dipole

a

a

P

-q

+q

x

Dipoles are so common in Nature that it

turns out to be useful to define a new

quantity called the electric dipole moment

L is a vector of length 2a,

from –q to +q

Dipole in an Electric Field

The +ve charge wants to move right, while

the –ve charge wants to move left

+q

O

Potential Energy of Dipole

Electric Field

where we have chosen U0= 0

+q

O

Potential Energy of Dipole

Electric Field

For a dipole we must multiply by 2

to get the potential energy

of the two charges

H2O has a

permanent

dipole