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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
topics
Topics
  • Coulomb’s Law
  • Electric Field
  • Motion in an Electric Field
  • Electric Field Lines
  • Electric Dipole Moment
vectors in 2d recap

O

Vectors (in 2D): Recap

Magnitude of

vector:

coulomb s law
Coulomb’s Law

q1

q2

Force of charge q1 on q2

slide5

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

slide6

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
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.

slide9

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
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 field1
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.

slide12

An ink-jet printer

uses an electric

field to alter the

path of charged ink

droplets

electric field lines
Electric Field Lines

A field line

shows the

direction of

the electric

force on a

positive

point charge

the electric dipole
The Electric Dipole

a

a

P

-q

+q

x

The electric field at point P far away is

the electric dipole1
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
Dipole in an Electric Field

The +ve charge wants to move right, while

the –ve charge wants to move left

potential energy of dipole2

+q

O

Potential Energy of Dipole

Electric Field

where we have chosen U0= 0

potential energy of dipole3

+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

slide23

H2O has a

permanent

dipole

moment of about 40 e.pm