Chapter 5

Electric fields

Properties of Electric charge

• Electric charge and electric forces play a major role in determining the behavior of the universe.

• The building blocks of matter, electrons and protons, have a property called electric charge. Electric charge is observed to have following characteristics; – An electric charge has a polarity; that is,

either positive or negative. Like charges repeal each other and opposite charges attract. (Benjamin Franklin model)

– An electric charge is conserved. It cannot be created or destroyed. We obtain charge by separating neutral object into a negative piece and positive piece.

– Charge is quantized (Robert Milikan model) and q is symbol for charge. It is always observed to occur as an integer of multiple of e (e=1.60 x 10-19 coulomb)

Insulators and Conductors• Electrical conductors are materials in which electric

charges move freely, whereas electrical insulators are materials in which electric charges cannot move freely.

• Examples of conductors : metals, plasmas, liquids containing ions (sulfuric acid, blood, salt Water). On the other hand vacuum, glass, distilled water paper and rubber are the kinds of insulators.

• Semi conductor are a third class of materials, and their electrical properties are somewhere between those of insulators and those of conductors. Silicon and germanium are well-known examples of semiconductors commonly used in the fabrication of a variety of electronic device

Coulomb’s Law

• Coulomb law is the law describing the relationship between electric forces and charge object.

• Stated by Charles Coulomb as conclusion of coulomb’s torsion balance experiment, this law confirm that the electric force between two stationary charged particles– is inversely proportional to the square of the

separation r between the particles and directed along the line joining them

– is proportional to the product of the charges q1 and q2 on the two particles;

– is attractive if the charges are of opposite sign and repulsive if the charges have the same sign

WhereF= Electric force (N)Ke= coulomb constantQ = charge (C)R = distance between two charge (m)

This constant is also written in the form

Example• The electron and proton of a hydrogen atom are

separated (on the average) by a distance of approximately 5.3 x10-11 m. Find the magnitudes of the electric force

• Three charges lie a long the x axis as show in figure

0,30 m 0,20 m

Q1=-8,0 μC Q2=+3,0 μC Q3=-4,0 μC

Determine the electric force of q3

Example

Find the electric force in q3

52 cm

60 cm

Q1 = -86 μC Q2 = +50 μC

Q3 = +65 μC

30o

30 cm

The Electric Field

• Electric field is one of way to describe electric effects. Developed by Michael Faraday, it is said to exist in the region of space around the a charge object.

• When another charged object enters this electric field, an electric force act on it

• The electric field is a vector field, and at every point in space it has a magnitude and direction. The magnitude of (=strength) of electric field at the location of the test charge to be the electric force per unit charge, or to be more specific the electric field E at a point in space is defined as the electric force Fe acting on a positive test charge qo placed at that point divided by the magnitude of the test charge:

E=Electric field (N/C)

+

r

E

+Q0

+Q0

+Q0+Q0

Description of electric field

Electric field lines• We can visualize electric field by means of

electric field lines or line of force.• These lines sprout out of positive charges

(positive charge as a sources of electric field), and they end on negative electric charge (negative as a sink)

• At a given point, the electric field direction is tangent to the electric field line passing through that point, and the magnitude of the electric field at point is proportional to the density of lines (the number per unit area measured in a plane perpendicular to the E line)

+++++

++

+

- --

---

--E

E

Electric field at a field point

• By comparing coulomb law equation and the electric field equation we can find the electric field of a point charge q at a field point a distance r from the charge

• E=Fe/qo E=kqoQ/r2/q = kQ/r2

If there are many charges,

02202

2012

01

1

0

ˆ||

ˆ||4

1 rr

rr

E QQ

1E

Q1

1r1̂r Q2

2r

2E

Example

52 cm

60 cm

Q1 = -50 mC

Q2 = +50 mC

Q3 = +65 mC

30o

Determine the electric field at B due to Q1 dan Q2

Dan Q3

B26 cm

30 cm

E: ………

Electric field of dipole

• An electric dipole is defined as a positive charge q and a negative charge q separated by some distance

Electric Field of a Continuous Charge Distribution

Electric Field of a Continuous Charge Distribution

ρ

Some Example

• The electric field Due To Charged Rod

Some Example

• The electric field of Uniform Ring of Charge

a. One side, b. two side

Motion of Charged particle in A Uniform Electric Field

Dipole in the an electric field

• When an electric dipole is placed in an electric field it tends to line up with its axis parallel to the field. If it is not parallel to the field, it experiences a torque (, associated with this torque is potential energy U, where; = p x E = pEsinU = p.E = pEcos