APPLIED PHYSICS AND CHEMISTRY ELECTRICITY LECTURE 3(I THINK)

Electric Fields

Electric Field

Definition: Michael Faraday (1791-1867) Electric field exists around any charged object Area where a test charge would be given a force by

the charged object Test charge is a positive charge of small magnitude Magnitude of field is a measure of the force exerted

by the charged object Area of field indicated by field lines

Show the path that would be taken by the test charge Note: electric field intensity is a VECTOR!

September 18, 2007

Electric Field Electric field is said to exist

in the region of space around a charged object: the source charge.

Concept of test charge: Small and positive Does not affect charge

distribution Electric field intensity:

Existence of an electric field is a property of its source;

Presence of test charge is not necessary for the field to exist;

0q

FE

+ + +

++

+ +

+

+

Some Typical Electric Fields

Near a charged, hard rubber rod

Television picture tube Field that will create a

spark in air At electron’s orbit in

hydrogen atom

1 x 103

1 x 105

3 x 106

5 x 1011

Field Value (N/C)

The electric field will be directly proportional to the charge setting up the field and inversely proportional to the square of the distance between that charge and where you are measuring the strength of the electric field.

In other words,

Electric field = (Coulomb’s constant) (Charge on object making the field)

(distance away from the charge)2

E = kq d2E is in N/C K=9x109 Nm2

C 2 Q is in Coulombs Distance is in meters

Example of Field Lines

Electric Field Lines

Problem 1

A positive test charge of 4.0 x 10-5 C is placed in an electric field. The force acting on the test charge is 0.60 N at 10o. What is the electric field intensity at the location of the test charge?

USE THE STEPS!What we know:

Q = 4.0 x 10-5C F = 0.60 N at 10o

Equation: E = F/q

Problem continued

Substitute in values: E = 0.60 N 4.0 x 10-5C

Solve the math: E = 1.5 x 104 N/C at 10o

Check answer! Reasonable answer? Units???

Problem : What is the electric field 0.2 meters away from a +4 Coulomb charge?

E = (9x109 Nm2/C2)(4C)

(0.2 m)2

E=9x1011 N/C

The direction of the electric field would be away from the +4C charge since the direction is always the direction

a small positive test charge would move.

Equation: E = kq/d2