Unlike charges ATTRACT

Like charges REPEL

+ FORCEF

OR

CE

We define the direction of the electric field at a particular place as being the direction of the force it produces on A POSITIVELY CHARGED OBJECT

Example:Charge and FieldExample:Charge and Field

Electric ForceElectric Force

Electric field of two charges systemElectric field of two charges system

Drawing/mapping electric field lines

Rules for Drawing Electric Field PatternsThere are a variety of conventions and rules to drawing such patterns of electric field lines. The conventions are simply established in order that electric field line patterns communicate the greatest amount of information about the nature of the electric field surrounding a charged object. One common convention is to surround more charged objects by more lines. Objects with greater charge create stronger electric fields. By surrounding a highly charged object with more lines, one can communicate the strength of an electric field in the space surrounding a charged object by the line density. This convention is depicted in the diagram below.

+++++

-----

The lines are equally spacedShowing the electric force between The plates are constant

mg

h

+

+

--++

EXAMPLE

vp = 9.3 x 104 m/s

Example

Example

A tiny ball of mass 0.60 g carries charge of magnitude 8 C. It is suspended by a thread in a downward electric field of intensity 300 N/C. What is the tension in the thread if the charge on the ball is

(a) POSITIVE (b) NEGATIVE?

Tension, T

E Field

Example

TE Field

qEweight

TE Field

qEweight

+

-

Definition of Electric Field An electric field is a region of space where an electric charge experiences an electrostatic force. The strength of the field is the ratio of the force experienced by the charge to the magnitude of the charge. The direction of the electric field is the direction of the force that would be experienced by a small postitive charge placed at that point.

Electric field intensity (sometimes just called electric field) is the force per unit charge experienced by a point charge somewhere in space.

E = F/q

Therefore, F = Eq

Concept of Field

Region throughout which a force may be exerted; examples are the gravitational, electric, and magnetic fields that surround, respectively, masses, electric charges, and magnets. Fields are used to describe all cases where two bodies

separated in space exert a force on each other.

Electric Potential Energy

The amount of energy required to move a charge from point to another within a region of electric field.

EPE = Work done in moving the charge = (force)(displacement)

= (qE)d

Electric Potential and Potential Difference (Voltage)

The amount of EPE per charge is the definition of electric potential.

The difference in electric potential between any two point defines the Voltage.

field) Econstant a(within Voltage Edq

qEd

q

EPE

Conservation of Energy

In a closed system, the amount of EPE possessed by the charge in moving between any two points will be converted into other forms of energy, i.e. Kinetic energy.

qEd = qV = ½ m v2

UNIFORM ELECTRIC FIELDS

•The field is uniform and constant except near the edge of the plates

•Since Electric Field Strength (E) is constant F = qE, the force on any charge q between the plates is also constant.

If the force acting is constant, any free charge of mass m, between the parallel plates would experience a constant acceleration , a (Newton’s Law)a = F/m

We could use the equations of constant accelerated motion to solve problems

Deflection of ChargeDeflection of Charge

SOLUTION:

ExampleExample

An electron (q = 1.6 x 10-19 C, m = 9.1 x 10-31 kg) is projected out along the +x axis with an initial speed of 3 x 106 m/s. It goes 45 cm and stops due to uniform electric field in the region. Find the magnitude and direction of the field.

An electron is shot at 106 m/s between two parallel charged plates. If Electric field strength between the plates is 1 kN/C, where will the electron strike the upper plate? Assume vacuum conditions.