Group Members

Umar Jamil Faiz Majeed

International Islamic University Islamabad Pakistan

The Concept of a FieldA field is defined as a property of space in which a material object experiences a force.

.PAbove earth, we say there is a gravitational field at P.

Because a mass m experiences a downward force at that point.

No force, no field; No field, no force!

m

F

The direction of the field is determined by the force.

Field is Property of Space

E

Electric Field

++

++

+

+++

Q

.

r

The field E at a point exists whether there is a charge at that point or not. The direction of the field is away from the +Q charge.

E

Electric Field

++

++

+

+++

Q

.

r

++q --qF

F

Force on +q is with field direction.

Force on -q is against field direction.

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:

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

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

0qFE

+ + +

++

+ +

+

+

September 18, 2007

Electric Field

0qFE

Magnitude: E=F/q0 Direction: is that of the force that acts

on the positive test charge SI unit: N/C

Situation ValueInside a copper wire of household circuits 10-2 N/CNear a charged comb 103 N/CInside a TV picture tube 105 N/CNear the charged drum of a photocopier 105 N/CElectric breakdown across an air gap 3×106 N/CAt the electron’s orbit in a hydrogen atom 5×1011 N/COn the suface of a Uranium nucleus 3×1021 N/C

A Point Charge in an Electric Field

When a charged particle, of charge q, is in an electric field, E, set up by other stationary or slowly moving charges, an electrostatic force, F, acts on the charged particle as given by the above equation.

The Electric FieldDue to a Point Charge

Coulomb's law says

... which tells us the electric field due to a point charge q is

1 22

12

q qF =k ,12 r

This is your third starting equation.

q 2

qE =k , away from +

r

…or just… 2

qE=k

r

The equation for the electric field of a point charge then becomes:

2

q ˆE=k rr

We define as a unit vector from the source point to the field point:

r̂

+source point

field point

r̂

You may start with either equation for the electric field (this one or the one on the previous slide).

A system of two equal and opposite charges separated by a certain distance

Dipole moment - a measure of the strength of electric dipole. It is a vector quantity represented by .

P

Magnitude of dipole moment - product of the magnitude of either charge and the separation between them.

P = q (2a)

SI unit – C m

Direction of dipole moment – it points from negative towards positive charge

Electric dipole

Torque on a dipole

In an electric field E, the force acting on charges will be equal and opposite. The parallel forces form a couple. Moment of the couple constitute a torque

τ = p×E ;where, p is the dipole moment

E

qE

–qE

q

2a

B C

A

t = force x perpendicular distance = qE x AC = qE x 2a sinq = PE sinq