Electrostatics

• Ancient Greeks were familiar with electrostatic phenomenon.

• A piece of amber rubbed with fur could attract small objects over a distance.

Think

• What is another force that acts over a distance?

Electric Nature of Matter• Atoms are made of protons and electrons.

• Electrons (negative charge) are free to move, while protons (positive charge) are not as free to move.

• Matter usually has a balanced charge.

Charges

Like charges repel. Unlike charges attract.

Static Electricity: Definition

• An imbalance of charge.

• Usually it is the electrons that move.

Mechanisms of Charge Imbalance

• Contact/Peeling:– Rub balloon on hair.

• Conduction:– Touch a charged object

to a neutral object.

• Induction:– Bring a charged object

close to a neutral object.

positive negative neutral

Van de Graff Generator

• Like amber and fur, a rubber belt transfers charge from a wool roller to the dome.

• The charge builds up and is distributed on the surface of the sphere.

Electrostatic Forces

• We have seen that electric charges exert forces on other electric charges.

Like charges repel. Unlike charges attract.

Think-Partner-Share

• What factors might affect the magnitude of this electrostatic force?

Electrostatic Forces

• Coulombs Law

Like charges repel. Unlike charges attract.

F = Kq1q2

r2

Coulomb’s Law: force between two charged particles

• K = 9 x 109 Nm2/C2 (electrostatic constant)• q = value of charge in Coulombs

• A comb has a charge of about 1 microcoulomb (10-6 C)

• r = distance between charges.• Note the inverse square relationship.

F = Kq1q2

r2

Think

• What is another inverse square relationships we have seen?

Electric Field   

"An electric field is a region in which charged particles experience a force"

ELECTRIC FIELD

FORCEFORCE

Lines of force show the direction of the force felt by a positive charge

+q-q

-+

Field Lines

• Field Lines start and end on charges.

• The density of field lines is a measure of the strength of the field.

This is defined as the force per unit charge acting at a point in the field.            

ELECTRIC FIELD

STRENGTH E

ELECTRIC FIELD STRENGTH E

q

FE UNITS = N/C

Hence the force on a charge of q coulombs in the diagram above is given by :

+q

F = E q

IT IS A VECTOR

Motion of Charge Initially Moving Perpendicularly to an Electric Field e.g. an electron in an oscilloscope beam.

- - - - - - - - - -

+ + + + + + + + + + +

Ex

y

--

vH

VH is the initial horizontal velocity and REMAINS CONSTANT - Like a projectile !!

For the vertical motion u = 0, som

Fa

2

2

1tay

For the horizontal motion, x = VH t

222x

mv

qEy

H

Hence which is a PARABOLA

where m = mass of the particle and F = Eq

What is electricity?

• The interaction between charged particles of matter.

• We discussed “static” electricity or charge imbalance – how about if charges are moving?

Current Electricity

• Current = flow of charge, such as electrons through a wire in a circuit.