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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.