The document discusses electrostatics and electric fields. It states that all charged objects have an electric field around them, which is a region where a test charge would experience a force. The electric field points in the direction that a positive test charge would move. The document then goes on to describe different patterns of electric fields, how field strength is calculated, and the elementary charge of an electron. It also discusses the principle of conservation of charge in closed systems.

1. Electrostatics & electric fields
All charged objects have an electric field
about themselves.
The electric field is a space or region in which
a charge would experience a force, if placed
in the electric field.
The direction of the field is the direction in
which a +1 C (test charge) would move, if
placed in the field.
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2. Electrostatics & electric
fields
Just as the earth has a
gravitational field about it,
so charged objects have
electric fields about them.
In order to establish the
direction of the field about
a charged object, place a
small + test charge near
the object and see which
way it would move – if left
near the charged object.
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3. Field Patterns in different electric fields
Field about a
point charge
Field about
like point
charges
Field about
opposite
point charges
Field between
parallel plates
Field on
outside of
hollow object
These are photos
& don’t show
directions
Basic electric field
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4. The following represents ideal patterns
of electric fields.
Field about
+ charge
Field about
2 positive
charges
Field
between
parallel
plates
Field between
2 opposite
charges
Electric field
lines
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5. The distribution of charge on a
conductor depends on its shape.
More charge gathers at the pointed, sharp end of a
conductor.
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6. Electric field strength
Just as the gravitational field of the earth (or a
planet) is determined by:
The electric field strength is found by placing a
charge in an electric field and measuring the force
experienced by the charge:
E =
F
q
Def.: Electric field strength at a point in an electric field is
the force experienced per unit charge placed at that point.
The unit is N.C-1 and it is a vector quantity.
Now do some calculations involving electric field strength.
g =
Fg
m F = qE
Both E & F are
vector quantities.
Electric field near point charge
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7. Electric field strength near a charged object
F = k
Q1Q2
r2
Consider: and F = qE
Since these forces are equal, we can combine the
equations and now we have:
kQ
E =
r2
Where: E = electric field strength (in N.C-1)
Q = charge in C.
r = dist. in m. & k = 9 x 109 N.m2.C-2
Charged object
Find the electric field
strength at this point
Electric field strength
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8. Electric field strength near
2 charged objects
B
A
C
-2µC
-1µC
kQ
E =
r2Using find the strength
of the electric at C relative to the
charge at A.
Using the same formula, find the
strength of the field at C, relative
to the charge at B.
Now find the resultant field
strength R by adding the two
vectors CA & CB. This the
resultant field strength at point C.
R
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9. The elementary charge
Objects obtain charges by the addition or removal of
electrons.
Robert Millikan established that the smallest possible
charge was the charge on the electron and it has the
value of -1,6 x 10-19 C.
It is called the elementary charge.
1 Coulomb of charge contains:
1 C
1,6 x 10-19 C
= 6,25 X 1018 ē
This is a very large number
of electrons!
The proton carries the same elementary charge as the
electron & is +1,6 x 10-19 C.
Elementary charge
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10. Conservation of charge
Since objects can become charged by adding or
removing electrons, it follows that the negative
charge removed equals the positive charge
remaining on the object.
The total amount of charge thus remains the same.
Charge = +10-6 C
Charge removed = -10-6 C
The total charge thus remains
constant & we say:
charge is conserved.
Principle of conservation of charge:
In a closed system, the total amount of charge is
constant.
Conservation of charge
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11. Conservation of charge
On touching 2 charged metal spheres, ē will flow
from the more negative sphere to the less negative
sphere and
if the spheres are identical, the charge on each
sphere after touching will be the same.
To find the charge on each after touching:
Q =
Q1 + Q2
2
+2 x 10-6 C +3 X 10-6 C
Before touching
After touching:
Charge on each: +2,5 x 10-6 C
Electrons flow from A to B.
A B
Find the number of
electrons that moved to B.
Charges
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