Download Electrostatic Fields and Coulombs Law File

Electric Fields and
Electrostatics
Electric fields are always directed away from positive by convention
A field around a point charge
diverges in a regular fashion
A uniform field exists between
two charged plates where the
field lines are parallel
Field Definition And
Coulomb’s Law
r
Q1
Q
Q2
For two point charges in space the force between them is
proportional to the product of each charge
F  Q1 Q2
As the distance between them increases the force reduces rapidly.
It is found that the Force is inversely proportional to the square of
the distance
1
F 2
r
r
Q1
Q
Q2
Putting these two statements
together we get
Q1 Q2
F 2
r
Introducing a constant of
proportionality
Q1 Q2
F k 2
r
Coulomb’s Law
• Gives us the rule for
dealing with two point
charges.
(in practice for two charges
whose separation is
much greater than the
radius of the charges.)
1 Q1Q2
F
40 r 2
r
Charge
Q1
ε0 is a constant known as the permitivity of free space in Fm-1. It is
defined for a vacuum and has a value of 8.854 x 10 -12 Fm-1
Charge
Q2
A
+++
B
+++
Two metal spheres are given a charge of 3 x 10-3C and 4 x 10-4C
each. Their distance of separation is 0.5m.
What is the electrostatic force acting on A?
What is the electrostatic force acting on B?
The field strength of an electric field
is defined by
F
E
Q
(Unit NC1)
That is the force exerted by the field on unit charge placed at that point.
+1C
F
Calculating electric field strength
A positively charged
polystyrene bead carrying
6 x 10-3C of charge
+++
+
is brought into proximity with
a charged metal sphere.
The ball is pushed away
with a force of
0.02 Newtons.
Calculate the electric field
strength at this point.
The diagram shows the path of an electron between two
charged metal plates. Label the positve and negative plates.
The electron experiences a constant force of 4.8 x 10 -14 N.
What is the strength of the electric field between the plates?
Add lines to the diagram to show why you would expect the
field strength between the plates to be constant.
Together with these relationships:
W
V
Q
Q
Ek 2
r
The definition of the volt
The electric field strength due to a point charge
k
V
E
d
1
40
The electric field strength in a uniform field
So the unit of E is Vm-1 as well as NC-1
Forms the basis of any solution to the electric fields questions you will be
asked
1.State Coulomb’s law for the electric force between two charged particles in free
space.
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(2)
What are the base units of e0 (the permittivity of free space)?
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(2)
(Total 4 marks)
Calculate the magnitude of the electric field strength at the surface of a
nucleus U (Z=92 M=238) . Assume that the radius of this nucleus is 7.4 × 10–15 m.
238
....................................................................................................................................
92
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Magnitude of electric field strength =.........................................
State the direction of this electric field.
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(4)
State one similarity and one difference between the electric field and the
gravitational field produced by the nucleus.
Similarity .....................................................................................................................
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Difference ...................................................................................................................
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Write down an equation for the force between two point charges, Q1 and Q2 ,
separated by a distance r
(1)
A speck of dust has a mass of 1.0 × 10–18 kg and carries a charge equal to that of
one electron. Near to the Earth’s surface it experiences a uniform downward electric
field of strength
100 N C–1 and a uniform gravitational field of strength 9.8 N kg–1.
Draw a free-body force diagram for the speck of dust. Label the forces clearly.
Calculate the magnitude and direction of the resultant force on the speck of dust.
(6)
(Total 7 marks)
The diagram shows a positively charged oil drop held at rest
between two parallel conducting plates A and B.
A
Oil drop
2.50 cm
B
The oil drop has a mass 9.79 x 10–15 kg. The potential difference between the plates
is 5000 V and plate B is at a potential of 0 V. Is plate A positive or negative?
………………………………………………………………………………………………
Draw a labelled free-body force diagram which shows the forces acting on the oil
drop.
(You may ignore upthrust).
(3)
Calculate the electric field strength between the plates.
Electric field strength =…………………………………
(2)
Calculate the magnitude of the charge Q on the oil drop.
Charge =……………………………………
How many electrons would have to be removed from a neutral oil drop for it to
acquire this charge?
………………………………………………………………………………………………
(3)
(Total 8 marks)