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Topic 5
Electric currents
TEST next
Thursday 2nd
Feb
Topic 6
Fields and
Forces
Definitions and
copy of the
syllabus
Electric Force and Electric field
We already
know that;
Electric Force and Electric field
1. There are two types of electric charge
(positive and negative)
Electric Force and Electric field
2. Static charges can be produced by the action
of friction on an insulator
Electric force and electric field
3. Conductors contain many free electrons
inside them (electrons not associated with
one particular atom)
Electric Force and Electric field
4. Charge is conserved. The total charge of
an isolated system cannot change.
I’m
indestructible!
So am I!
Electric Force and Electric field
The force
between two
charges was
investigated by
Charles Augustin
Coulomb in 1785
Electric Force and Electric field
Coulomb found that the force between two point
charges is proportional to the product of the two
charges
F α q1 x q 2
and inversely proportional to the square of the
distance (r) between the charges
F α 1/r2
Coulomb’s law
It follows that
F α q1q2
r2
or
F = kq1q2
r2
Coulomb’s law
F = kq1q2
r2
The constant k is sometimes written as
k = 1/4πεo
where εo is called the permittivity of free
space.
Calculations using Coulomb’s law
The force between two charges is 20.0 N.
If one charge is doubled, the other charge
tripled, and the distance between them is
halved, what is the resultant force between
them?
F = 20N
q2
q1
r
2q1
F=?N
3q2
r/2
Calculations using Coulomb’s law
F = kq1q2/r2 = 20.0N
x = k2q13q2/(r/2)2 = 6kq1q2/(r2/4) = 24kq1q2/r2
x = 24F = 24 x 20.0 = 480 N
F = 20.0N
q2
q1
r
2q1
x = 480 N
3q2
r/2
Electric field
An area or region where a charge feels a
force is called an electric field.
The electric field strength at any point in
space is defined as the force per unit
charge (on a small positive test charge) at
that point.
E = F/q (in N.C-1)
Force on a charge
• This means the force on a charge q is
given by
• F = Eq
• If the charge is a proton or electron
• F = Ee where e = 1.6 x 10-19 C
Electric field around a point charge
If we have two charges q1 and q2 distance
r apart
F = kq1q2
/r2
q1
q2
Looking at the force on q1 due to q2, F = Eq1
F = kq1q2/r2 = Eq1
E (field due to q2) = kq2/r2
NOT in data
book
Electric field
Electric field is a vector, and any
calculations regarding fields (especially
involving adding the fields from more than
one charge) must use vector addition.
Field here due to both
charges?
q1
q2
Electric field
Electric field is a vector, and any
calculations regarding fields (especially
involving adding the fields from more than
one charge) must use vector addition.
Field due to q1
Field here due to both
charges?
q1
q2
Electric field
Electric field is a vector, and any
calculations regarding fields (especially
involving adding the fields from more than
one charge) must use vector addition.
Field due to q1
Field due to q2
q1
Field here due to both
charges?
q2
Electric field
Electric field is a vector, and any
calculations regarding fields (especially
involving adding the fields from more than
one charge) must use vector addition.
Field due to q1
Field due to q2
Resultant field
q1
q2
Electric field patterns
An electric field can be represented by
lines and arrows on a diagram, in a similar
ways to magnetic field lines.
Electric field patterns
An electric field can be represented by
lines and arrows on a diagram , in a
similar ways to magnetic field lines.
The arrows
show the
direction of force
that would be
felt by a positive
charge in the
field
Electric field patterns
An electric field can be represented by
lines and arrows on a diagram , in a
similar ways to magnetic field lines.
The arrows
show the
direction of force
that would be
felt by a positive
charge in the
field
Electric field patterns
An electric field can be represented by
lines and arrows on a diagram , in a
similar ways to magnetic field lines.
This is an
example of a
radial field
The closer the
lines are
together, the
stronger the
force felt.
Field around a charged metal
sphere
E = 0 inside
the sphere
Field around two point charges
Field around two point charges
Field between charged parallel plates
NOT in data
book
d
“Edge effects”
Uniform field E = V/d
V
Remember!
The force F on a charge q in a field E is
F = Eq
Parralel plates
• E = V/d and E = F/q
• So V/d = F/q
• Useful!!!!
Electric field hockey!
•
http://phet.colorado.edu/sims/electric-hockey/electric-hockey_en.jnlp
Let’s do some reading
and try some questions
Read pages 285 to 287
and 289 to 295.
Questions:
Page 287 qs 3, 4, 5,
11.
Page 296 qs 4,5,6,7,11