Transcript 2.4-Fields - Mr. Schroeder

 Michael Faraday developed
the idea of a field as being a
sphere of influence to explain
how a force could affect an
object at a distance without
contact (such as gravity)
 Fields can be scalar or vector
 Gravity, electric and magnetic fields are vector fields
because they have magnitude and direction
 When an object is placed in the field, both objects
experience a force.
e.g. gravity
 An electric field exists around a charged particle.
 When another charged particle enters this field an electric
force is exerted on both particles
-
Fe
+
-
Fe
 The direction of the electric field is defined as the
positive particle
direction a
would move if
placed in the field. (Often called a test charge)
 Electric field lines begin at positive charges and end on
negative charges and are at right angles to the charged
object.
 Field lines show the path that a positive test charge
would follow if placed in the field.
• The closer the electric field lines, the stronger the
electric field
• Field lines do not cross
MULTIPLE CHARGES: the net electric force
determines the shape of the field lines
Two like charges:
Fnet
Fnet
• The net electric field
is the vector sum of
each charge’s electric
field.
Two opposite charges:
Fnet
+
Example: The figure below shows the electric field lines for
a system of two point charges.
(a) What are the relative magnitudes of the charges?
(b) What are the signs of the charges?
Solution:
(a) There are 32 lines coming
from the charge on the left, while
there are 8 converging on that on
the right. Thus, the one on the
left is 4 times larger than the one
on the right.
(b) The one on the left is positive;
the one on the right is negative.
•ELECTRIC FLUX is an measure of how many field
lines there are per unit area
The Shape of a Conductor:
•The charges on a conductor are spread out as far as
possible in order to make the energy of system as low
as possible
• On a solid conductor, the charges are on the surface
of the conductor (draw pointy conductor)
• On a hollow conductor, the charges are on the outer
surface
• The electric field inside a hollow conductor is zero
• The charges tend to be closer
together at edges and corners,
so the field lines are closer
together
• The electric field can be so
strong at corners it can force
electrons off molecules in the air
(pointed lightning rods cause
lightning to spark from the
clouds to the rod and then into
the earth and not flow through
houses)
ELECTRIC FIELD STRENGTH
 The definition: electric field
strength: E measured in (N/C)
Fe
E
q
Electric force acting
on the particle in
the field (N)
Charge of the particle
placed in the field (C)
Electric field strength (N/C)
e.g. How much force is exerted on a 3.00 μC
balloon placed in a cat’s electrical field
where the field strength is 4.00 x 105 N/C?
Fe  Eq
=1.2 N
A class of drugs
which target HIV-I
reverse
transcriptase, called
non-nucleoside
inhibitors, all bind to
a specific pocket on
the enzyme. Shown
is a close-up of the
binding of the drug
TIBO-R86183 inside
this pocket. The
electrostatic
characteristics of
reverse transcriptase
in the vicinity of the
non-nucleoside
inhibitor binding site
is also shown. The
electrostatic
potential is mapped
onto a solventaccessible surface,
along with some
representative
electric field lines
Finding electric field strength at a certain distance from a
charged object:
Coulomb’s
Constant
|E|
kq
r
The magnitude
of the electric
field strength
(N/C)
2
Charge of the
object
producing the
field (C)
Distance from
the object (m)
Example
Find the magnitude of the electric field at a point 0.450
m from a 5.00 x 10-6 C charge.
2.22 x 105 N/C
2. A –2.00 x 10-6 C charge is placed in an electric field.
The charge experiences a force of 5.30 x 10-2 N to the
left. Determine the magnitude and direction of the
electric field. (Hint: draw a diagram)
2.65 x 104 N/C
3. A –3.50 x 10-6 C charge is 0.440 m to the left from a
3.00 x 10-6 C charge. Determine the magnitude and
direction of the electric field at point P halfway between
the charges.
P
0.440 m
-3.50 x 10-6 C
3.00 x 10-6 C
1.21 X 106 N/C [left]