Download Electric Charges, Forces and Fields

Electric Charges, Forces and
Fields
Chapter 16
Electric Charge
 Two types of charge,
positive and negative
 Electrostatic Force: Like
charges repel, unlike
charges attract
 Charge is measured in
Coulombs [C]
 qe = -1.6 X 10-19 C
 qp = +1.6 X 10-19 C
Electric Charge
 Electricity involves the study of electrically charged
objects.
 Static electricity – charges that aren’t moving
(static)
 Electric current – charges that are moving.
 Conservation of Charge: The net charge of an
isolated system remains constant.
Example
 You shuffle across a carpeted floor on a dry
day and the carpet acquires a net positive
charge. Will you have a deficiency of
electrons or an excess of electrons?
If the carpet acquired charge of magnitude
2.15 nC, how many electrons were
transferred?
Electrostatic Charging
 Conductors – have valence electrons which are loosely
bound. Electrons can be removed relatively easily,
allowing the flow of electric charge. Excess charge added
to a conductor will distribute throughout the conductor.
 Insulators – have valence electrons which are tightly
bound. Electrons cannot be easily removed and will not
allow the flow of charge. Excess charge added to an
insulator will sit in one place and not redistribute.
 Semiconductors - materials with conductivity between that
of insulators and conductors.
Detecting Static Charge
 An electroscope is a
device used to detect
static charge. The
leaves of the
electroscope are made
of conducting material.
Leaves spread in the
presence of electric
charge.
Electroscopes
Electrostatic Charging
 Charging By Friction
 Charging by
Conduction (Contact)
 Charging by Induction
(no contact)
– Charge separates –
polarization of charge
Grounding
 The Earth is a huge reservoir of electric
charge – it will accept a huge amount of
excess charge. Items which are grounded
are connected to the ground so that excess
charge will be removed.
Millikan
 Millikan devised an
experiment to measure the
charge on one electron!
 He used an atomizer to
spray droplets of oil. The
spray process causes the
droplets to be charged due
to friction.
 Millikan balanced the
weight of the drop with the
force of attraction
Coulomb’s Law – Electric Force
 Coulomb’s Law is used
to calculate
quantitatively the
electric force
(attraction or repulsion)
between charged
objects.
 k = 8.99 X 109 Nm2/C2
Coulomb’s Law
 Force is directly
proportional to charge
 Force is inversely
proportional to square
of distance
 Force is negative for
attractive, positive for
repulsive
Example
 Two point charges of -1.0 nC and +2.0 nC
are separated by a distance of 0.30 m.
What is the electric force on each particle?
Example – Coulomb’s Law with
Vectors
 A system of three charges is shown on page
514 , Figure (b). Find the net force acting
on charge 3 due to charges 1 and 2.
Homework
 Read Sections 15.1 and 15.2
 Read Examples 15.4, 15.5 on pages 515,
516.
 Do # 1, 3, 5, 6, 10, 11, 12, 14, 15, 17, 24,
25, 29, 32, 33, 35, 36, 38, 39 page 530
Force Fields
 A force field shows the force that would exist on a
mass or a charge in a region of space, due to the
presence of another mass or charge.
 Gravitational force field is measured in N/kg since
Fg is a force between masses.
 Electric force field is measured in N/C since Fe is a
force between charges
Gravitational Force Field
 Gravitational field
points towards the
mass since Fg is
always attractive.
 Gravitational field gets
weaker as distance
increases
 g = Gm/r2
Electric Field
 Electric field, E, shows
the force that would
exist on a positive test
charge placed in that
region of space.
 E = Fe/q [N/C]
 E = kq/r2
 E is a vector quantity
Example
 Two point charges are placed on the x-axis.
q1 = 1.5 μC located at x=0 and q2= 6.0 μC at
x=0.60m. Find all locations where the
electric field is zero.
Example
 Three charges are located as follows:
q1 = 1.00 μC located at (3.0m, 0);
q2 = 2.00 μC located at (-5.0m, 0) and
q3 = -1.50 μC located at (0, 4.0m).
Find the electric field at the origin.
Field Lines
Electric field is depicted using ‘lines of force’:
 Lines closer together shows stronger electric field.
 Electric field is tangent to the field lines.
 Electric field lines point from positive charge
towards negative charge.
 The number of field lines entering (exiting) a charge
is proportional to the magnitude of the charge.
 Field lines never cross.
Parallel Plates
 The electric field between two parallel plates
is
E = 4πkQ/A
where Q is the charge on one plate and A is
the area of one plate. Parallel plates are
used as capacitors in electronics.
Homework
 Read 15.4, particularly example 15.9 on
page 524.
 Do # 41, 43, 45, 47, 50, 51, 54, 58, 59, 60
Conductors and Electric Fields
 Inside a conductor, charges
have the ability to move.
They will spread out as far
as possible in order to
minimize forces of repulsion.
 Any excess charge on a
conductor resides entirely
on the outer surface only!
 The electric field inside a
conductor is zero.
Conductors and Electric Fields
 Electric field lines are always
perpendicular to the surface of
the conductor.
 On a conducting sphere, excess
charge spreads evenly. The
electric field lines outside the
conducting sphere are evenly
spaced.
 On an irregular shape, the
charges accumulate at sharp
points or regions of high
curvature. Electric field lines in
these regions are more dense.
Electric Field near Conductor