Download Electric and Magnetic Forces

Electric and Magnetic Forces
and Properties of Materials
Chapter 16 and 24
Static Electricity
• Better known as Electrostatics comes from
the roots electro and statics.
– “Electro” is Greek for amber, a petrified tree resin
which when rubbed will attract other objects.
When it was discovered that the movement of a
sub-atomic particle was responsible for this
attraction, the particles were called electrons; the
force was called electric.
– “Stati” is Greek for standing or place.
• Thus, electrostatics is the study of electrical
forces at rest.
What do the forces do?
• First – they are not like gravity
• The electrical forces can either attract
or repel one another.
• Ben Franklin named the two types of
forces positive and negative.
Like charges repel each other,
unlike charges attract each other.
Opposites attract!
Coulomb’s Law
The force between two electrically
charged particles (q) is proportional
to the product of their charges
divided by the square of the
distances between them.
K is the universal electrostatic
constant. It is equal to 9.0 x 109 Nm2/C2.
q2 q2
F k 2
d
Which is stronger,
gravitational forces or
electric forces?
Problem: Assume that you have two objects, one with a mass of
10 kg and the other with a mass of 15 kg, each with a charge of
–3.0 x 10-2 C and separated by a distance of 2 meters. Compare
the electrical and gravitational forces, which is greater?
The Solution



2
2
q1q2
9
2
2  3.0  10 C  3.0  10 C
6
F  k 2  9.00  10 N  m C

2
.
03

10
N
2
d
2.0 m 

qmqm  6.7  10 N  m kg310
.0 kg
1015Ckg
3.02.51
10 10
C N
F G
k
 9.00  10 N  m C 
 2.03  10 N
d

1 12 2
22
d
F G
911
2 2 2
2
2
2
2.0 m
 m
2.0
2
9
6
2
m1m2
11
2
2 10 kg 15 kg 
9

6
.
7

10
N

m
kg

2
.
51

10
N
2
2
d
2.0 m 


What causes the charge?
• The movement of electrons
grade at JGMS)
(remember back to sixth
• Atom basics
– The atom is made from protons and neutrons in
the nucleus and electrons that rotate around the
nucleus.
– Electrons have a negative charge, protons have a
positive charge.
• If an atom loses a negative electron or gains
one, it becomes an ion (a charged particle)
How are materials classified?
Conductors
• permit electrons to flow
freely from atom to atom
and molecule to molecule
• permit charge to be
transferred across the entire
surface of the object
• Conductors allow for charge
transfer through the free
movement of electrons.
Insulators
• materials which impede the
free flow of electrons from
atom to atom and molecule
to molecule
• The particles of the insulator
do not permit the free flow
of electrons; subsequently
charge is seldom distributed
evenly across the surface of
an insulator
How can an object be charged?
• By friction, when electrons are transferred by
friction from one object to another.
• By conduction, when electrons are transferred
from one object to another by direct contact
without rubbing.
• By induction, when electrons are caused to
gather or disperse by the presence of nearby
charge.
Law of Conservation of Electric Charge
• In a closed system, the net amount of charge
produced in any process is zero
• The strength of charged particles is measured in coulombs.
An electron and a proton have the same magnitude of
charge, just opposite signs. The magnitude of the charge of
either of these two particles is:
– 1.602x10–19 coulombs So, we can say that an electron has a charge
of –1.602x10–19Coulombs, and a proton has a charge of
+1.602x10–19Coulombs.
A little more on the conservation of charge
How much energy required to tear away electrons
varies from substance to substance.
– Rubber holds electrons more firmly than fur. When
rubber and fur are rubbed together, electrons transfer
from the fur to the rubber rod.
• The rubber has excess electrons and is negatively charged.
– A glass or plastic rod rubbed with pure silk will transfer
its electrons to the silk. Giving the silk a negative
charge.
• The electrons are not created or destroyed, they
are simply transferred from one object to another.
Polarization
• Charge polarization occurs in insulators
that are in the presence of a charged
object.
• A realignment of charge rather than a
migration of charge occurs.
Electric Fields and Potentials
The Van de Graaff is coming!!!
Electric Field & Field Lines
• The space around every electrical charge
• Has both magnitude and direction, a vector
quantity
• Exploring Electrical Fields
Charge Distribution
How does charge distribute itself on an
object?
•Charges spread out as much as they
can.
•In order to be in equilibrium, charges
will bunch up at corners. (they gather
so the net force in the center of a
conductor is zero)
Charge distribution car
What should you do if a broken power
lands on top of your car?
Electric Shielding
• Static charge occupies only the outer
surface of a conductor; inside the
conductor the electric field is zero.
Electrical Potential Energy
• The work required to push a charged
particle against the electric field of a
charged object increases the particle’s
electrical potential energy.
•Work done is equal to the
energy gained.
•Similar to how a mass’s PE
depends on its location within the
Earth’s gravitational force field
Electric Potential
• Electrical potential is the electrical potential
energy per charge.
• Electric potential = electrical potential
energy/charge
• Other terms for Electric
Potential:
– Voltage
– EMF (electromotive force)
• 1 volt = 1 joule/coulomb
What is Lightning?
Lightning on Science Joy Wagon