“[Because] the cathode rays are deflected by an electrostatic force as if they were
negatively electrified, and are acted on by a magnetic force in just the way in which this
force would act on a [moving] negatively electrified body, . . . I can see no escape from
the conclusion that they are charges of negative electricity carried by particles of
matter.”
~ J. J. Thompson (1897)
Electrical Forces
► The
famous physicist Benjamin Franklin decided to
call the two types of charge negative and positive.
ƒ rubber/fur leaves negative charge on rod
ƒ glass/silk leaves positive charge on rod
► objects
with a like charge repel each other (two
rubber or two glass rods)
► objects with opposite charge attract each other
(one rubber and one glass rod)
Chapter 4 - The Electromagnetic Interaction
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Chapter 4 - The Electromagnetic Interaction
The Electrical Model of Matter
Electrical Model of Matter
► normal
(neutral) matter consists of equal
numbers of positively charged particles
(protons) and negatively charged particles
(electrons)
► electrons can be moved from one object to
another by rubbing, producing a net charge
► protons remain in the solid object
► At
first electricity
was thought to be a
fluid, like water.
► Experiments done
by J. J. Thompson
showed this “fluid”
fluid”
to be a stream of
charged particles.
ƒ He broke atoms into
positive and negative
parts.
Chapter 4 - The Electromagnetic Interaction
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3
Chapter 4 - The Electromagnetic Interaction
Relative Sizes of Electrical and
Gravitational Forces
4
The Electric Force Law
► Put
the three fundamental forces that we
deal with in this class in order of strength.
►
►
ƒ Two protons: electrical repulsion is 1036 times
stronger than the gravitational attraction.
ƒ Protons in a nucleus are held together by the
strong force in spite of the electric repulsion.
►
Gravity only pulls. And it works on objects with
mass.
The electromagnetic force both pulls and pushes and
it works on objects with charge.
But the force law is otherwise the same:
F = kQq/d2
Q
q
Q and q represent charge.
d
which can be positive or
negative. As with gravity, k is just a number.
Chapter 4 - The Electromagnetic Interaction
5
Chapter 4 - The Electromagnetic Interaction
6
1
Lets predict some behaviors
You reach for a metal door knob
You scuff across a wool carpet wearing
rubber shoes. What happens?
►
► Why
do you get ‘sparked’
sparked’?
► Is metal positively charged…
charged…How can we
find out?
► Why would both positive and negative
charges attract metal?
A. You end up negatively charged because
protons are rubbed off of your shoes
B. You end up negatively charged because your
shoes pick up extra electrons
C. Both A and B
Chapter 4 - The Electromagnetic Interaction
7
that allow electrons to move freely are
► Materials
that electricity cannot pass through are
8
What happens if I bring a negatively
charged finger near a metal door
knob?
Insulators and Conductors
► Materials
Chapter 4 - The Electromagnetic Interaction
conductors.
insulators.
__
__
__
__
__
__
__
__
► If
I bring any charge near a piece of metal, what
will happen?
Chapter 4 - The Electromagnetic Interaction
9
Contact Force
+ +
+
Chapter 4 - The Electromagnetic Interaction
10
Contact Force
► This
is air friction, sliding friction, cutting,
touching, pushing, resting on something,
connected by a rope, etc.
► It only exists when things touch.
Chapter 4 - The Electromagnetic Interaction
+
+
► Electromagnetic
repulsion
between atoms
causes the force
of contact and
sliding friction
between
objects.
11
Chapter 4 - The Electromagnetic Interaction
12
2
Magnetism
Magnetism and Field Lines
► It
has been known for
thousands of years
that some metals had
the ability to
permanently attract
other similar metals.
► Such metals are called
“magnetic”
magnetic” or
“ferromagnetic”
ferromagnetic”.
Chapter 4 - The Electromagnetic Interaction
► The
extent of magnetic influence is
readily described by “field lines”
lines”. Such
lines can be traced by iron filings.
13
Chapter 4 - The Electromagnetic Interaction
14
Permanent Magnets
Electromagnetic Interaction
► Permanent
magnetism arises when electric
currents flow in tiny domains inside a metal.
► When metal is heated to temperatures above
the Curie temperature the domains go away.
► magnetic
forces come from moving charges
nE •dA = Q/,0
due to moving and
► complete interaction
stationary charges is called
nB •dA = 0
electromagnetic
interaction
► Electromagnetic forces obey a set of four
nE •ds = dM
B/dt Maxwell’
equations
called
Maxwell’s equations.
nB •ds = :0dQ/dt + ,0 :0dME/dt
Chapter 4 - The Electromagnetic Interaction
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Chapter 4 - The Electromagnetic Interaction
16
3