press and hold the down arrow key until the green light on the remote turns red
DESCRIPTION
The radio channel number for this room is “09” (zero, nine). It is STRONGLY recommended to login your remote for every class just to be sure it is on the correct radio channel and working before class. - PowerPoint PPT PresentationTRANSCRIPT
It is against the honor code to “click” for someone else-violators will loose all clicker pts.HITT RF Remote Login Procedure:
1. PRESS AND HOLD THE DOWN ARROW KEY until the GREEN light on the remote turns RED.
2. PRESS THE “0” KEY and you will see the RED light flash GREEN.
3. PRESS THE “9” KEY and you will see the RED light flash GREEN.
4. PRESS AND RELEASE THE DOWN ARROW KEY again and you will see the red light search for the receiver, if it BLINKS GREEN MULTIPLE TIMES you are logged in.
The radio channel number for this room is “09” (zero, nine). It is STRONGLY recommended to login your remote for every class justto be sure it is on the correct radio channel and working before class.
The Electric Vector in Light WavesPolarization of Light Waves
Each atom produces a wave with its own orientation of
All directions of the electric field vector are equally possible and lie in a plane perpendicular to the direction of propagation
E
This is an unpolarized wave
Polarized Light A wave is said to be linearly
polarized if the resultant electric field vibrates in the same direction at all times at a particular point
It may vibrate in any fixed direction
Polarization can be obtained from an unpolarized beam by
selective absorptionreflectionscattering
Polarization by Selective Absorption
The most common technique for polarizing light Uses a material that transmits waves whose
electric field vectors in the plane are parallel to a certain direction and absorbs waves whose electric field vectors are perpendicular to that direction
E. H. Land discovered a material that polarizes light through selective absorption
He called the material Polaroid
Polarization by Reflection When an unpolarized light beam is reflected
from a surface, the reflected light is Completely polarized Partially polarized Unpolarized
It depends on the angle of incidence If the angle is 0° or 90°, the reflected beam is unpolarized For angles between this, there is some degree of
polarization For one particular angle, the beam is completely polarized
The angle of incidence for which the reflected beam is completely polarized is called the polarizing angle θp
θp is called Brewster’s Anglesin tancos
pp
p
n
Liquid Crystal Displays (LCDs) One use of polarized Light
A liquid crystal is intermediate between a crystalline solid and a liquid
The molecules of the substance are more orderly than those of a liquid but less than those in a pure crystalline solid
In a display, the liquid crystal is placed between two glass plates with electrical contacts
A voltage is applied across any segment in the display and that segment turns on
V=0 rotates Polarization 90° making it Bright
Light passes through the polarizer on the right and is reflected back to the observer, who sees the segment as being bright
The light is absorbed by the polarizer on the right and none is reflected back to the observer
V≠0 produces no rotation making it Dark
Magnetism-Magnetic Fields Poles of a magnet are the ends where objects
are most strongly attracted Two poles, called north and south
Like poles repel each other and unlike poles attract each other Similar to electric charges
Magnetic poles cannot be isolated If a permanent magnetic is cut in half
repeatedly, you will still have a north and a south pole
This differs from electric charges There is some theoretical basis for
monopoles, but none have been detected
Sources of Magnetic Fields The region of space surrounding a
moving charge includes a magnetic field The charge will also be surrounded by
an electric field A magnetic field surrounds a properly
magnetized magnetic material Soft magnetic materials, such as iron, are
easily magnetized They also tend to lose their magnetism
easily Hard magnetic materials, such as cobalt and
nickel, are difficult to magnetize They tend to retain their magnetism
Magnetic Fields Symbolized by Direction is given by the direction a north pole
of a compass needle points in that location Magnetic field lines can be used to show how
the field lines, as traced out by a compass, would look
B
Earth’s Magnetic Field The Earth’s geographic north pole corresponds
to a magnetic south pole The Earth’s geographic south pole corresponds
to a magnetic north pole Strictly speaking, a north pole should be a
“north-seeking” pole and a south pole a “south-seeking” pole
Quick Quiz The red end of a compass needle which points
“North” is which end of a dipole magnet? A. North pole B. South pole
Electric Charges in Magnetic Fields Moving charges feel magnetic force
perpendicular to path of This force has a maximum value when
the charge moves perpendicularly to the magnetic field lines This force is zero when the charge moves
along the field lines This force is zero if the charge is stationary
Superconducting magnets 300000 Gauss or 30 Tesla
Earth’s magnetic field 0.5 G or 5 x 10-5 T
sinBqvF Quick Quiz How many G in a T?A. 10-5 D. 105
B. 10-4 E. 0.5 x 10-5
C. 104
Right Hand Rule Place your fingers in the
direction of Curl the fingers in the
direction of the magnetic field,
Your thumb points in the direction of the force, , on a positive charge
If the charge is negative, the force is opposite that determined by the right hand rule
v
B
F
Magnetic Force on a Current Carrying Conductor A force is exerted on a current-
carrying wire placed in a magnetic field The current is a collection of many
charged particles in motion The direction of the force is given
by right hand rule #1
Force on a Wire The blue x’s indicate the
magnetic field is directed into the page
The x represents the tail of the arrow
Blue dots would be used to represent the field directed out of the page
The • represents the head of the arrow
In this case, there is no current, so there is no force
Force on a Wire,cont B is into the page The current is up
the page The force is to the
left
Force on a Wire,final B is into the page The current is
down the page The force is to the
right
Force on a Wire, equation The magnetic force is exerted on each
moving charge in the wire The total force is the sum of all the
magnetic forces on all the individual charges producing the current
F = B I ℓ sin θ θ is the angle between and the direction
of I The direction is found by the right hand
rule, placing your fingers in the direction of I instead of
B
v
Force on a Charged Particle in a Magnetic Field
Consider a particle moving in an external magnetic field so that its velocity is perpendicular to the field
The force is always directed toward the center of the circular path
The magnetic force causes a centripetal acceleration, changing the direction of the velocity of the particle
Force on a Charged Particle
Equating the magnetic and centripetal forces:
Solving for r:
r is proportional to the momentum of the particle and inversely proportional to the magnetic field
Sometimes called the cyclotron equation
rmvqvBF
2
qBmvr
Particle Moving in an External Magnetic Field
If the particle’s velocity is not perpendicular to the field, the path followed by the particle is a spiral The spiral path is
called a helix
Hans Christian Oersted 1777 – 1851 Best known for
observing that a compass needle deflects when placed near a wire carrying a current
First evidence of a connection between electric and magnetic phenomena
Magnetic Fields – Long Straight Wire
A current-carrying wire produces a magnetic field
The compass needle deflects in directions tangent to the circle
The compass needle points in the direction of the magnetic field produced by the current
Direction of the Field of a Long Straight Wire Right Hand Rule
#2 Grasp the wire in
your right hand Point your thumb
in the direction of the current
Your fingers will curl in the direction of the field
Magnitude of the Field of a Long Straight Wire The magnitude of the field at a
distance r from a wire carrying a current of I is
µo = 4 x 10-7 T.m / A µo is called the permeability of free
space
2oIBr
Ampère’s Law André-Marie Ampère found a procedure
for deriving the relationship between the current in an arbitrarily shaped wire and the magnetic field produced by the wire
Ampère’s Circuital Law B|| Δℓ = µo I Sum over the closed path
Ampère’s Law, cont Choose an
arbitrary closed path around the current
Sum all the products of B|| Δℓ around the closed path
Ampère’s Law to Find B for a Long Straight Wire
Use a closed circular path
The circumference of the circle is 2 r
This is identical to the result previously obtained
2oIBr
André-Marie Ampère 1775 – 1836 Credited with the
discovery of electromagnetism Relationship
between electric currents and magnetic fields
Mathematical genius evident by age 12
Magnetic Force Between Two Parallel Conductors
The force on wire 1 is due to the current in wire 1 and the magnetic field produced by wire 2
The force per unit length is:
1 22o I IF
d
Force Between Two Conductors, cont Parallel conductors carrying
currents in the same direction attract each other
Parallel conductors carrying currents in the opposite directions repel each other
Magnetic Field of a Current Loop The strength of a
magnetic field produced by a wire can be enhanced by forming the wire into a loop
All the segments, Δx, contribute to the field, increasing its strength
Magnetic Field of a Current Loop – Total Field
Magnetic Field of a Current Loop – Equation The magnitude of the magnetic field
at the center of a circular loop with a radius R and carrying current I is
With N loops in the coil, this becomes2
oIBR
2oIB NR
Magnetic Field of a Solenoid
If a long straight wire is bent into a coil of several closely spaced loops, the resulting device is called a solenoid
It is also known as an electromagnet since it acts like a magnet only when it carries a current
Magnetic Field of a Solenoid, 2 The field lines inside the solenoid
are nearly parallel, uniformly spaced, and close together This indicates that the field inside the
solenoid is nearly uniform and strong The exterior field is nonuniform,
much weaker, and in the opposite direction to the field inside the solenoid
Magnetic Field in a Solenoid, 3 The field lines of the solenoid resemble
those of a bar magnet
Magnetic Field in a Solenoid, Magnitude The magnitude of the field inside a
solenoid is constant at all points far from its ends
B = µo n I n is the number of turns per unit length n = N / ℓ
The same result can be obtained by applying Ampère’s Law to the solenoid
Magnetic Field in a Solenoid from Ampère’s Law A cross-sectional
view of a tightly wound solenoid
If the solenoid is long compared to its radius, we assume the field inside is uniform and outside is zero
Apply Ampère’s Law to the blue dashed rectangle
Magnetic Effects of Electrons – Orbits An individual atom should act like a
magnet because of the motion of the electrons about the nucleus Each electron circles the atom once in about
every 10-16 seconds This would produce a current of 1.6 mA and
a magnetic field of about 20 T at the center of the circular path
However, the magnetic field produced by one electron in an atom is often canceled by an oppositely revolving electron in the same atom
Magnetic Effects of Electrons – Orbits, cont The net result is that the magnetic
effect produced by electrons orbiting the nucleus is either zero or very small for most materials