•   Today: Induction & transformers
•   Wednesday: Finish transformers, start light
•   Reading: review Fig. 26.3 and Fig. 26.8
•   Recall: N/S poles (opposites attract)
•   Moving electrical charges produce magnetic fields
•   RHRs
•   Moving electrical charges:
–   Produce an electric field, E = kq/d 2
–   Produce a magnetic field (related to q, velocity, and ~ 1/d 2 )
•   Currents produce magnetic fields
•   Moving magnetic fields cause produce currents!
EM Induction: Current is Induced
Why is current induced?
Remember the RHR for force on a charge moving through a magnetic field.
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The induced voltage in a coil is proportional to the number of loops , the size of the loops, and the rate at which the magnetic field changes within the loops.
Example: Electric Guitar Pickups
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•   It takes work (mechanical energy) to push a magnet through a loop.
•   Try to push it faster, and it’s more difficult to push.
•   How come? !
The new magnetic field of each current loop resists the motion of the magnet!
•   Usually it’s easier to move a wire than to move a big magnet
•   A generator converts energy
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•   Number of magnetic field lines passing through the loop changes over time, so current changes over time.
•   Magnetic field decreases: voltage drops
•   Magnetic field increases: voltage increases
Transformers (No, not those ones)
•   By passing a CHANGING current through a coil, we can induce a (changing) magnetic field. The second coil feels it, too!
•   The induction doesn’t have to come from a completely different coil (as in a transformer), it can come from other loops in the same coil!
•   The voltage induced by the coils opposes the change (Lenz’s law)
Demo: http://www.youtube.com/watch?
v=aSmMFog10D0
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•   V
1
/N
1
= V
2
/N
2
–   V = voltage in each coil
–   N = number of loops (or “turns”) in each coil
•   Example: A typical cell phone charging cable contains a transformer that “steps down” the voltage from the wall socket
120V to 4.6V. If the primary coil has 200 turns, how many turns are in the secondary coil?
•   If we have some large voltage that we want to step down (say for household appliances) using a transformer, should the secondary coil have fewer loops, or more loops than the primary coil?
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•   If we have some small voltage that we want to step up (say for long-distance transmission) using a transformer, should the secondary coil have fewer loops, or more loops than the primary coil?
•   These days, all power transmission is done through AC. (Thomas Edison and
Nikola Tesla argued about this hotly!)
•   Power is transmitted over long distance wires at low current, but high voltage.
(Remember power = current x voltage)
•   We’re also starting to develop wireless energy transmission, using induction!
•   Induction: Moving a wire through a magnetic field (or moving a magnetic field near a wire) will induce a current in the wire
•   Faraday’s Law
•   Transformers: V
1
/N
1
= V
2
/N
2
•   Maxwell’s counterpart to Faraday’s Law
•   Today: Light and EM waves
•   Friday: Light in materials and color
•   Reading: CH 27, focus on Fig. 27.14 and the concept of “scattering”
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•   Ok, so a changing magnetic field causes a current (Faraday’s law)
•   Why do we have currents in the first place? !
electric fields of the charges
•   Changing current produces a magnetic field, too! (Maxwell’s counterpart to
Faraday’s Law)
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Electromagnetism is a 2-way street
•   An EM wave is the result of the mutual induction of electric and magnetic fields
•   If the wave has wavelengths between
~400-700 nanometers, it will be a visible
EM wave: Light!
•   The speed of light is the wave speed for
EM waves.
•   Moving charges (currents) create magnetic fields, and changing magnetic fields create electric fields
•   These effects create electromagnetic waves
•   Wavelength is the distance between two wave peaks
•   Frequency is the number of times per second that a wave vibrates up and down wave speed = wavelength x frequency
•   c = 3.0 x 10 8 meters/sec in a vacuum
•   Example: What is the wavelength of radio waves being broadcast at a frequency of
88.5 megahertz?
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•   Gamma Rays
•   X-rays
•   Ultraviolet Light
•   Visible Light (ROY G BIV)
•   Infrared Light
•   Microwaves & Radio Waves
•   Important distinction: sound waves need a medium in order to travel.
•   Light waves need no medium, they can travel even in a vacuum.
•   We experience light and sound in different ways wavelength x frequency = speed of light = constant
How do light and matter interact?
•   Emission
•   Absorption
•   Transmission
–   Transparent objects let light through
–   Opaque objects block or absorb light
•   Reflection or Scattering
Mirror reflects light in a particular direction
Movie screen scatters light in all directions
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Gamma rays and radio waves are at opposite ends of the EM spectrum. Which form of light:
A. Carries more energy?
B. Has a longer wavelength?
C. Travels faster through a vacuum?
•   Today: Light in materials, color perception
•   Monday: Reflection and Refraction
•   Reading: Chapter 28, focus on figs. 28.6 and 28.25 on reflection and refraction
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•   Light travels at 3.0 x 10 8 m/s in a vacuum
•   Light waves in matter travel more slowly, depending on the material and the frequency of the wave
•   Slowing of light !
Dispersion
–   High f = more dispersion
–   Low f = less dispersion
Transparent vs. Opaque Materials
•   When light hits the atoms in a material, the electrons absorb it and are forced to vibrate
•   The vibrating electron either emits a photon or transfers the energy as heat
•   Time-delay between absorption and reemission is why the speed of light
is lower in the material.
•   Average speed of light through different materials
–   Vacuum: c (300,000,000 m/s)
–   Atmosphere: slightly less than c (but rounded off to c)
–   Water: 0.75 c
–   Glass: 0.67 c, depending on material
–   Diamond: 0.41 c
•   Most things around us are opaque; they absorb light without re-emitting it.
•   Vibrations given by light to their atoms and molecules are turned into random kinetic energy (into internal energy).
–   These materials become slightly warmer when light hits them
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Materials can be opaque to some kinds of light and transparent to other kinds of light
How does your eye form an image?
Cornea
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•   Refraction is the bending of light
•   Eye uses refraction to focus light
•   Refraction can cause parallel light rays to converge to a focus
•   The focal plane is where light from different directions comes into focus
•   The image behind a single (convex) lens is actually upside-down!
•   Two different kinds of antennae that pick up light in our retinas
•   Rods: handle vision in low light
•   Cones: handle color vision and detail
–   Three types of cones, sensitive to different frequency ranges.
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•   Since only rods fire under low light, we mostly see in b&w when it’s dark
•   Edges of the retina are very sensitive to motion
•   Our retinas have lateral inhibition so we can see details even under high contrast
•   Our eyes highlight edges and differences
(this is the source of many optical illusions)
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