Announcements. EM Induction. Faraday s Law 4/24/15. Why is current induced? EM Induction: Current is Induced

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1 Announcements Today: Induction & transformers Wednesday: Finish transformers, start light Reading: review Fig and Fig Recall: N/S poles (opposites attract) Moving electrical charges produce magnetic fields RHRs EM Induction 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. Faraday s Law 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 1

2 Faraday s Law: No Free Lunch 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! Generators and AC Usually it s easier to move a wire than to move a big magnet A generator converts energy Generators and AC 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! Self-Induction 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: v=asmmfog10d0 2

3 Transformers 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? Conceptual check: 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? Conceptual check: 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? Power Transmission 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! Main Points: 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 Announcements Today: Light and EM waves Friday: Light in materials and color Reading: CH 27, focus on Fig and the concept of scattering 3

4 4/24/15 Chapter 26: Properties of Light Field Induction 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) 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 ~ nanometers, it will be a visible EM wave: Light! The speed of light is the wave speed for EM waves. Recall: Properties of Waves Electromagnetic Waves Moving charges (currents) create magnetic fields, and changing magnetic fields create electric fields These effects create electromagnetic waves Speed of Light c = 3.0 x 108 meters/sec in a vacuum Wavelength is the distance between two wave peaks Frequency is the number of times per second that a wave vibrates up and down Example: What is the wavelength of radio waves being broadcast at a frequency of 88.5 megahertz? wave speed = wavelength x frequency 4

5 The Electromagnetic Spectrum Gamma Rays X-rays Ultraviolet Light Visible Light (ROY G BIV) Infrared Light Microwaves & Radio Waves Wavelength and Frequency Light vs. Sound 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? Reflection and Scattering 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 5

6 Get out a piece of paper, write your name on it and answer the following: 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? Announcements Today: Light in materials, color perception Monday: Reflection and Refraction Reading: Chapter 28, focus on figs and on reflection and refraction Light Waves in Matter 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. Transparent Materials 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 Opaque Materials 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 6

7 Materials can be opaque to some kinds of light and transparent to other kinds of light Cornea How does your eye form an image? Refraction Focusing Light Refraction is the bending of light Eye uses refraction to focus light Refraction can cause parallel light rays to converge to a focus Image Formation The focal plane is where light from different directions comes into focus The image behind a single (convex) lens is actually upside-down! Rods & Cones 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. 7

8 4/24/15 Seeing and Perception 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) 8