anyone got pre-prepared answers to these paragraph stlye questions? I was thinking about just writing them on my sheet.
AOS1:
Explain the role of a commutator in a DC motor
Explain why voltage is stepped up across transmission lines
AOS2:
Explain the difference between continuous/discrete wavelength light sources
Explain how emission/absorption occur
Explain how the photo-electric effect supports the photon theory and not the wave theory
Explain how Young's 2-slit experiment supports the wave theory
DS (Sound):
Explain how this microphone works (all of them)
Explain why low notes can be heard better by a person outside the room
Explain the role of a baffle
Explain the role of a port
Explain how resonance occurs
Explain why several different sized speaker cones are used
In a DC motor, a commutator reverses the direction of the current every 180 degrees. This allows the pair of forces to be reversed, and results in them creating continuous torque.
In transmission lines, as in all ohmic resistances, P=I^2R, therefore power loss is exponentially dependant on the magnitude of the current. By stepping up the voltage using a transformer, the current goes down (as P=VI, and P is constant in an ideal transofrmer) Lower values of I mean exponentially lower values of P.
(never heard it called continuous/discrete... but you mean coherent/incoherent?)
Coherent light sources are light sources that provide the same level/intensity of light at all frequencies. While Incoherent provide varying/random levels at each frequency. (not very happy with this definition, anyone help?)
Electrons around an atom 'orbit' according to their de broglie wavelength. They require energy to jump up a wavelength, and when they move down a wavelength they give off energy. This energy is in the form of light. Absorption is when it receives light and the elctrons can move up, emission is when the electrons fall back down a level and give off light.
The photoelectric effect supports photon theory. When light of any intensity but sufficient frequency strikes a metal, electrons are emitted. Wave theory states that any frequency light of sufficient intenstiy would do so. Ie. emission is dependant on frequency of light, NOT intensity. (Intensity only affects NUMBER of electrons, not their Ekmax)Also, the wave model predicts that there would be a time delay for less intense light, this is not the case
Youngs double slit experiment demonstrates an interference pattern, including constructive interference (where crestmeets crest or trough meets trough) as well as destructive interference (where crest meets trough), shown by antinodes and nodes respectively. These interference patterns are similar to those shown by water or sound waves, and as such provide evidence for the wave theory of the nature of light.
MICROPHONES:
-Dynamic (moving coil)
a coil is mounted over the pole of a permanent magnet, this coil is connected to a light diaphragm. Sound waves cause the diaphragm to vibrate, which movese the coil (Inside a magnetic field). This induces a current (according to faradays Law), and this current exhibits a rate of frequency/intensity that are representative of the sound waves.
-Crystal
Piezoelectric crystals (such a quartz) are atatched to a diaphragm. These crystals produce an electric current when subjected to pressure. Ths electrical signal is representative of the sound waves (change in pressure)
-Electret-condenser (capacitance)
diaphragm is positioned over one side of a capacitor. As the diaphragm vibrates, the gap between the capacitor changes, and so more or less voltage crosses accordingly. The change in amount of current that makes it across the capacitor represents the sound wave.
-Velocity (ribbon)
Thin ribbon of aluminium suspended in a magnetic field, sound waves make it vibrate. This changes flux across ribbon + causes induced current. ( same principles as dynamic mic, but more primitive + no diaphragm )
Lower frequency notes have higher wavelenght, due the equation v=fl (pretend l is lambda, lol) as lambda is greater, more diffraction occurs around walls/through gaps. This is due to equation diffraction: lambda/gap width. Soemone standing outside a room can hear lower notes better because the sound waves diffract more than for higher notes.
When a speaker generates a sound wave, there is also an equal and opposite generated sound wave behind it. The baffle blocks this + prevents it from bouncing forward and (according to phase) creating random interference that would lower the fidelity of the speaker. The baffle traps the reverse wave inside teh speaker, and it eventually runs out of energy.
(im not sure what ports are...
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Resonance is due to sound waves superimposing with an objects natural frequency, creating a standing wave inside an open ended or close ended pipe. This standing wave has a constructively caused antinodes, and destructively caused nodes, which are located at the ends of the pipes and this results in an amplification of the sound. (or something like that... havent studied soudn yet, incase you cant tell
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We use different size speakers for lower/higher frequencies because of diffraction. Diffraction occurs according to lambda/gapwidth. so the higher frequencies have lower wavelenghts, thus need smaller gap widths to receive the same diffraction as lower frequencies + hgher wavelengths.