Explain Questons

[dooder]

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

[Timtasticle]

Yeah I was going to put down worded qn answers on my cheat sheet also.

Are there any other likely topics for worded qns??

[Galelleo]

I havent written mine yet... but when i do, if i remember, ill chuck em up here and we can discuss ?

[brendan]

Explain the role of a commutator in a DC motor
DC generators cannot use slip rings because they must produce DC, and therefore a commutator is essential.
The purpose of the commutator is to reverse the direction of the current every half cycle. (With no commutator the coil would remain static in a position perpendicular to the magnetic field.)
Thus, by reversing the direction of the current every half cycle, a continuous torque is maintained.

Explain why voltage is stepped up across transmission lines
Increasing the voltage reduces the current in the transmission and distribution lines and hence the size of conductors required and distribution losses incurred.


Explain how emission/absorption occur
? When a gas is heated or a large current is passed through it, the gas glows and emits a characteristic diffraction pattern called an emission line spectrum. The emission line spectrum suggests that only certain wavelengths of light are emitted. Each line in the spectrum corresponds to a particular wavelength. A different gas produces a different spectrum.
? Emission and absorption line spectra are evidence for quantised atomic energy levels where electrons move around a nucleus with discrete energies.
? Take for example the hydrogen atom where absorption spectra shows that the hydrogen atom is only capable of absorbing a small number of different frequencies, and therefore energies, of very specific values, i.e. the absorbed energy was quantised.
? Accordingly, atoms can only emit/absorb in energies which correspond to the differences between energy levels.
? If the f, and therefore E, of the incident light were below a certain value for the hydrogen atom, the light would simply pass straight through the gas without any absorption occurring
? All of the photons of light with energies above the ionisation energy vale for hydrogen are continuously absorbed.



Explain how the photo-electric effect supports the photon theory and not the wave theory
? Ek(max) does not depend on the light intensity, it depends only on the frequency of the light. This is in accordance with Einstein?s photon model Ek(max) = hf ? W, where hf is the energy of a photon in the light beam.
? The ?stopping potential? for the photoelectrons is independent of the light intensity.
? The cut-off frequency below which there is no photoelectric effect contradicts the prediction of the wave model. According to the wave model, it should occur at any frequency of the incident light, provided the light is intense enough.
? Wave theory predicts that any frequency of light should produce the photoelectric effect given sufficient time. This does not happen. It was observed that only light above a particular frequency will eject photoelectrons, irrespective of the intensity; this in agreement with the photon model which suggests the emission of photoelectrons if the frequency is above the threshold frequency.
? The wave theory also predicts a time lag between the turning on of the light source and the ejection of photoelectrons especially very weak sources.  The actual time delay before photoelectrons were produced was random, and often there was an almost instantaneous production of photoelectrons.


Explain how Young's 2-slit experiment supports the wave theory
? The formation of the bright and dark regions was due to an interference pattern where the bright bands resulted from constructive interference (i.e. path difference of 0λ, λ, 2λ, etc.) and the dark bands from destructive interference (i.e. path difference of ? λ, 1 ? λ, etc.).
? The pattern was in fact a diffraction/interference pattern and, as such, suggests a wave property. This tells us that the individual photons have wave properties.
? G.I. Taylor used light of such low intensity so that there was only one photon in the apparatus at any time.  This was then used as evidence that individual photons exhibit wave-like behaviour.

[bilgia]

oo this is great material...im gona just be writing cheat sheet & studying physics on tuesday...cant wait to copy this stuff down..
thanks guys!

[winter_89]

Explain how this microphone works (all of them)

The answer can be found in the itute cheat sheet.

Also, check this
      http://community.boredofstudies.org/421/physics/149798/answers-commonly-asked-questions.html

[Galelleo]

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... )

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 )

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.