Model answers

[trinon]

I'm trying to compile a list of model answers for expected "explanation" questions that are bound to be on the exam. This is my list so far:

Q: In Young's double slit experiment, why does the middle of the screen show a bright band?
A: The light coming from the slits are in phase and reach the middle of the screen with a path difference of 0 wavelenghts, therefore the waves constructively interfere to cause a bright fringe in the middle.

1 mark = in phase
1 mark = path difference
1 mark = constructive interference hence bright band

Q: Explain how a commutator works.
A: A commutator reverses current every half turn, and hence reverse the force acting on the armature or coil every half turn. Therefore, the torque acting on the coil stays in one direction, keeping the coil rotating continuously.

1 mark = reverses current every half turn
1 makr = hence reverses force acting on the armature/coil every half turn
1 mark = therefore the torque acting on the coil stays in one direction keepin the coil rotating

Q: Explain the term incandescent.
A: Incandescent refers to the thermal vibration of electrons that lose energy as photons.

1 mark = mention thermal
1 mark = electrons lost energy as photons

Q: Why does the motor stop rotating after 90 degrees? (Assume no commutator)
A: The force on side CD (the side opposite to that of the side closest to the north pole) remains in the same direction as the coil rotates beyond 90 degrees. This causes the coil to change direction and rotate back in the opposite direction, continuing to oscillate around the 90 degrees mark until it stops.

Q: Why are transformers used to step-up voltage?
A: Step-up transformers are used so that power is transmitted at high voltage in power lines. This is useful as high voltage means low current and less power loss in transmission due to the resistance of the power lines. This is shown through

Q: What is the fundamental difference between an alternator and a generator?
A: Generators have a coil rotating a magnetic field, alternators have a magnet(rotor) rotates between coils(stator).

Q: Explain why the input of a transformer must be AC rather than AC.
A: Secondary voltage is due to electromagnetic induction. This is caused by a changing flux through the coil of the transformer. If the changing flux doesnt vary, there will be no emf induced, hence no output so AC must be used.

Thanks to fredrick, rowshan for contributions.

[cara.mel]

They're all accurate (I think)

This one could be clearer though:

Q: Explain how a commutator works.
A: A commutator reverses current every half turn, and hence reverse the force acting on the armature or coil every half turn. Therefore, the torque acting on the coil stays in one direction, keeping the coil rotating continuously.

reverses current through what every half turn? Examiners pretend they know nothing, is it reversing current through the whole circuit? (ie the battery as well)
You could draw pictures to help explain, showing the current through the coil going the opposite way after 180 degrees, but because the whole thing has flipped over the force is still the same way with respect to the magnets etc

[fredrick]

Q)Explain why the input of a transformer must be AC rather than AC.
A).Secondary voltage is due to electromagnetic induction
   .This is caused by a changing flux through the coil of the transformer.
   .If the changing flux doesnt vary, there will be no emf induced, hence no output
    so AC must be used

[trinon]

Thanks. Anyone else?

[danielf]

A quote from the VCAA 2006 Physics Exam about the role of speaker boxes:
"A common misconception was that the rearward sound wave was reflected from the back of the box, became in phase and constructively interfered with the front wave."
I always thought that this was true (and I just did a TSFX 06 exam and this was a correct answer)
The answer is just that is prevents the sound at the rear of the cone from diffracting around the diaphragm and cancelling the front wave.
Can anyone improve on this/explain to me why the first thing is a 'misconception'?

[vce08]

Because it is out of phase

[Mao]

A quote from the VCAA 2006 Physics Exam about the role of speaker boxes:
"A common misconception was that the rearward sound wave was reflected from the back of the box, became in phase and constructively interfered with the front wave."
I always thought that this was true (and I just did a TSFX 06 exam and this was a correct answer)
The answer is just that is prevents the sound at the rear of the cone from diffracting around the diaphragm and cancelling the front wave.
Can anyone improve on this/explain to me why the first thing is a 'misconception'?

it is possible. this is the case with the "ports" in sub-woofers typically found in cars, that use reflection of sound from the rear to constructively interfere with the output.

but normally [assuming this was the question], infinite baffles [enclosed boxes] are used so that out-of-phase sounds at the back cannot diffract around the baffle and destructively interfere with the output.

[rowshan]

q.What is the fundamental difference between an alternator and a generator?
a.Generators have a coil rotating a magnetic field, alternator have a magnet(rotor) rotates between coils(stator).

not hundred percent sure; but this looks like a valuable thread and i wanted to keep it alive.

[onlyfknhuman]

How does an intensity graph vary between, an interference experiment with ONE SLIT to TWO slit.

Cant really explain verbally... easier to draw a diagram unless someonelse can explain this.

[IMG]http://img530.imageshack.us/img530/3715/14460682ex5.th.png[/img][IMG]http://img530.imageshack.us/images/thpix.gif[/img]

FCUK LATEX LOOKS SICK!!

EDIT: make sure the curves are round not pointy

[rowshan]

anyone want to have a crack at explaining why?

[ReVeL]

*cough* Mao *cough*

[Mao]

oh hi.

you might want to take a look at single slit and double slit [click on the intensity profile in both and see the graphs]

basically, the key features are

 - there is always an antinode at the centre
 - the central peak for single slit is twice as wide as the other peaks
 - the intensity of single-slit diffraction decreases as you move away from the centre. [though in the exam, don't draw it as dramatically as the java applet, the gradual decrease in the picture posted above should be fine, and make sure that the peaks are ROUND, not triangles ]
 - the intensity of double-slit diffraction tend to decrease less gradually [with the central peaks of about the same intensity], and they generally don't punish you if you draw them to the same size. In double-slit, the width of each peak is about the same.

the java applets does the visual explanation a lot better, so yeah...

[rowshan]

lol those things were fun to play with, but could you explain in terms of interference?

[onlyfknhuman]

oh hi.

you might want to take a look at single slit and double slit [click on the intensity profile in both and see the graphs]

basically, the key features are

 - there is always an antinode at the centre
 - the central peak for single slit is twice as wide as the other peaks
 - the intensity of single-slit diffraction decreases as you move away from the centre. [though in the exam, don't draw it as dramatically as the java applet, the gradual decrease in the picture posted above should be fine, and make sure that the peaks are ROUND, not triangles ]
 - the intensity of double-slit diffraction tend to decrease less gradually [with the central peaks of about the same intensity], and they generally don't punish you if you draw them to the same size. In double-slit, the width of each peak is about the same.

the java applets does the visual explanation a lot better, so yeah...

HAHA yeah forgot to note that, paint is too hard to do curves

[Mao]

lol those things were fun to play with, but could you explain in terms of interference?

for double slits, the textbook explanation should be adequate.

for single slits, I personally have never seen a good explanation in terms of interference... so the answer to that is NFI.
sorry.