VCE Physics Question Thread!

[FallingStar]

Would someone be able to summarise the key ideas and differences between DC and AC generators?

AC generator:
  -Uses a slip ring (no gaps).
  -Current reverses every half turn, as shown in the graph.
  -Sinusoidal wave (trig function)
  -Graph of current produced:

  -Diagram:



DC generator:

  -Uses split rings commmutators (there is a gap between the rings)
  -Current does not reverse: Modulus/absolute function
  -Sinusoidal wave, but in absolute values.
  -Graph of current produced:

  -Diagram:



And just remember:
Slip ring = AC
Split ring commutator = DC

Edit: Added commutator to the "split rings" since my teachers said that VCAA is going to be pedantic about that

[MB_]

AC generator:
  -Uses a slip ring (no gaps).
  -Current reverses every half turn, as shown in the graph.
  -Sinusoidal wave (trig function)
  -Graph of current produced:
(Image removed from quote.)
  -Diagram:
(Image removed from quote.)


DC generator:

  -Uses split rings (there is a gap between the rings)
  -Current does not reverse: Modulus/absolute function
  -Sinusoidal wave, but in absolute values.
  -Graph of current produced:
(Image removed from quote.)
  -Diagram:
(Image removed from quote.)


And just remember:
Slip ring = AC
Split ring = DC

Would you also be able to explain the purpose of the slip rings in the AC generator and the purpose of the split rings in the DC generator. I've read quite a few different things but I'm not entirely sure of their main purpose. Thanks for the help

[sweetiepi]

Would you also be able to explain the purpose of the slip rings in the AC generator and the purpose of the split rings in the DC generator. I've read quite a few different things but I'm not entirely sure of their main purpose. Thanks for the help

Hey MB_!
A split ring commutator changes the current's direction every half-rotation so that the torque is in the same direction throughout the rotation.
This makes the current/time graph look like this

On the otherhand, slip rings allow the current to alternate, so the torque is changing direction every half turn.
This makes the current time graph look like this

Edit: I couldn't get the pics to work- will try later!

[wyzard]

Would you also be able to explain the purpose of the slip rings in the AC generator and the purpose of the split rings in the DC generator. I've read quite a few different things but I'm not entirely sure of their main purpose. Thanks for the help

A slip ring ensures the sides of the coil always remain in contact with same side the wire. Say the left side of the coil, which we will call A for now. After rotating 180 degrees, A will be on the right side, but the slip ring ensures A remains in contact with the same side of the wire.

A split ring on the other hand reverses this and A is now connected to the other terminal of the wire.

Due to this, a slip ring generates AC, while split ring generates DC.

This will take some thinking and visualisation to get it, but once you get it, it's actually a really smart design

[HopefulLawStudent]

Would you be able to please explain why the light source for the photoelectric experiment needs to be coherent? Checked several textbooks and they just say that it has to be coherent, no explanation why. 

[Swagadaktal]

Would you be able to please explain why the light source for the photoelectric experiment needs to be coherent? Checked several textbooks and they just say that it has to be coherent, no explanation why. 

Yoyo hls,

It's gotta be coherent coz you want only 1 frequency wave length hitting the plates, coz if there were other wavelengths with diff energies they could cause some photoelectrons to be emitted - makes no sense to have 8 different frequency lights to test the photoelectric effect coz how are you gonna gauge the work function etc

Is that what you're asking?

[wyzard]

Would you be able to please explain why the light source for the photoelectric experiment needs to be coherent? Checked several textbooks and they just say that it has to be coherent, no explanation why. 

Pretty sure monochromatic's the right word. I think in the context in photoelectric effect it means light of a single frequency/wavelength striking the surface of the metal. Reason for this is that if there were multiple frequencies, the energy of the ejected electrons will be more chaotic and difficult to study.

The term coherent light source is more applicable to the double slit experiment, where the light diffracted from the two slits are coherent. In this context, it means that the electromagnetic wave leaving the slits are in phase.

You can think of light waves oscillating side by side in the slits, if one goes up, the other goes up as well. The crest and the trough will leave the slits exactly at the same time. The coherence of light diffracted from the double slit is necessary to produce the double slit interference pattern observed.

[Swagadaktal]

Pretty sure monochromatic's the right word. I think in the context in photoelectric effect it means light of a single frequency/wavelength striking the surface of the metal. Reason for this is that if there were multiple frequencies, the energy of the ejected electrons will be more chaotic and difficult to study.

The term coherent light source is more applicable to the double slit experiment, where the light diffracted from the two slits are coherent. In this context, it means that the electromagnetic wave leaving the slits are in phase.

You can think of light waves oscillating side by side in the slits, if one goes up, the other goes up as well. The crest and the trough will leave the slits exactly at the same time. The coherence of light diffracted from the double slit is necessary to produce the double slit interference pattern observed.

A google search defined coherent light source as being a light source of a singular wavelength and not multiple wave lengths in the context of physics- is this incorrect?

[wyzard]

A google search defined coherent light source as being a light source of a singular wavelength and not multiple wave lengths in the context of physics- is this incorrect?

Yeap you're right, having a single wavelength is one of the criteria for the light source to be coherent.The other is that they are in phase.

Here's the full definition I've found:
"Two sources of light are said to be coherent if the waves emitted from them have the same frequency and are 'phase-linked'; that is, they have a zero or constant phase difference."

From: http://schools.matter.org.uk/Content/Interference/coherent.html

They also provide a nice gif to illustrate this 

[Swagadaktal]

Yeap you're right, having a single wavelength is one of the criteria for the light source to be coherent.The other is that they are in phase.

Here's the full definition I've found:
"Two sources of light are said to be coherent if the waves emitted from them have the same frequency and are 'phase-linked'; that is, they have a zero or constant phase difference."

From: http://schools.matter.org.uk/Content/Interference/coherent.html

They also provide a nice gif to illustrate this 

+1 for gif

[FutureDoctor2k16]

2010 Exam 2, Question 6

Would my explanation have been sufficient?

Gradient = Planck's constant = a constant
Therefore, the magnesium and selenium graphs should be parallel
Therefore not C
The two lines cannot be collinear either otherwise magnesium and selenium graphs would have the same work function.
Therefore not D
Magnesium's Work function < Selenium's Work function.
Therefore, the answer is A.

[JellyBeanz]

2010 Exam 2, Question 6

Would my explanation have been sufficient?

Gradient = Planck's constant = a constant
Therefore, the magnesium and selenium graphs should be parallel
Therefore not C
The two lines cannot be collinear either otherwise magnesium and selenium graphs would have the same work function.
Therefore not D
Magnesium's Work function < Selenium's Work function.
Therefore, the answer is A.

Yeah that seems solid enough. Although might want to clarify why < work function results in the graph in this case. (value of y intercept smaller than Selenium's and same gradient)

[Gogo14]

Helen and Emily conduct the following experiment from a skyscraper. Helen drops a platinum sphere from a vertical height of 122 m while at exactly the same time Emily throws a lead sphere with an initial downwards vertical velocity of 10.0 m s−1 from a vertical height of 140 m. Assume g = 9.80 m s−2 and ignore friction.
a Determine the time taken by the platinum sphere to strike the ground.
b Calculate the time taken by the lead sphere to strike the ground.
c Determine the average velocity of each sphere over their respective distances.

how do you do b and c?
I got b=4.42s and c= 31.67ms for lead

Answer for b is 4.08s and for c for lead is 29.9ms.

[trystan.paderno]

how many decimal places should i correct to on the physics exam? they dont specify
also how do i know if my calculator is truncating answers and will this cause problems in the exam if it is? and how do i fix it if so? im using the casio fx-82AU plus
 

[Swagadaktal]

In Young's double slit experiment, why does the intensity of the bright fringes decrease as the path difference increases?

Can someone confirm whether this is within the scope of the SD?

But basically intensity = power/ area, both waves are travelling a greater distance so their intensity decreases