HSC Physics Question Thread

[Sukakadonkadonk]

is nanometers x 10-6, whats 10*-9?

Millimetres is x10^-3
Micrometres is x10^-6
Nanometres is x10^-9

[scienceislife]

How was a standing wave produced by Hertz in his experiment?

[jamonwindeyer]

Hey Guys,

Does anyone know what the difference is between 1 rayl and 1 kg m^-2 s^-1
in terms of acoustic impedance? Can they be used interchangeably?

or is 1 rayl just equal to 10^6 1 kg m^-2 s^-1

Thanks.

Hey! They are interchangeable - The rayl is just a more convenient name for that unit of measure, \(kgs^{-1}m^{-2}\) - Kind of like how the Newton is interchangeable with \(kgms^{-2}\) (but of course we always use Newton, and just the same, we usually use Rayl, purely because it is easier)

[jamonwindeyer]

Hi,
Does anyone know how to write a 6 mark response for this hsc question, because I'm pretty stuck rn?
(and no I don't have the success one book)

Hey! So the sorts of things to cover here would be:
- The nature of p-type and n-type semiconductors, specifically the majority charge carriers for each. Just briefly, enough to explain how the diode works.
- What happens when you bring the PN junction together, specifically the formation of the depletion layer. Most of the response will probably be here.
- The photoelectric effect, and what happens when light shines on the P-type semiconductor
- The net result of the current flow from the above step

There's a lot to understand here, be sure to let us know if there's anything you don't quite understand

[jamonwindeyer]

How was a standing wave produced by Hertz in his experiment?

Hey! Basically, he directed the radio waves at a reflecting screen. When the waves reflected, they superimposed with the waves being sent to the screen. You don't really need to know the specifics of how, but basically this causes the waves to sort of, partially 'cancel' each other out, and you get the formation of a standing wave as a result!

[gilliesb18]

Ak so another question on forces....
Consider the object shown in the diagram. Several forces act on this object which is at rest on a smooth, horizontal surface. Calculate the net force on the object.
BTW if the left force is too hard to read, its 26 N

[jamonwindeyer]

Ak so another question on forces....
Consider the object shown in the diagram. Several forces act on this object which is at rest on a smooth, horizontal surface. Calculate the net force on the object.
BTW if the left force is too hard to read, its 26 N

Hey gillies! So the total force on this object will be \(26-6=20N\) to the right, and \(15N\) upwards. We use those two values to form a right angled triangle, with 20 on the horizontal and 15 on the vertical. The magnitude of the force will be the hypotenuse:



The direction of the force can be found by using simple right angled trigonometry:



This will cause the object to accelerate in that direction does this make sense?

[gilliesb18]

Can some one help me on a few wave questions??
a) How come loud noises from bass loud speakers can cause windows to rattle??
b) Solar hot water systems heat water
c) waves at the beach can knock you over
d) Some public telephones are powered by photovoltaic cells (whatever they are...)
Thanks!!!!
BG

Oh and thanks heaps for the last reply Jamon!!! It helps heaps!!!

Mod Edit: Post merge

[jamonwindeyer]

Can some one help me on a few wave questions??
a) How come loud noises from bass loud speakers can cause windows to rattle??
b) Solar hot water systems heat water
c) waves at the beach can knock you over
d) Some public telephones are powered by photovoltaic cells (whatever they are...)
Thanks!!!!
BG

Hey! So let's see:

a) I'm reasonably certain that what is happening could be for two reasons. First, human hearing gets less sensitive at lower frequencies. To hear it, you need a more intense sound wave and thus more air pressure. So it is possible that you are getting vibrations purely due to that greater air pressure. More likely though is that the frequency happens to correspond to the resonant frequency of the object.

All objects have a resonant frequency, which is a frequency they respond to more readily than others. Picture a park swing - You know how you settle into the rhythm? That's the resonant frequency of that swing. You can try and swing faster or slower, but it never works quite as well! Indeed, if you expose an object to sounds of its resonant frequency, then it will vibrate far more strongly at that frequency (note, objects always vibrate and rattle when exposed to sound, but we never perceive it unless it is strong).

This idea of resonant frequency is something that engineers have to take very seriously, because all objects do it. If you've not seen it, this video shows what ignoring resonant frequency can do to what you build.

b) Yes, they do! The workings of a solar panel specifically are reasonably complex, but oncoming light waves are converted into electrical energy, which is then converted into heat to radiate into the water!

c) Yes, they can! They carry energy/momentum, its a mass of water moving with a velocity, so there is kinetic energy and momentum there. Get hit, get bowled over if the force is large enough

d) Photovoltaic Cells = Solar Cells

[bsdfjnlkasn]

Hey there,

With the I2I outcome which focuses on whether cathode rays were charged particles or electromagnetic waves, who are the main profiles we need to know to accurately detail the development of the theory? So far I've got Hertz and Thomson but elsewhere I've read about Varley, Stoney, Goldstein and Helmholtz. Did these four make notable contributions that would be worth discussing in a question if one were to come up? The outcome doesn't exactly detail the historical development so it would be nice to have some confirmation as to where to head with the info for this outcome.

Any help would be greatly appreciated!

[jakesilove]

Hey there,

With the I2I outcome which focuses on whether cathode rays were charged particles or electromagnetic waves, who are the main profiles we need to know to accurately detail the development of the theory? So far I've got Hertz and Thomson but elsewhere I've read about Varley, Stoney, Goldstein and Helmholtz. Did these four make notable contributions that would be worth discussing in a question if one were to come up? The outcome doesn't exactly detail the historical development so it would be nice to have some confirmation as to where to head with the info for this outcome.

Any help would be greatly appreciated!

Hey! You don't actually need to know the personalities of the story; just the experiments here. So, know a few experiments that were used to 'show' that electrons were particles, a few experiments that were used to 'show' that electrons were waves, and the definitive experiments that prove it one way or the other!

[jamonwindeyer]

Hey there,

With the I2I outcome which focuses on whether cathode rays were charged particles or electromagnetic waves, who are the main profiles we need to know to accurately detail the development of the theory? So far I've got Hertz and Thomson but elsewhere I've read about Varley, Stoney, Goldstein and Helmholtz. Did these four make notable contributions that would be worth discussing in a question if one were to come up? The outcome doesn't exactly detail the historical development so it would be nice to have some confirmation as to where to head with the info for this outcome.

Any help would be greatly appreciated!

Hey! As Jake said, you'll never need the people that conducted the research into these properties, you only really need the properties themselves. But just adding that Hertz and Thomson are good to know because they link to other dot points, I wouldn't worry about remembering the rest either

[bsdfjnlkasn]

Awesome, thanks for the help!

I'm just looking at the formula F = qvBsina and am still unsure of what the designated angle is, perhaps a diagram might help but I understand if it'd be too hard to sketch so a written explanation is cool . Also, my textbook says that if a charged particle enters a uniform magnetic field at 90°, it will move in the arc of a circle. Is there a way to see if the charge enters at 90° or will we always be told this? Also, I don't entirely understand why this would happen, even if it's a minor detail, I would like to know why as it'll probably stick better that way.

Thank you!!

[jamonwindeyer]

Awesome, thanks for the help!

I'm just looking at the formula F = qvBsina and am still unsure of what the designated angle is, perhaps a diagram might help but I understand if it'd be too hard to sketch so a written explanation is cool . Also, my textbook says that if a charged particle enters a uniform magnetic field at 90°, it will move in the arc of a circle. Is there a way to see if the charge enters at 90° or will we always be told this? Also, I don't entirely understand why this would happen, even if it's a minor detail, I would like to know why as it'll probably stick better that way.

Thank you!!

Hey! So the diagram of the angle is attached below, it is the angle between the field lines and the direction of movement



You'll always be able to see that angle, if you aren't told it. It is very often 90 degrees usually it is the field going into or out of the page - If that happens, if the particle is moving left/right/up/down, or in any direction around the page, the velocity is always perpendicular to the field

The reason the particle moves in a circle if it enters at 90 degrees, is because the force on the object stays perpendicular to its motion. That's just like orbits when gravitational force is perpendicular to orbital velocity. What happens in that circumstance? It is uniform circular motion!

[Sukakadonkadonk]

Hi,

Does anyone know what the voltage time graph looks like for an AC generator which is turned twice as fast?

Thanks.