HSC Physics Question Thread

[Bri MT]

A puzzle says 1000 pieces...?

https://puzzel.org/en/jigsaw/play?p=-MSuld_tea2uknHFln2-

All of this work will pay off.

[david.wang28]

The wheel has 22 cuts on it. We also know the radius of the wheel to be 0.250 metres. From the radius, we can find the diameter of the wheel, and hence the distance between 2 cuts (assume they are equally spaced).

This value can then be used for part b

In c, wencan consider the time taken for the light to travel to the mirror and back & use that it needs to pass through a slit both times to find the period


Let us know how you go after these hints and where/if you get tripped up

Not sure which formulas to use?

[jamonwindeyer]

Not sure which formulas to use?

The first one just needs your formula for the circumference of a circle, \(C=2\pi R\). The second one is the tangential velocity formula:



And finally, the last one will also use the tangential velocity formula, as well as the basic \(v=\frac{d}{t}\). What you are essentially doing is figuring out how fast the wheel needed to be spinning to allow light travelling at \(3.15\times10^8\) to pass back through on the adjacent slit. Of course, we know the light wasn't travelling that fast, but that was the measurement so we do the maths based on that

(a) and (b) are fairly easy, but C is tricky! Have a crack at it though and if not I can step you through it

[david.wang28]

The first one just needs your formula for the circumference of a circle, \(C=2\pi R\). The second one is the tangential velocity formula:



And finally, the last one will also use the tangential velocity formula, as well as the basic \(v=\frac{d}{t}\). What you are essentially doing is figuring out how fast the wheel needed to be spinning to allow light travelling at \(3.15\times10^8\) to pass back through on the adjacent slit. Of course, we know the light wasn't travelling that fast, but that was the measurement so we do the maths based on that

(a) and (b) are fairly easy, but C is tricky! Have a crack at it though and if not I can step you through it

For c), I used v = d/t and subbed in the values to get 6.35*10^-5 secs. I then multiplied that value by 22 to get 1.4*10^-3 secs (there are 22 cuts in the wheel for the light to travel through). Is this the right answer?

[jamonwindeyer]

For c), I used v = d/t and subbed in the values to get 6.35*10^-5 secs. I then multiplied that value by 22 to get 1.4*10^-3 secs (there are 22 cuts in the wheel for the light to travel through). Is this the right answer?

Unless I'm mistaken, I think that is bang on nice work!

[david.wang28]

Unless I'm mistaken, I think that is bang on nice work!

Thanks Jamon! Means a lot coming from you

[david.wang28]

Hello,
I'm having trouble answering this question in the link below. Can anyone help me please? Thanks

[r1ckworthy]

Hey all,

This is question 19 from HSC 2017, and it is really stuffing me up. I'm not quite sure how the right hand palm/ slap rule works in this case, so I hope you guys could explain it to me.

[zhudiac]

Hi,

At point A and C, the wire is being swung downwards which moves the conductor through equipotential lines; i.e. the B-field strength changes as the conductor is swung at points A and C, as can be seen in the Earth diagram. This is not the case for B and D, where the wire moves along equipotential lines (into/out of the page) and thus moves such that the B-field strength does not change. Thus, we can immediately eliminate B and D, as current is induced when there is a change in flux over change in time; which only occurs during A and C, by Faraday's Law.

To determine between A and C, we must use Lenz's law. By Lenz's law, the induced current must oppose the change in B-field which caused it; or to oppose the direction of motion. Thus, by applying right hand palm rule, we can see that by applying a current from P to Q at C (which interacts with the external Earth's B-field), a force is generated so as to oppose the direction of motion (which is down the page). Thus, it must be C and not A that produces a positive current from P to Q. Thus the answer is C.

[jamonwindeyer]

Hello,
I'm having trouble answering this question in the link below. Can anyone help me please? Thanks

Hey! Gleaming context from the question, essentially what this refers to is the absorption spectrum of the star! The frequencies of the lines represent energy differences in the atoms of the star, which can help characterise their elemental composition. Further, the density of the star can be gleamed from the width of the spectral lines (I explain this more here, but it is probably unnecessary to understand)

[Crystine_16]

Can someone please explain to me how AC induction motors work. I've spent nearly five hours on youtube and websites trying to understand it but I srsly can't

Thanks in advance

[fun_jirachi]

Not too sure if this is how you explain it, but this is how I learned.

Basically, current induces a magnetic field. An AC motor produces an alternating current, so it produces a constantly shifting magnetic field by RHGR. Then because the magnetic field is constantly moving, it induces a change in flux over a period of time (because the field is always changing! By the definition of flux, which is magnetic field permeating an area, if the field is changing while the area that gets permeated is the same, the flux changes!). By Faraday's Law, we get an induced EMF, which again induces a potential difference. This potential difference forms a current proportional to the resistance of whatever it flows through, and then you get an induced current flowing from the current.

Hope this helps

[jamonwindeyer]

Can someone please explain to me how AC induction motors work. I've spent nearly five hours on youtube and websites trying to understand it but I srsly can't

Thanks in advance

Hey! Jirachi's response above is great, it basically highlights a bit of a process. You need to understand each step before you move to the next:

1. Three phase AC input to stator creates rotating magnetic field
2. Rotating magnetic field creates changing magnetic flux for rotor
3. EMF (and so current) is generated in the rotor via electromagnetic induction
4. Force produced due to this current interacting with the field (motor effect)

If you were having trouble with a specific step here, we might be able to hone in a bit more

[david.wang28]

Hello,
I wanted to ask why Young's experiment had two slits instead of one. Can someone clarify this please? Thanks

[jamonwindeyer]

Hello,
I wanted to ask why Young's experiment had two slits instead of one. Can someone clarify this please? Thanks

Hello! Young's experiment was aiming to demonstrate the interference and diffraction of light waves. For that to happen, you need two light 'sources.' That's what the two slits approximate - Two light sources which then interfere with each other to produce the interference patterns Young observed in the experiment!