HSC Physics Question Thread

[jamonwindeyer]

Hey guys!
Just a quick question about "other consequences of Special Relativity": Does this mean time,length and mass dilations? What does it mean by some other consequences?
Thank you!:D

Definitely just means time dilation, length contraction and mass dilation  - As well as the relativity of simultaneity!

[f_tan]

Hi, can anyone help me with this question? I'm not sure what else to write, other than to check that information is consistent across a range of sources, but the question is a 3 marker:

Thank you!

[sidzeman]

Question 12 - Shouldn't the force on the parallel field be a curved graph, F = qvbsintheta - and theta is slowly rising to 90?

For the transformer question - the MG calculate power loss with the equation of P = I^2 R - I attempted it by calculating the power of the primary coil (VpxIP) and the power of the secondary coil (VsIs), and then subtracting the 2. Why doesn't this approach work as well?

Thanks in advance!

[winstondarmawan]

Just want to check if my understanding of this question is correct:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22290522_1344113975714122_1473090637_n.png?oh=5b467e24114901787583256d3160a4fa&oe=59DB1CEA
As the motor becomes faster, the back EMF reduces the current being fed into the motor and as torque is directly proportional to current, the answer is D.
However, I'm just wondering what happened to the traditional sin/cos curve that is used for torque.
Thanks in advance.

[Iminschool]

Question 12 - Shouldn't the force on the parallel field be a curved graph, F = qvbsintheta - and theta is slowly rising to 90?

For the transformer question - the MG calculate power loss with the equation of P = I^2 R - I attempted it by calculating the power of the primary coil (VpxIP) and the power of the secondary coil (VsIs), and then subtracting the 2. Why doesn't this approach work as well?

Thanks in advance!

The C.C.C is perpendicular to the magnetic field lines at all times and is thus at a maximum at all points in the journey (reverses every half cycle). Thus the answer should be C.

[Iminschool]

Hi, can anyone help me with this question? I'm not sure what else to write, other than to check that information is consistent across a range of sources, but the question is a 3 marker:
(Image removed from quote.)
Thank you!

I'd probably add that MRIs produce functional images where certain abnormalities may be detected ehich woukd otherwise not be viewable through structural images (such as those produced in X-rays)

[armtistic]

Just want to check if my understanding of this question is correct:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22290522_1344113975714122_1473090637_n.png?oh=5b467e24114901787583256d3160a4fa&oe=59DB1CEA
As the motor becomes faster, the back EMF reduces the current being fed into the motor and as torque is directly proportional to current, the answer is D.
However, I'm just wondering what happened to the traditional sin/cos curve that is used for torque.
Thanks in advance.

nothing's happened to the sine curve normally used, because that curve represents "Torque vs Time", in this case we have "torque vs motor speed" thats all

[Maraos]

Hello,
I would just like to make sure that my understanding of the wilson cloud chamber track paths is correct. My teacher didn't actually teach us this in class so I've had to research it.

Alpha particles: Leave a thick straight track
Beta Particles: Leave a thin line that curves at the end
Gamma rays: Leave short swiggly lines

I used this image as a reference: http://1.bp.blogspot.com/_FAgA6-CHYr8/SoggUi2sWYI/AAAAAAAAAKA/316jS1V70s0/s400/track+patten.bmp

Is this correct?

Thanks! 

** forgot to add this is for quanta to quarks**

[blasonduo]

Hi, can anyone help me with this question? I'm not sure what else to write, other than to check that information is consistent across a range of sources, but the question is a 3 marker:
(Image removed from quote.)
Thank you!

Your point is 100% correct, I would add checking the quality of the information, If it is from reputable sources such a .gov sites or peer-reviewed scientific journals, you'd know the information is accurate and reliable!

Also, a point would be to know when the article was published (ie sources from 1999 might not be as reliable as sources from 2017)

Hope this helps

[sidzeman]

Hey can someone explain stopping voltage to me and its relevant formula (max KE = qv) or something
Thanks!

[blasonduo]

Question 12 - Shouldn't the force on the parallel field be a curved graph, F = qvbsintheta - and theta is slowly rising to 90?

For the transformer question - the MG calculate power loss with the equation of P = I^2 R - I attempted it by calculating the power of the primary coil (VpxIP) and the power of the secondary coil (VsIs), and then subtracting the 2. Why doesn't this approach work as well?

Thanks in advance!

Hello!

For the transformer, Its because you aren't calculating power, you are showing a RATIO.

Which is odd, because, when you minus one from the other you should get 0. (when I calculated this, I got that)

Just remember that you are showing that one formula equals another;

VpIp = VsIS

If I have not made this clear, just remember equals sign means exactly the same! So it is the equivalent of

24 = 24

and if you minus one from the other, you will always end with 0

[bsdfjnlkasn]

Hey can someone explain stopping voltage to me and its relevant formula (max KE = qv) or something
Thanks!

Hey I just learnt this

Stopping voltage (V_s) - voltage required to stop current.

Remember when you were investigating the photoelectric effect at school (or if not, you can still read the following explanation and hopefully it'll make sense) you had a set up like this?



So the cathode here is the stopping electrode. We are able to make the cathode more and more negative, by increasing the voltage supplied by the external supply. This then begins to repel the photoelectrons (because like charges repel) and eventually, the flow of charge will be stopped. Hence the name stopping electrode. When it is high enough, the voltage will be such that electrons will be able to just reach the cathode but not flow into the external circuit. When this happens, the voltage being applied is known as the stopping voltage i.e. V = V_s.

Now onto making sense of: E_k(max) = qV_s

Recall that .

E_k(max) refers to the kinetic energy of the fastest moving photoelectron. The electrons which are furthest away from the nucleus are held the weakest and so require the least amount of energy to be released when a photon strikes the metal surface. When an incoming photon of light (carrying E = hf) has sufficiently high frequency, and so energy, these outermost electrons will be emitted with the greatest kinetic energy.

Spoiler

For further clarity and so, to continue: This is because the photon's energy is larger than the energy required to overcome the electrostatic attraction acting between the electron and nucleus. Because not all of the photon's energy is absorbed in the process of liberating the electron (it only needs enough to overcome the attractive force) it still has some energy left over which is then converted into kinetic energy. The outermost electrons will have the greatest amount of energy left after interacting with the incoming photon of light because the attractive force is smallest and so the energy required to overcome this force is the smallest.

At the stopping voltage, V_s, only the fastest moving electrons are reaching the (stopping) anode (note, they're just reaching, not flowing into external circuit so current reading is 0), because all the slower moving ones don't have enough E_k to overcome the large potential difference (i.e. they are repelled). These emitted electrons (photoelectrons) have the highest kinetic energy as:





Really hoped that helped convey a more conceptual explanation - it really puts the formulas into context and saves you the time of memorising the formula (plus it'll have more meaning to you too)

Let me know if you have any more questions

If you need me to, i'm happy to also explain the formula:

[bsdfjnlkasn]

Hey there,

I'm still a bit confused about how Bohr's model of the atom helped explained the hydrogen emission spectra so would like some clarification with close reference to his 3 postulates.

I'm struggling with piecing the 3rd postulate into it all (without de Broglie's wavelength equation)

[hobocop]

Hi, I am having trouble understanding this question. Could I get some help?

[pikachu975]

Hi, I am having trouble understanding this question. Could I get some help?

Using right hand grip rule the magnetic field is out of the page in the loop. Since the current is decreasing constantly then the magnetic field strength is decreasing, hence a magnetic field will be induced out of the page due to the loop to balance it. Hence the current will be anticlockwise using right hand grip rule.

Hey there,

I'm still a bit confused about how Bohr's model of the atom helped explained the hydrogen emission spectra so would like some clarification with close reference to his 3 postulates.

I'm struggling with piecing the 3rd postulate into it all (without de Broglie's wavelength equation)

1st postulate was that electrons existed in stationary states and possessed an unexplained stability. This means that the specific orbits of electrons meant that there is a line spectra rather than a continuous spectra which would occur if the electrons could orbit at any radii.

2nd postulate is that the transition between stationary states is accompanied by the emission or absorption of EMR. This basically explains how the spectral lines are produced as hydrogen is excited by heat/current causing emission of EMR which shows in the spectrum.

3rd postulate is the quantisation of angular momentum in multiples of h/2pi. This also shows how orbits are at fixed radii, explaining the line spectra rather than a continuous one.