HSC Physics Question Thread

[jamonwindeyer]

Hi! I am having troubles doing the HSC Question in 2016 Q 30 b). (Sorry I can't post the question b/c for some reason it says the screenshot is too big to post. idk). I wrote about how the circuit was incomplete so the mass would fall faster but Idk the other reason for difference in behaviour.
Thanks

Hey! So you've pretty much got it. When the circuit is complete, the generator is powering the lightbulb. As the coil spins in the magnetic field, electromagnetic induction causes an opposing torque. This is what is resisting the fall of the mass, because the coil can't spin as quickly (thus limiting the speed the rope can unwind).

When the switch is open, no generator. No opposing torque. No restriction, the mass falls freely under gravitational forces

So your answer would probably just explain these things (probably referencing Lenz's Law as well)

[beau77bro]

help the answer is  1.03 x 10^-2 Nm and i have no idea how

[jamonwindeyer]

help the answer is  1.03 x 10^-2 Nm and i have no idea how

Hey! Formula for torque is:



Now the minimum torque is just when \(\theta=90\), the max is when \(\theta=0\). So we just ignore the cosine term:



Now \(N=1\), \(B=0.5\), \(I=1.5\). All we need is the area, which we can get from the radius:



Put it all together to find maximum torque (minimum should be zero):



Hmm, I can't match it either mate! Can anyone spot what we're missing?

[arunasva]

Im kinda confused help !!!! The answer's apparently B ?

Also in BCS conduction, how are phonons produced ? Is it that the two positive lattices repel and then get attracted to the electron so it vibrates and hence sound energy ?

Mod Edit: Post merge

[arunasva]

Tha

Hey! This is a really common question, and the way I always answer is, don't worry. It is a really complicated piece of Physics, I only learned it fairly recently in Electrical Engineering and it's still something I struggle with, have to rewatch videos or rescan notes a lot of the time.

In the HSC, they'll never assess you beyond knowing that it can be used for amplification. They'll never ask how

This is the best video I've ever seen that explains how a transistor works as a SWITCH (not an amplifier), and it covers a lot of HSC relevant material too!

Thanks

[bsdfjnlkasn]

Hey can I please get some help with the following (because I'm being expected to answer these questions before being taught )

We can detect light when our eyes received 2 x 10^-17 J of energy. How many photons of light, of wavelength 550nm, is this?

Any help would be appreciated! (I'm sure it's easy, I just don't know where to start)

Thank you!

[jakesilove]

Im kinda confused help !!!! The answer's apparently B ?

Hey! This questions asks us to understand the difference between mass and force.

Mass is an absolute(ish) quantity, that will be the same no matter where you are. Weight force, on the other hand, is the mass of an object times the acceleration due to gravity.

So, on Mars, the weight force would be


On Earth, the weight force would be



However, the weight itself doesn't change. Just the weight FORCE!

In BCS conduction, how are phonons produced ? Is it that the two positive lattices repel and then get attracted to the electron so it vibrates and hence sound energy ?

Sort of! However, this is definitely way beyond the curriculum. All you need to know is that a phonon is produced due to vibration energy; nothing more!

[jakesilove]

Hey can I please get some help with the following (because I'm being expected to answer these questions before being taught )

We can detect light when our eyes received 2 x 10^-17 J of energy. How many photons of light, of wavelength 550nm, is this?

Any help would be appreciated! (I'm sure it's easy, I just don't know where to start)

Thank you!

Hey! On the formula sheet, we are given the relation



Where E is the energy, h is Planck's Constant, and f is frequency. First, we need to convert wavelength into frequency.





Now, we can plug the frequency into our Energy equation





Finally, if we want 2*10^(-17)J,



Subject to mathematical error, but that's the method!

[itssona]

why does electric potential energy for a positive charge decrease when its in the direction of an electric field??

alsooo for Alternating current, would you say that this is a good definition:

 AC (Alternating Current) terminals periodically change their polarities. (I.e first terminal starts by being the positive one, then the other.) The AC terminals provide an alternating voltage. If a circuit is connected to the AC terminal, a current will be produced in which the charge carriers move backwards and forwards periodically.

is that basically what AC is?

thank you

[Syndicate]

why does electric potential energy for a positive charge decrease when its in the direction of an electric field??

alsooo for Alternating current, would you say that this is a good definition:

 AC (Alternating Current) terminals periodically change their polarities. (I.e first terminal starts by being the positive one, then the other.) The AC terminals provide an alternating voltage. If a circuit is connected to the AC terminal, a current will be produced in which the charge carriers move backwards and forwards periodically.

is that basically what AC is?

thank you

I can answer question 1. So basically, the positive charge is being attracted towards the negative charge, since it's going in the direction of the electric field (positive to negative), it loses Potential Energy, as it moves with no repulsion (instead it gains kinetic energy). On the other hand, if the positive charge was forced to go against a repulsive force, it would gain P. E (think of it as a compressed spring).

[bsdfjnlkasn]

Hey!

Can I get some help with the following:

A light beam has 270 photons passing a given point in one second. If this beam of light is rated as 2.23 x 10^-16W, calculate the frequency of this light beam.

Wouldn't it just be 270Hz? I don't understand what the light rating means and if there's a formula we can use considering no wavelength was given ...

Could someone please explain the connection between photons and energy. I also don't see how the number of photons can be relevant (as it has come in previous questions). Like this has been really poorly explained and i'm really confused - like I don't even know how to ask my question. Hopefully someone can help

Thank you!!

[jamonwindeyer]

alsooo for Alternating current, would you say that this is a good definition:

AC (Alternating Current) terminals periodically change their polarities. (I.e first terminal starts by being the positive one, then the other.) The AC terminals provide an alternating voltage. If a circuit is connected to the AC terminal, a current will be produced in which the charge carriers move backwards and forwards periodically.

is that basically what AC is?

thank you

Yep you've got it! It's just reversing the polarity of voltage (and thus direction of current) at a regular interval (with the number of times it switches back and forth per second being the frequency of the source/signal)

[jamonwindeyer]

Hey!

Can I get some help with the following:

A light beam has 270 photons passing a given point in one second. If this beam of light is rated as 2.23 x 10^-16W, calculate the frequency of this light beam.

Wouldn't it just be 270Hz? I don't understand what the light rating means and if there's a formula we can use considering no wavelength was given ...

Could someone please explain the connection between photons and energy. I also don't see how the number of photons can be relevant (as it has come in previous questions). Like this has been really poorly explained and i'm really confused - like I don't even know how to ask my question. Hopefully someone can help

Thank you!!

Hey! Don't worry, this trips a lot of people.

When considering EM waves in terms of photons, you can't think of frequency in the usual "the number of waves passing by per second" sense, because it gets confusing. Instead, think of it more abstractly as purely a property of the photons.

Photons are 'packets' of electromagnetic waves, each containing a set amount of energy. That energy is equal to \(E=hf\) - Each photon has an energy proportional to the frequency! The frequency affects energy per photon.

Analysing your question should help clear up the confusion. We have 270 photons passing per second, and the laser is rated for \(2.23\times10^{-16}W\). Now remember, 1 Watt is equivalent to 1 Joule per second. This means that those 270 photons coming past per second, must be delivering a total of \(2.23\times10^{-16}\) joules of energy. To get the energy per photon, divide by 270:



Then, to find the frequency, we use the energy formula for a photon, since we know that each photon must have an energy equal to that we found just then:



So the frequency has nothing to do with photons passing per second - It is instead determined based on the energy per photon required to deliver the specified amount of energy per second (and thus power) does this make sense?

I wrote this brief guide on all this Quantum Physics stuff you do for the HSC!

[itssona]

I can answer question 1. So basically, the positive charge is being attracted towards the negative charge, since it's going in the direction of the electric field (positive to negative), it loses Potential Energy, as it moves with no repulsion (instead it gains kinetic energy). On the other hand, if the positive charge was forced to go against a repulsive force, it would gain P. E (think of it as a compressed spring).

omg thank youuuu!!!

[Bubbly_bluey]

Hi! I'm having trouble visualising what Black body radiation is. (like what it physically is). Are they just normal objects that can absorb all radiations?
Also the graph that goes with it, intensity VS wavelength. I'm not completely understanding what it all means.
Basically the whole concept of black body radiation is confusing.:/
Thanks