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March 29, 2024, 01:14:58 am

Author Topic: HSC Physics Question Thread  (Read 1030444 times)  Share 

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beau77bro

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Re: Physics Question Thread
« Reply #2070 on: April 24, 2017, 01:42:31 pm »
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I'm just trying to work a bit more on my understanding. would i be correct in saying that superconductivity is viable for the transmission of electricity as the action of dropping the resistance to zero removes any power loss as per P = I2R. but would prevent a conductor from doing work, as without resistance there is no potential difference, which is a measure of the work done between two points. so a superconducting light would be pointless, as resistance creates the heat which produces a given light.

 But how would this apply to say an electric motor? the lack of resistance would change what exactly?

also does anyone know any good bcs explanations? a youtube video or explanation on line would be great thankyou

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« Last Edit: April 24, 2017, 04:35:52 pm by jamonwindeyer »

jamonwindeyer

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Re: Physics Question Thread
« Reply #2071 on: April 24, 2017, 04:47:43 pm »
+1
I'm just trying to work a bit more on my understanding. would i be correct in saying that superconductivity is viable for the transmission of electricity as the action of dropping the resistance to zero removes any power loss as per P = I2R. but would prevent a conductor from doing work, as without resistance there is no potential difference, which is a measure of the work done between two points. so a superconducting light would be pointless, as resistance creates the heat which produces a given light.

Exactly correct on all counts ;D

Quote
But how would this apply to say an electric motor? the lack of resistance would change what exactly?

Hmm, that's an interesting question! I believe that using a superconductor for the coil wouldn't actually achieve the desired effect, because superconductors exclude magnetic flux. This means that you don't get forces on current carrying wires in a magnetic field - Because the current doesn't interact with the magnetic field. I could be wrong there - Perhaps the Meisner effect would manifest in such a way as to create the force by some other means. I'm honestly not sure!

What we can (and do as of fairly recently I believe) use superconductors for in motors is to generate the magnetic fields. Superconductors, having zero resistance, are an extremely efficient way to generate powerful magnetic fields - Because they can carry HUGE amounts of current ;D and that is with, again according to the formula, no power loss!! ;D

Quote
also does anyone know any good bcs explanations? a youtube video or explanation on line would be great thankyou

I wrote a guide on Superconductors here - The BCS theory is really tough to understand just from reading though. Not sure of any good video explanations - I reckon some of the Notes in the free notes section would have good diagrammatical ones! Otherwise, a handy Youtube search will set you straight I am sure ;D

Edit: Rathin made some recommendations here ;D

smile123

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Re: Physics Question Thread
« Reply #2072 on: April 26, 2017, 03:19:10 pm »
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HELP PLEASE!!! :)

jamonwindeyer

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Re: Physics Question Thread
« Reply #2073 on: April 26, 2017, 03:20:24 pm »
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HELP PLEASE!!! :)

Hey smile123! This definitely isn't a question you need to know how to do in HSC Physics - Where did you find it? ;D

Rathin

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Re: Physics Question Thread
« Reply #2074 on: April 26, 2017, 09:11:37 pm »
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How to do Part C?
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Re: Physics Question Thread
« Reply #2075 on: April 26, 2017, 09:22:15 pm »
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How to do Part C?

Hey Rathin! Can't quite step through the Math right this second, but I reckon you'd be okay with a breakdown:

- The mass being placed puts a force on the coil due to its gravitational acceleration. Call this \(F_m\), and \(F_m=mg\).
- This force is applied tangentially to the rotation of the coil, so produces a torque. \(\tau=F_md=mgd\) where \(d\) is the 'radius' of the coil, the distance from the centre, 0.4 metres.
- You need to produce an equal and opposite torque to balance the torque caused by the mass, and thus hold the mass in place. Torque due to a motor is \(\tau=BAIn\) (the coil is parallel to the field so we don't need the cosine term, \(\cos{90}=1\)). So you are solving the following equation for \(I\):



You have everything you need in terms of values there I think! Hope this helps ;D

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Re: Physics Question Thread
« Reply #2076 on: April 28, 2017, 10:16:23 pm »
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To be honest, I have no idea how to approach this question:

Negatively charged latex spheres are introduced between two charged plates and are held stationary by the electric field. Each sphere has a mass of 2.4 × 10^−12 kg and the strength of the field required to counter their weight is 4.9 × 10^7 NC^−1. Sketch this arrangement, identifying the positive and the negative plate, and determine the charge on the spheres.

I drew the diagram (I think...), but I don't know which formula to start with given the information...  :-\
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pikachu975

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Re: Physics Question Thread
« Reply #2077 on: April 29, 2017, 02:15:41 am »
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To be honest, I have no idea how to approach this question:

Negatively charged latex spheres are introduced between two charged plates and are held stationary by the electric field. Each sphere has a mass of 2.4 × 10^−12 kg and the strength of the field required to counter their weight is 4.9 × 10^7 NC^−1. Sketch this arrangement, identifying the positive and the negative plate, and determine the charge on the spheres.

I drew the diagram (I think...), but I don't know which formula to start with given the information...  :-\


So basically you draw horizontal parallel plates first and a sphere in between it (as it is falling down). Now, you consider gravitational force because it is falling down, F = mg. The electric plates HOLD IT STATIONARY as given, so it must be attracted to the top plate. Since the negatively charged spheres will attract to the positive plate, the positive plate is up, because it would counter the downward force due to gravity.

F = mg
F = 2.4x10^-12 x 9.8
F = 2.352 x 10^-11 N

Now to counter this downward force, the electric field exerts the same force upwards. We can now just sub into E = F/q to find the sphere's charge.

E = F/q
q = F/E
q = 2.352x10^-11 / 4.9x10^7
q = 4.8 x 10^-19

Therefore the top plate is positive, bottom plate is negative, and the sphere's charge is -4.8x10^-19 (negative because it's given).

Hope this helped!

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Re: Physics Question Thread
« Reply #2078 on: April 29, 2017, 04:15:42 pm »
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How are triodes exactly used for amplification ?
:3

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Re: Physics Question Thread
« Reply #2079 on: April 29, 2017, 04:40:56 pm »
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How are triodes exactly used for amplification ?

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!
« Last Edit: April 29, 2017, 04:45:42 pm by jamonwindeyer »

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Re: Physics Question Thread
« Reply #2080 on: April 30, 2017, 10:31:23 am »
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heeey i was wondering if someone could please confirm my understanding-

so when a car accelerates, we push on the accelerator, and the driving force increases, and because the driving force is more than the resistive forces, there is a net force to accelerate it?

alsooo, when a car slows down- we push the brakes, the friciton from the brakes slows the wheels? im confused because (its hard to word it) but like, what about driving force? does driving force become zero when braking? and does driving force basically control the speed of the wheels? :/

sorry for writing in such a messy way, any help is appreciated :)
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gilliesb18

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Re: Physics Question Thread
« Reply #2081 on: May 01, 2017, 10:00:49 am »
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Hello- Can someone please help me with this one??? I've had someone explain it to me already :-[ (not on this forum) but for some reason i forgot how to do it!!!
Thanks in advance!!!
Sam is on his push bike riding at 4 m/s. He comes across a huge hill, with a gradient of 50 degrees, and realises his breaks don’t work. He goes speeding down the hill in a panic for and minute and two seconds, until he comes face to face with a brick wall. If he, plus his bike has a mass of 70 kg, and there was a friction force of 113N, then what was his velocity before he hit the wall?

jamonwindeyer

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Re: Physics Question Thread
« Reply #2082 on: May 01, 2017, 11:19:18 am »
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Hello- Can someone please help me with this one??? I've had someone explain it to me already :-[ (not on this forum) but for some reason i forgot how to do it!!!
Thanks in advance!!!
Sam is on his push bike riding at 4 m/s. He comes across a huge hill, with a gradient of 50 degrees, and realises his breaks don’t work. He goes speeding down the hill in a panic for and minute and two seconds, until he comes face to face with a brick wall. If he, plus his bike has a mass of 70 kg, and there was a friction force of 113N, then what was his velocity before he hit the wall?

Hey gillies! Welcome to the forums! ;D

This is a really tough question for HSC level! But here we go ;D

So we are considering forces along the direction of the hill. So, we need to see what component of gravity will act along the hill (picture a vector at 53 degrees to the horizontal, then another perpendicular to that forming a right angled triangle with gravity as the hypotenuse. We'll call this \(F_h\):



Now, we have a frictional force acting against this, so the total force acting down the hill:



Now this force acts for 1 minute and two seconds, starting at a velocity of 4 metres per second. We know the acceleration will be given by:



So we can use the velocity formula:



Now I may have stuffed up some maths above (I'm procrastinating in a lecture, lol) but I get about 370 metres per second, exceeding the speed of sound. This doesn't quite seem sensible, but the question seems a little 'humorous' so maybe! If it is the answer, Sam is... Not in a good state ;)

Does this make sense?

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Re: Physics Question Thread
« Reply #2083 on: May 01, 2017, 05:13:26 pm »
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Hello! :)
I've got a question regarding one of the physics syllabus dot points (3.2.1)

What would be the best example to this dot point: "Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current."

Could you speak about the experiment that involves moving a magnet in and out of a solenoid that is hooked up to a galvanometer. Because aren't you technically producing an alternating current?
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Re: Physics Question Thread
« Reply #2084 on: May 01, 2017, 06:04:43 pm »
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Hello! :)
I've got a question regarding one of the physics syllabus dot points (3.2.1)

What would be the best example to this dot point: "Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current."

Could you speak about the experiment that involves moving a magnet in and out of a solenoid that is hooked up to a galvanometer. Because aren't you technically producing an alternating current?


Yep, you're 100% right; that's the perfect practical task to use! The Galvanometer clearly shows that the current changes directions, so you've produced AC current :)
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