HSC Physics Question Thread

[jakesilove]

Heey would someone be able to see if my answer makes sense?? Im in prelim so i dont think i have to go in depth with the motor effevt thing but here it is

Explain how curcuit breakers work

Electricity magnetises the electromagnet (in electromagnets a magnetic field is produced by an electric current). Moving charge i.e. current in a magnetic field experiences EMF and thus whrn the current is too high, the electromagnet has a strong EMF to pull down a lever connected to the switch.

Hey! Your answer is good; I would go back through and read it out loud, just to clean up the phrases etc. However, since I don't know how many marks this question is, it's sort of hard to judge whether you've included enough information. However, you've certainly got the correct information! You might add a sentence on why they are important?

[itssona]

Hey! Your answer is good; I would go back through and read it out loud, just to clean up the phrases etc. However, since I don't know how many marks this question is, it's sort of hard to judge whether you've included enough information. However, you've certainly got the correct information! You might add a sentence on why they are important?

Ahh okay im glad i understand it well - thank you Jake!!:)

Ill certainly add that too

[beau77bro]

Thanks jamon (from ages ago)

Also is this seriously what happens, I thought the "overlap" was just conceptual for band gaps so that we could understand it. Is this just worded poorly?

[jakesilove]

Thanks jamon (from ages ago)

Also is this seriously what happens, I thought the "overlap" was just conceptual for band gaps so that we could understand it. Is this just worded poorly?(Image removed from quote.)

Nope, the actual valence bands in metals overlap, insofar as ALL valence bands are connected, allowing the electrons orbiting around the nucleus to move freely from one nucleus to another.

[Bubbly_bluey]

Hey guys! having trouble with this question. I was able to deduce possible answers to B or D.
answer:

Spoiler

B

Also quick question. If an exam questions asks to draw the transmission lines tower what would be a sufficient way of drawing it with all the relevant parts? Also what is it meant by "arcing"?
Thanks

[yuriques]

Hey! If you have a 5 mark question asking you to assess XXXXXXXX on Society and the Environment, I would make sure to split up your discussion of the two (Society and the Environment) approximately equally. Perhaps you'd give three advantages/disadvantages for Society, and two for the Environment, but like I wouldn't go overboard with one and minimise the other.

I would also ALWAYS have as many advantages as disadvantages. That's a method I've personally stuck with; would be interested to hear what others do.

When it comes to your table specifically, I would be drawing two: one for Society (advantages in one column, disadvantages in another, taking up 4 lines in total. One is the title, three are advantages/disadvantages) and one for the Environment (advantages in one column, disadvantages in another, taking up 3 lines in total. One is the title, two are advantages/disadvantages).

Does that sort of make sense? If you do the above, and are still struggling, post up a picture of your answer for us to mark!

Jake

Ok then! I guess I'll go look for some more disadvantages!
Can I ask how detailed should the points be to fit up a single line each? Are they just key words like "long distance transmission possible"? I'm worried that it wasn't a detailed enough answer so I would wrote "AC could be stepped up or down for more efficient, long distance transmission. By stepping up voltage, the current could be reduced (P=IV) and subsequently power loss was reduced (Ploss=I^2R). Thus price of electricity is dropped" and that went over 8 lines for one point...
Other than that, I'll redo it and see how it goes

[winstondarmawan]

Hey guys! having trouble with this question. I was able to deduce possible answers to B or D.
answer:

Spoiler

B

Lenz Law states that the EMF will be induced in a manner that opposes the initial change.
In a generator, the initial change is the rotation. So the current (due to EMF) will be induced so that it opposes the initial change.
Let's look at B. As the generator rotates clockwise, the current generated will be generated so that it moves anticlockwise.
Using the right-hand palm rule, we can see that the current moves up and ends at terminal X, making thus making it have a "positive" voltage (remember it is the potential difference). This matches with the graph.
(If I'm wrong someone please correct me)

[jamonwindeyer]

Lenz Law states that the EMF will be induced in a manner that opposes the initial change.
In a generator, the initial change is the rotation. So the current (due to EMF) will be induced so that it opposes the initial change.
Let's look at B. As the generator rotates clockwise, the current generated will be generated so that it moves anticlockwise.
Using the right-hand palm rule, we can see that the current moves up and ends at terminal X, making thus making it have a "positive" voltage (remember it is the potential difference). This matches with the graph.
(If I'm wrong someone please correct me)

I'm intrigued by this question and answer, because I think the answer is D (maybe it is 1:21am and I'm missing something but I'll throw this out there nonetheless)

Your working is totally correct right until the end Winston (note we'll want B or D because those are slip rings and we need them for AC) - But if we have current flowing from Y to X, that means the voltage at Y is higher. Current flows from points of a higher potential to points of a lower potential (remember back to basic circuit theory). For this reason, I'd go for D!

Would love for someone to weigh in - Especially if it means calling me out on a mistake (people never do this, and I want them to!)

[jamonwindeyer]

Also quick question. If an exam questions asks to draw the transmission lines tower what would be a sufficient way of drawing it with all the relevant parts? Also what is it meant by "arcing"?
Thanks

Ok then! I guess I'll go look for some more disadvantages!
Can I ask how detailed should the points be to fit up a single line each? Are they just key words like "long distance transmission possible"? I'm worried that it wasn't a detailed enough answer so I would wrote "AC could be stepped up or down for more efficient, long distance transmission. By stepping up voltage, the current could be reduced (P=IV) and subsequently power loss was reduced (Ploss=I^2R). Thus price of electricity is dropped" and that went over 8 lines for one point...
Other than that, I'll redo it and see how it goes

I think the detail you are providing there is good detail - The use of formula particularly is nice. Perhaps just adjust your phrasing, so I'd say that like:

Ability to transform AC power allows power loss to be minimised by minimising current (\(P=I^2R\)), thus increasing efficiency and reducing the cost of electricity

Though in truth, if the question is asking for impact on society and environment, even the formula may not be worth your space. Just saying:

Ability to transform allows reduced power loss, maximising efficiency, reducing energy cost and enabling distribution of power to remote regions to improve quality of life

This is more society focused, which is more your aim

[ekhan_01]

Hi Jake 

I'm confused as to finding the value of max torque. The answers say it is 1.03x10^-2 Nm but i keep getting 5.89x10^-3 Nm.

Perhaps I've done something wrong but a second answer on this would much be appreciated

Thank You

[jakesilove]

Hi Jake 

I'm confused as to finding the value of max torque. The answers say it is 1.03x10^-2 Nm but i keep getting 5.89x10^-3 Nm.

Perhaps I've done something wrong but a second answer on this would much be appreciated

Thank You

Hey! The formula for torque is



Here, our area is



Note that I have converted from cm to m: this is absolutely crucial to getting the answer right, and COULD be where your error comes from. Subbing all of this into our equation, we get



Whiiiiich is what you got. Great job on that conversion. I suspect the answers are wrong!

[ekhan_01]

Hey again!

Thx for the prev answer gr8 reassurance

I'm just wondering how do you do b? (answer is anticlockwise) & I have no clue going abt this question

Ty

[kiwiberry]

Hey again!

Thx for the prev answer gr8 reassurance

I'm just wondering how do you do b? (answer is anticlockwise) & I have no clue going abt this question

Ty

Hello!
Here we have to use the right hand palm rule to determine the force on either side of the coil. Fingers represent the direction of the magnetic field, thumb points in the direction of current and your palm will face the direction of the force. On the left, current is going into the page and magnetic field is going right, so point your thumb into the page with your fingers pointing right - you'll end up with your palm facing downwards. This means that the left side of coil will experience a force downwards. Similarly, on the right, current is going out and the field is in the same direction, so using the same rule again you'll find that the right side will experience a force upwards! Together, these forces will make the coil spin anticlockwise.

[yattmoani]

Hey guys! I was doing a past paper today and I just couldn't wrap my head around these 3 questions. Loving this thread and thanks for the help guys!

Correct answers are:
8 - B
9 - D
19 - A

[kiwiberry]

Hey guys! I was doing a past paper today and I just couldn't wrap my head around these 3 questions. Loving this thread and thanks for the help guys!

Correct answers are:
8 - B
9 - D
19 - A

Hey!
8- The force on a charged particle in a magnetic field is given by F=qvBsintheta, where theta is the angle between the direction of the field and the particle's motion. Looking at B, the particle's motion is parallel to the magnetic field, hence theta=0 and F=0. Therefore this should be the one which exerts the least force because there is none! As a note, the force on a charged particle due to an electric field is given by F=qE, so the velocity of the particle will not affect the force of the electric field exerts on it.

9- First we need to find the force that P and Q exert on R. P and R are parallel, meaning that P will exert an attractive force on R towards the left
However, Q and R are anti-parallel, meaning that Q will exert a repulsive force on R towards the right
To find the net force, we need to add these forces together, keeping in mind that one of the forces must be negative as they are in opposite directions. Letting right be the positive direction.
19- B is a fact, but it doesn't explain why the astronaut stays in orbit. C is incorrect as the force due to gravity takes the astronaut's mass into account, which is clearly different to the spacecraft's. D is also incorrect as \(a=\frac{GM}{r^2}\), meaning that their accelerations are independent of their individual masses. Their acceleration is only dependent on the mass of the central body and the distance they are from its centre of mass, which are both the same, so A is correct.

Hope this helps!