# HSC Physics Marathon 2016 (1 Viewer)

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#### leehuan

##### Well-Known Member
According to Faraday's Law a change in magnetic flux will induce emf as according to the formula emf = (ceebs latexing formula). As current does not reach maximum instantaneously, there is a brief building up period. According to the relationship B = kI/d, current is proportional to B, and hence as the current builds up (therefore changes) similarly there is a change in magnetic flux inducing emf on the phone. As the build up interval is very small, the change in magnetic flux would be very high and hence the repeated emf induced on the phone would be very high, which could lead to damage of the phone's internal components. This will occur both when the charge is turning on and when it is turning off, meaning that the emf is being repeatedly applied to the phone, potentially causing damage to the internal components.
An interesting thing to keep in mind is that the semiconductors (integrated circuits) in phones don't really want high voltages. Indeed though, correct

#### Drsoccerball

##### Well-Known Member
An interesting thing to keep in mind is that the semiconductors (integrated circuits) in phones don't really want high voltages. Indeed though, correct
I think it's the opposite high voltages are better. If there is a low voltage the current is higher and thus has a higher chance of causing the semi-conductor to no longer possess its super conductive properties.

#### leehuan

##### Well-Known Member
I think it's the opposite high voltages are better. If there is a low voltage the current is higher and thus has a higher chance of causing the semi-conductor to no longer possess its super conductive properties.
Typo here.
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Isn't the input power what is technically constant?

#### Nailgun

##### Cole World
I think it's the opposite high voltages are better. If there is a low voltage the current is higher and thus has a higher chance of causing the semi-conductor to no longer possess its super conductive properties.
Does this mean my answer was wrong lol?

#### Drsoccerball

##### Well-Known Member
Does this mean my answer was wrong lol?
No. Your answer is correct. Just pointed out something with what Lee said.

#### loje

##### New Member
Quick question:

A cannon shot was fired at an elevation angle of 40. A second shot was fired at the same speed, but at an elevation angle of 60. Compared with the first shot, what would the second shot expected to have:

A. shorter range
B. longer range
C. shorter time of flight
D. longer time of flight

I picked A, because if the 2nd shot was fired at the same speed but a higher angle wouldn't Ux be smaller (and hence range is smaller)? The answer is apparently D (not from textbook, just a teacher's answer)

Cheers

#### Drsoccerball

##### Well-Known Member
Quick question:

A cannon shot was fired at an elevation angle of 40. A second shot was fired at the same speed, but at an elevation angle of 60. Compared with the first shot, what would the second shot expected to have:

A. shorter range
B. longer range
C. shorter time of flight
D. longer time of flight

I picked A, because if the 2nd shot was fired at the same speed but a higher angle wouldn't Ux be smaller (and hence range is smaller)? The answer is apparently D (not from textbook, just a teacher's answer)

Cheers
Worst question in HSC physics multiple choice...

#### loje

##### New Member
Worst question in HSC physics multiple choice...
Why? Too easy?

Its actually from a school past paper and the answer might be wrong...

#### ml125

##### Well-Known Member
Why? Too easy?

Its actually from a school past paper and the answer might be wrong...
Nah the answer is right.. but A is right as well.

Lel currently in the car and was bothered doing this bc bored if it makes it clearer hahaha.

#### wu345

##### Member
$\bg_white I\quad am\quad testing\quad my\quad new\quad cannon.\quad \\ I\quad fire\quad the\quad cannonball\quad upward\quad off\quad a\quad 135m\quad tall\quad cliff\quad and\quad it\quad lands\quad 235m\quad from\quad the\quad base\quad of\quad the\quad cliff.\\ If\quad the\quad cannonball\quad has\quad a\quad range\quad of\quad 136m\quad when\quad it\quad is\quad level\quad again\quad with\quad the\quad cliff,\quad find\quad the\quad initial\quad speed\quad of\quad the\quad cannonball.$

#### loje

##### New Member
Nah the answer is right.. but A is right as well.

Lel currently in the car and was bothered doing this bc bored if it makes it clearer hahaha.

(image clipped)
Thank you haha

#### InteGrand

##### Well-Known Member
Quick question:

A cannon shot was fired at an elevation angle of 40. A second shot was fired at the same speed, but at an elevation angle of 60. Compared with the first shot, what would the second shot expected to have:

A. shorter range
B. longer range
C. shorter time of flight
D. longer time of flight

I picked A, because if the 2nd shot was fired at the same speed but a higher angle wouldn't Ux be smaller (and hence range is smaller)? The answer is apparently D (not from textbook, just a teacher's answer)

Cheers
$\bg_white \noindent A quick way to do this without calculation is to just compare the vertical components of the initial velocity of each shot (the horizontal component doesn't affect the time of flight). Since they have the same speeds and the second one was shot up at a higher angle than the first, the second one has a greater vertical component of initial velocity. Hence the second shot has a longer time of flight than the first. Hence the answer is (D).$

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#### Jeff_

##### Member
Hey just a quick question about something I wrote in the exam:

Would it be correct if I wrote that when a magnet is dropped through a copper pipe, the eddy currents (... and here I state that its from Lenz's Law) will make the magnet fall slower and kinetic energy is converted into heat? The question was basically asking to compare the magnet falling in a copper pipe, to the induction cooktop. (So here it is somewhat similar that they both produce heat but in different ways).

Thanks

#### Bestintheworld

##### Active Member
^ not too sure.

Discuss the effects on the environment of the development of AC generators. (6 Marks)

#### leehuan

##### Well-Known Member
^ not too sure.

Discuss the effects on the environment of the development of AC generators. (6 Marks)
Woah what... is this seriously a 6 marker?

I would've made it 6 if you also included social though..

#### Drsoccerball

##### Well-Known Member
^ not too sure.

Discuss the effects on the environment of the development of AC generators. (6 Marks)
This should be 3 marks.

#### Jeff_

##### Member
Hey just a quick question about something I wrote in the exam:

Would it be correct if I wrote that when a magnet is dropped through a copper pipe, the eddy currents (... and here I state that its from Lenz's Law) will make the magnet fall slower and kinetic energy is converted into heat? The question was basically asking to compare the magnet falling in a copper pipe, to the induction cooktop. (So here it is somewhat similar that they both produce heat but in different ways).
^ not too sure.
Just found this:

When a magnetic field moves through a conductor a current called an Eddy current is induced in the conductor due to the magnetic field’s movement. The flow of electrons in the conductor creates an opposing magnetic field to the magnet which results in damping of the magnet and causes heating inside of the conductor similar to heat buildup inside of power cords. The loss of energy used to heat up the conductor is equal to the loss of kinetic energy by the magnet.
(from http://thekidshouldseethis.com/post/75596084570)

#### leehuan

##### Well-Known Member
That's just conservation of energy, I think

Honestly I reckon the main similarity is that in both cases on top of Faraday's and Lenz's law it's eddy currents being induced cause you obviously don't have a complete circuit

Clearly, the application of the induced currents differs, as one is used for heating whereas the other is used to slow down something. But because the question says to compare you would've had to include that as well because compare implies give similarities AND differences

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#### Jeff_

##### Member
That's just conservation of energy, I think

Honestly I reckon the main similarity is that in both cases on top of Faraday's and Lenz's law it's eddy currents being induced cause you obviously don't have a complete circuit

Clearly, the application of the induced currents differs, as one is used for heating whereas the other is used to slow down something. But because the question says to compare you would've had to include that as well because compare implies give similarities AND differences
Yeah that would make sense that the main similarity is the eddy currents. I guess I went onto the more specifics, thinking how heat is produced in the induction cooktop and therefore somehow the teacher might want us to talk about heat and eddy currents with magnet/copper pipe.

#### Bestintheworld

##### Active Member
Woah what... is this seriously a 6 marker?

I would've made it 6 if you also included social though..
This should be 3 marks.
It was bloody 6 marks in the HSC 2011 lol

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