HSC Physics Question Thread

[smiley2101]

last one!! haha

[jakesilove]

last one!! haha

Hey Smiley!

The answer to this multiple choice is going to be C. We can figure this out by process of elimination: gravity is actually the MAIN force acting on any spacecraft orbiting our planet, as it causes the orbit to occur in the first place (the shuttle is dragged down to earth, but is traveling so fast that it 'misses' the ground due to the curvature of the planet!), so the answer can't be A. The 'orbiting around the Sun' etc. thing doesn't even make any sense, and the whole 'inversely proportional' relationship thing would require some sort of mathematics that told you WHEN the astronaut would stay with the spacecraft, and when it would not. As the forces on the two objects are the same (namely, gravity), there is no reason that their ACCELERATION would be different!.

Remember, gravity has units that are meters per second, per second. There is no 'kilograms' or anything like that in the units, meaning that it operates INDEPENDENTLY of the mass of the object!

Jake

[jakesilove]

why is the answer C not D? thank you so much

For this question, you need to look at the formula for Force on a current carrying wire: namely



By creating a formula of I versus B, it is pretty clear that there will be an inverse (hyperbolic) relationship.



Therefore, the relevant graph will be the hyperbolic one!

Jake

[jakesilove]

I always stuff up the frame of reference like in this question. Do you have any tips or tricks that you used to know what tv or to should be substituted by?

The main trick I use is to first think about whether your value should be getting smaller or bigger. If you are dividing a number by the whole square root thing (which will equal less than one), then you are making the number bigger. If you are multiplying by the whole square root thing, you are making the number smaller. Therefore, if you are given a time and you are expecting it to get bigger, sub it into whichever time gets divided by the square root factor. If you get a length and you are expecting it to get smaller, sub it into whichever length gets multiplied by the square root factor!

Jake

[jakesilove]

why is the answer B?

thank you!

We know that the force between Q and P is F newtons. The question wants us to find the total force on R.
The relevant formula we need to use is



When the distance double, the force will clearly halve. Therefore, the force on R due to P will be half that of Q due to P, and in the opposite direction (as the currents have switched directions). Therefore, the force on R due to P will be attractive.

The force on R due to Q will be the same as the force on Q due to P. Therefore, the force on R due to Q will be repulsive.

So, the total force will be repulsive, or to the right.

Jake

[jakesilove]

sorry I have so many questions!!!
I just don't get it haha

why is the answer D and not c?

Remember that the output voltage is going to be proportionate to the speed of the rotation (ie. the change in flux), as per Faraday's law. Therefore, the faster the thing is moving, the greater the output voltage will be! If it is slowed down, therefore, the output voltage has to decrease, making the answer D (note also that the amount it is slowed down is proportionate to the amount that the voltage decreases, ie. a factor of half).

Jake

[jakesilove]

Also, as a final note, welcome to the forum! Hope we are able to help out as much as possible

Jake

[jamonwindeyer]

Also, as a final note, welcome to the forum! Hope we are able to help out as much as possible

Jake

For future reference to all forum users, this man is a machine 

[jakesilove]

For future reference to all forum users, this man is a machine 

I have to procrastinate writing up my Physics lab somehow! But thank you Jamon, one of the few times I could have a crack at some Physics questions before you've snapped them all up with beautifully LaTexed responses.

[jamonwindeyer]

I have to procrastinate writing up my Physics lab somehow! But thank you Jamon, one of the few times I could have a crack at some Physics questions before you've snapped them all up with beautifully LaTexed responses.

I see you are trying your hand at LaTex yourself! It will change your life!

[brontem]

Hey I'm slightly confused about this question, I kinda get it, kinda don't.. and the answer in the book is really vague. Can anyone help me?? Thanks!!

[FallonXay]

Hiya!

I was wondering (In regards to the photoelectric effect), If a metal surface is hit with an incident EMR above the threshold frequency, electrons are emitted. What happens to the missing electrons in the metal? Are they replaced? How so?

thanks.

[jamonwindeyer]

Hey I'm slightly confused about this question, I kinda get it, kinda don't.. and the answer in the book is really vague. Can anyone help me?? Thanks!!

Hey Brontem!! Don't worry, I hated these questions in the HSC, and I think a lot of people do, because a lot of books give vague answers for these sorts of things 

I actually think the question is vague in this case as well, since the direction of the magnetic field is not given. The plates are identical except for their orientation, so the direction of the field makes a pretty big difference.

Essentially though, the principle is this. As you may have guessed, the aluminium plates being dropped in the magnetic field will generate eddy currents. These eddy currents will decelerate the plates, because, by Lenz's Law, the induced currents will oppose the change that created them. Therefore, dropping the plates will cause an induced current which pushes them back up (opposes the changes). This isn't enough, and the plate still falls, but they will both fall at a slower rate.

The question is, what role do those slots play? The closest comparison is probably laminations in the core of a transformer. Essentially, those gaps limit the size of the induced eddy currents in the plate, if they are at the appropriate orientation.

The way to tell how much slots/gaps in the conductor will impede eddy currents is to draw the magnetic field lines through it. The diagram below shows the way the eddy currents are affected by slots. Smaller circles = Less Eddy Currents = Less Deceleration = Plate Falling Faster.

Without seeing the field it is being dropped into I can't give a specific answer, but does this help elaborate on your answer at all? Anything specific in their answer that you wanted elaborated? I really hope this helps a little!! 

[jamonwindeyer]

Hiya!

I was wondering (In regards to the photoelectric effect), If a metal surface is hit with an incident EMR above the threshold frequency, electrons are emitted. What happens to the missing electrons in the metal? Are they replaced? How so?

thanks.

Hey FallonXay!!

It depends on the context a little bit. If we are just talking about a metal, totally isolated from anything else, then no! The electrons are not replaced, the metal will gradually build up a positive charge.

If the metal is earthed in any way, either as part of a circuit or just in the circumstances, then electrons from earth will be attracted to the positive charge in the metal (caused by things called holes, which is covered in the next section) and replace the missing electrons. Remember, earth just defines an infinite reservoir of electrons, so if the metal is attached to anything like this, the electrons are replaced I hope this helps!!

[brontem]

Hey!! Thanks so much that definitely helps, way better than the 2 line answer in the book