HSC Physics Question Thread

[jamgoesbam]

Hi!
Could someone please explain these questions? GPE confuses me 

Answers:
1.6.6 A --> for this question I thought that it went into a lower orbit as 3E = 3 x -GMm/r (since it is more negative, it would be closer to Earth!)
1.6.11 C (could you also please show working, thanks!)
1.6.12 B

[bethjomay]

Could someone remind me why AC is better for use in transformers? Thank you!

[nimasha.w]

Could someone remind me why AC is better for use in transformers? Thank you!

Hey! sorry if this explanation is pretty basic, just trying to test my knowledge
From what i understand a transformer requires an alternating current to create a changing magnetic field in the iron core. It is this changing magnetic field that induces an emf in the secondary coil, hence, DC can't be used in a transformer because it the current produced is only in one direction so it can't create a changing magnetic field

[Spencerr]

Could someone remind me why AC is better for use in transformers? Thank you!

Hey there, Nimasha gave a solid answer! Here's my 2cents. In an exam, i think this would be 3 marks. I've tried to allocate the marks below in an explanation.

Transformers are electrical devices which are used to step up or step down the voltage of electricity.
They operate via the principle of electromagnetic induction and Faraday's Law (quote his equation) whereby a changing magnetic flux produced by an oscillating magnetic field in a primary coil  induces an emf and current in a secondary coil. (1) In order to continuously produce this change in magnetic flux, an oscillating magnetic field is required. The constant switching in direction of the AC current produces the oscillating magnetic field required. (1) In contrast, the unidirectional current of DC does not produce an oscillating B field, and thus does not subject the secondary coil to a changing magnetic flux. Hence DC cannot be used (unless by switching the current on and off to produce the change in flux), making AC much better for use in transformers. (1)

[bethjomay]

Thank you both! I knew it was something simple, I just couldn't clarify it!

[teapancakes08]

Any extra bits of information you find out you highlight.

As in, over the black print they gave you. Highlight any bit of information that you suspect may be useful. Or even go that extra mile and use highlighting for suspicions and a tick for actual usage of it.

You can also choose to make use of the side of the paper in writing any things you need to sub into the formulae. Some people use this because the markers may or may not care about what's to the left of the lines they give you

Also as obvious as it seems break down any formulas you use. Make sure that what you used makes sense. With practice you should be able to see when things go missing.

Ah, noted. Thanks for the advice.

[teapancakes08]

Lots and lots of practice, so that when you are doing it you flash back to your earlier work and go, "Yo, this seems a bit off." It's all about experience! You might also want to check out this article!

Thanks, I'll keep that in mind 

[jamonwindeyer]

Hi!
Could someone please explain these questions? GPE confuses me 

Hey! So for your first question, I agree with your interpretation! I think the question is a little off in that way, but that said, if you swap back to your more simplistic version of \(GPE=mgh\), that gives you the correct answer. This little subtlety comes up a lot, and I don't they press on it like this in HSC Exams!

The second question, I can't match any of the answers actually! Anyone else who wants to have an attempt please feel free

Your last question is best answered by considering what is wrong with the other answers.
- A is incorrect, air resistance would need to be overcome either way
- C is incorrect, centripetal force IS our gravitational force, they are the same thing
- D is incorrect, doesn't really say anything relevant

B is the only remaining answer, we require extra energy to give the satellite enough horizontal velocity to enter an orbit

[katnisschung]

IM STUCK!!!
Object A of mass M sits on the surface of planet α
and experiences a force of X due to gravity.

Object B, also of mass M, sits on the surface of planet β
also experiences a force of X due to gravity.

Planet α is twice as massive as planet β

calculate the radius of planet  β in terms of planet α.

seems like nothing i do gets this answer
rβ= √2 times rα

[jakesilove]

IM STUCK!!!
Object A of mass M sits on the surface of planet α
and experiences a force of X due to gravity.

Object B, also of mass M, sits on the surface of planet β
also experiences a force of X due to gravity.

Planet α is twice as massive as planet β

calculate the radius of planet  β in terms of planet α.

seems like nothing i do gets this answer
rβ= √2 times rα

Hey! We can calculate g using the equation



M is the mass of the planet, r is the radius of the planet, G is a constant.
You need to be able to derive this; equate gravitational force and force due to gravity!

Subbing in our values, we get



From this,




As required! I definitely skipped lots of steps here, so let me know if you need any clarification

[teapancakes08]

I'm slightly confused as how to answer this question:

"A satellite has 4000J of work done on it. Does it move to a higher or lower altitude? Explain your answer."

....which is also related to this question:

"Explain the relationship between the work done on an object which changes its positon in a gravitational field and its gravitational potential energy (GPE)"

My understanding of the latter is that the change of GPE is related to the work done on an object in that it is converted into kinetic energy as is falls back to the object/body (if I worded it correctly that is...), although I can't quite grasp how it works or even if I got the concept. Actually, I'm not even sure if I'm making any sense...

If anyone can help explain that'll be great. Sorry for the long winded question ^^;;

[jakesilove]

I'm slightly confused as how to answer this question:

"A satellite has 4000J of work done on it. Does it move to a higher or lower altitude? Explain your answer."

....which is also related to this question:

"Explain the relationship between the work done on an object which changes its positon in a gravitational field and its gravitational potential energy (GPE)"

My understanding of the latter is that the change of GPE is related to the work done on an object in that it is converted into kinetic energy as is falls back to the object/body (if I worded it correctly that is...), although I can't quite grasp how it works or even if I got the concept. Actually, I'm not even sure if I'm making any sense...

If anyone can help explain that'll be great. Sorry for the long winded question ^^;;

Hey! If an object DOES WORK, it's going to have to work AGAINST SOMETHING. If an object has work DONE ON IT, that work has to be produced by SOMETHING. So, if a satellite has work DONE ON IT, how could that happen? Well, the gravitational field will do the work, thus PUSHING the satellite downwards! So, it moves to a lower orbit.

It sounds like you actually do have a good understanding of the topic! I think it's easiest to use a diagram in situations like this; hopefully the below gives you all the information you need! Let me know if I can clarify anything.

[Rikahs]

I realise that when a DC motor is running at full speed, the back emf induced due to the combination of Faraday's Law and Len'z law will oppose the supply emf. I was wondering whether they complete negate each other i.e. the net current in the coils would be 0? or is the back emf just a little under the supply emf i.e. the net current would be slight over zero?

[jakesilove]

I realise that when a DC motor is running at full speed, the back emf induced due to the combination of Faraday's Law and Len'z law will oppose the supply emf. I was wondering whether they complete negate each other i.e. the net current in the coils would be 0? or is the back emf just a little under the supply emf i.e. the net current would be slight over zero?

No, the back EMF is substantially smaller than the supplied current. This results in a net current that is less than the supplied current (due to the Back EMF/Induced current), however there is still current flowing! Otherwise, the motor wouldn't turn.

[student123456]

Hey guys!
Hoping somebody may be able to help me out with solving these problems. I never really know how to handle them. Do you it mathematically? Thanks

A spaceship at a distance r metres from the centre of a star experiences a gravitational force of x newtons. The spaceship moves a distance of r/2 metres towards the star. What is the gravitational force acting on the spaceship in this new location?