HSC Physics Question Thread

[jamonwindeyer]

Hi! I'm having a trouble understanding the answer to this question.

Initially I chose B because the galvanometer would deflect more if the current in the secondary increase by the formula. Isecondary/ Iprimary= n primary/ n secondary. And so a decrease in n secondary??

Hey Bubbly!!

So this is probably going to be the only time I'll ever say this - Don't worry about this question. It's really stupid, the given answer (A) can only be obtained through some really backwards thinking that I'm honestly not too sure about, because it makes assumptions about the questions I don't like. Some think the Galvanometer is actually measuring voltage, some think it is to do with the resistance introduced by adding more coils (the transformer is ideal, so don't buy that) - The one thing I believe about the question is that the voltage source is DC not AC, as indicated by the lines. But that shouldn't affect the answer, and of course, BOSTES hasn't given a solution either

So, don't worry about it. It's either a really convoluted/poorly written question, an incorrect answer by BOSTES, or perhaps the Illuminati. But your reasoning is solid, it makes the most sense, and it shows you understand the content!

[Bubbly_bluey]

Hey Bubbly!!

So this is probably going to be the only time I'll ever say this - Don't worry about this question. It's really stupid, the given answer (A) can only be obtained through some really backwards thinking that I'm honestly not too sure about, because it makes assumptions about the questions I don't like. Some think the Galvanometer is actually measuring voltage, some think it is to do with the resistance introduced by adding more coils (the transformer is ideal, so don't buy that) - The one thing I believe about the question is that the voltage source is DC not AC, as indicated by the lines. But that shouldn't affect the answer, and of course, BOSTES hasn't given a solution either

So, don't worry about it. It's either a really convoluted/poorly written question, an incorrect answer by BOSTES, or perhaps the Illuminati. But your reasoning is solid, it makes the most sense, and it shows you understand the content!

Oh Yes i see what you mean!!! But I thought a galvanometer measures only current like an ammeter. If it was measuring voltage wouldn't it be a volt meter?

[jamonwindeyer]

Oh Yes i see what you mean!!! But I thought a galvanometer measures only current like an ammeter. If it was measuring voltage wouldn't it be a volt meter?

I totally agree, hence why I hate those explanations 

[Sukakadonkadonk]

Hey guys,

Is there any difference between a metal being magnetic and a metal being Ferromagnetic?

Thanks.

[jakesilove]

Hey guys,

Is there any difference between a metal being magnetic and a metal being Ferromagnetic?

Thanks.

Hey! Generally, Ferromagnetism is just a term used to describe metals that exhibit strong, permanent magnetic properties (most notable, Iron). In the HSC syllabus, though, you can basically use the two terms interchangeably as far as I know

Jake

[kiwiberry]

This is from HSC 2013, why is the answer D?

[RuiAce]

This is from HSC 2013, why is the answer D?

A gets ruled out because the magnetic field is radial.

Now, note that the x-axis represents the speed of the motor. The speed of the motor will (because of electromagnetic induction and Lenz's law and blah) cause back EMF to appear in the motor. The back EMF is pretty much reducing the value of I in \(\tau = nBIA\cos \theta\). Hence, as the motor speeds up, the greater back EMF produced will work against the effect of the torque, thus bringing it back down.

Poor explanation because I'm shit at physics now.

[kiwiberry]

A gets ruled out because the magnetic field is radial.

Now, note that the x-axis represents the speed of the motor. The speed of the motor will (because of electromagnetic induction and Lenz's law and blah) cause back EMF to appear in the motor. The back EMF is pretty much reducing the value of I in \(\tau = nBIA\cos \theta\). Hence, as the motor speeds up, the greater back EMF produced will work against the effect of the torque, thus bringing it back down.

Poor explanation because I'm shit at physics now.

Thanks Rui!!

[Aaron12038488]

Just wondering what I should aim for (marks overall) at the end of Yr 11 and 12. I want to get >95.

[jamonwindeyer]

Just wondering what I should aim for (marks overall) at the end of Yr 11 and 12. I want to get >95.

100 ☺

Seriously, if you are really aiming high, you should just be aiming to get the absolute highest mark you can. Why restrict yourself to getting a mark of X, just aim for 100!

Realistically you'd want to aim for a Band 6, but you shouldn't think that way ☺

[Iminschool]

Simple question here but my teacher marked it weirdly last year
Q. The function of a split-ring commutator in a motor is to:
a) Prevent back EMF from building up
b) Keeps the rotor spinning by converting AC to DC
c) Keeps the rotor spinning by converting DC to AC
d) None of the above

Thanks,

[Alexicology]

Simple question here but my teacher marked it weirdly last year
Q. The function of a split-ring commutator in a motor is to:
a) Prevent back EMF from building up
b) Keeps the rotor spinning by converting AC to DC
c) Keeps the rotor spinning by converting DC to AC
d) None of the above

Thanks,

The Answer is c) because a DC motor uses DC voltage and a split ring commutator is needed to reverse the direction of the current so that the force on each side of the coil is constant. therefore AC current.

[jamonwindeyer]

Simple question here but my teacher marked it weirdly last year
Q. The function of a split-ring commutator in a motor is to:
a) Prevent back EMF from building up
b) Keeps the rotor spinning by converting AC to DC
c) Keeps the rotor spinning by converting DC to AC
d) None of the above

Thanks,

It is definitely not A or B because they don't make sense - But I don't really like the wording of C. It needs to reverse the direction of current every half turn, but that isn't really converting DC to AC. It is very inaccurate wording, and that makes me think it is D...

The Answer is c) because a DC motor uses DC voltage and a split ring commutator is needed to reverse the direction of the current so that the force on each side of the coil is constant. therefore AC current.

Undoubtedly this is correct too and makes sense, but I think they are trying to trick you with wording! But I wouldn't be surprised if the answer was C either

[Alexicology]

It is definitely not A or B because they don't make sense - But I don't really like the wording of C. It needs to reverse the direction of current every half turn, but that isn't really converting DC to AC. It is very inaccurate wording, and that makes me think it is D...

Undoubtedly this is correct too and makes sense, but I think they are trying to trick you with wording! But I wouldn't be surprised if the answer was C either

 I kind of think it is D too because the wording of the answer is weird, its function is reversing the current every half turn so thank you for correcting me

[jamonwindeyer]

 I kind of think it is D too because the wording of the answer is weird, its function is reversing the current every half turn so thank you for correcting me

Oh definitely not a correction, your reasoning is correct too! Like, from the point of view of the coil it might as well be AC just tricky questions being tricky