HSC Physics Question Thread

[jess-steenbeeke]

Einstein痴 1905 theory of special relativity made several predictions that could not be verified for many years. 
(a) State ONE such prediction. 
(b) Describe an experiment to test this prediction. 
(c) Explain how technological advances since 1905 have made it possible to carry out this experiment.    (HSC 2005)

So I said for part a about time dilation, and then I talked about the atomic clock experiment for (b) but what do I say for part C? kinda stuck

thanks guys

okay so basically the atomic clocks weren't around in his day- so we didn't have way to measure time that accurately (because effects of time dilation would be very very small) also you need to get somewhat fast to be able to measure a difference anyway... the atomic clock experiment was performed in high speed jets. Hope this helps

[justwannawish]

Hi everyone (it's been so long haha), I just have a question about determining current direction for AC generators.

I just have a question about how to determine the velocity/current direction for AC generators. I know the direction of the magnetic field is from north to south and I'm trying to use the right hand palm rule, but i cannot determine which is the negative/positive end?

Any help will be appreciated

[beatroot]

Hey!
Knowing the magnetic field already, we need either the current or force to find the other. The diagram shows a clockwise direction, so we値l know that the force on the wire MN is down.

Using the righthand palm rule, we can determine that the current runs from N to M. (because of induction)

We also know that current runs from the positive terminal to the negative terminal, so following the current will reveal the negative terminal!

From this, we can see that the top terminal is positive and the bottom is negative.

I hope I知 right

I値l be happy to explain anything that doesn稚 make sense.

Hope this helps!

[blasonduo]

Woah! That was great!
Beat me to it!

[Aaron12038488]

Europa, a moon of Jupiter, has an orbital diameter of 1.34 x 10^9m, and a period of 3.55 days.

What is the mass of Jupiter?
i get 1.89 x 10^27 kg but the answer is 1.51 x 10^28 kg.

[RuiAce]

Europa, a moon of Jupiter, has an orbital diameter of 1.34 x 10^9m, and a period of 3.55 days.

What is the mass of Jupiter?
i get 1.89 x 10^27 kg but the answer is 1.51 x 10^28 kg.

\begin{align*}\frac{(1.34\times 10^9)^3}{(3.55 \times 24\times 3600)^2}&= \frac{6.67\times 10^{-11} \times M}{4\pi^2}\\ M &\approx 1.51\times 10^{28}\, \text{kg}\end{align*}
Make sure you've converted everything to SI units. The period is given in days, but we need to work in seconds.

[Aaron12038488]

\begin{align*}\frac{(1.34\times 10^9)^3}{(3.55 \times 24\times 3600)^2}&= \frac{6.67\times 10^{-11} \times M}{4\pi^2}\\ M &\approx 1.51\times 10^{28}\, \text{kg}\end{align*}
Make sure you've converted everything to SI units. The period is given in days, but we need to work in seconds.

shouldn't divide diameter by 2? to get radius?

Also is it better to do HSC space questions, or from the THSC half-yearly bank? which is more likely to be similar for my half-yearly?

[RuiAce]

shouldn't divide diameter by 2? to get radius?

Oh, very good point. Thanks for the pick-up. In that case you should certainly divide by 2; the given answers are wrong

[jamonwindeyer]

Also is it better to do HSC space questions, or from the THSC half-yearly bank? which is more likely to be similar for my half-yearly?

Either! Half Yearlies all try and replicate the HSC to an extent so pretty much anything you practice that is similar will be helpful

[justwannawish]

hey, thank you for your answer.

I still don't understand where the current runs, sorry. Could you please explain how you used induction to find it flows from N to M?

Hey!
Knowing the magnetic field already, we need either the current or force to find the other. The diagram shows a clockwise direction, so we値l know that the force on the wire MN is down.

Using the righthand palm rule, we can determine that the current runs from N to M. (because of induction)

We also know that current runs from the positive terminal to the negative terminal, so following the current will reveal the negative terminal!

From this, we can see that the top terminal is positive and the bottom is negative.

I hope I知 right

I値l be happy to explain anything that doesn稚 make sense.

Hope this helps!

[beatroot]

hey, thank you for your answer.

I still don't understand where the current runs, sorry. Could you please explain how you used induction to find it flows from N to M?

Hey there!

Assuming you know the right-hand palm rule, You'd see that it would flow from M to N, however, by lenz's law, current will always be induced in a way to oppose the change in flux, so it must flow from N to M. (kinda think of it like if there was current flowing through the wire as with a motor. The induced current will always oppose the supply current, but in this case, there is no supply current, so the "back emf" is the current in generator.)

Hope this makes sense!

[justwannawish]

Hey there!

Assuming you know the right-hand palm rule, You'd see that it would flow from M to N, however, by lenz's law, current will always be induced in a way to oppose the change in flux, so it must flow from N to M. (kinda think of it like if there was current flowing through the wire as with a motor. The induced current will always oppose the supply current, but in this case, there is no supply current, so the "back emf" is the current in generator.)

Hope this makes sense!

yep, that makes sense, but if there wasn't a torque direction, would it be possible to find the direction of the current flow?

Also, is motor effect the flow of positively charged particles or negatively?

[beatroot]

yep, that makes sense, but if there wasn't a torque direction, would it be possible to find the direction of the current flow?

Also, is motor effect the flow of positively charged particles or negatively?

Hey!

That would be impossible, as we would have 2 unknowns, and because of this, we could never really know the direction of current (because if the force on MN was up, then the current would flow in the opposite direction)

Remember, current is the flow of electrons, and electrons are negatively charged

[justwannawish]

Hey!

That would be impossible, as we would have 2 unknowns, and because of this, we could never really know the direction of current (because if the force on MN was up, then the current would flow in the opposite direction)

Remember, current is the flow of electrons, and electrons are negatively charged

Hey, thank you. It's just that we learned it in class as positively charged particles as my teacher said it was in terms of conventional current rather than electric flow so I was a bit confused about that

[Bri MT]

Hey, thank you. It's just that we learned it in class as positively charged particles as my teacher said it was in terms of conventional current rather than electric flow so I was a bit confused about that

Conventional current which is shown on diagram could be described as the theoretical flow of positively charged particles; however, actual current is related to electrons (and thus negative charge).

So it can be useful when learning right hand rules etc. to do so in terms of conveniently current but physical phenomenon should be decided in terms of current actually is. 

(I studied VCE so feel free to correct me if the HSC course says differently )