HSC Physics Question Thread

[12carpim]

Hey, I'm just wondering if someone is able to explain how Proton density images are obtained in MRI imaging, is it from the combination of T1 and T2 relaxation or is it a totally different imaging?
Thanks 

[austv99]

Why is the answer C?
I thought it was D since electrical resistance is required to dissipate heat?

[kiwiberry]

Why is the answer C?
I thought it was D since electrical resistance is required to dissipate heat?

Assuming that voltage is constant, current and resistance are inversely proportional by V=IR - so if current is low, resistance must be high and vice versa. Since P_loss=I²R, the amount of heat is determined by both current and resistance, but mostly current. So, we want resistance to be low so current can be high, in order to maximise power losses and therefore have faster heating

[winstondarmawan]

Would appreciate help with the following:
5. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21557295_1321052091353644_1671431094_o.jpg?oh=0055bf0754276894538f81a3e740e1da&oe=59B53747
21. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21557082_1321052058020314_955487187_o.jpg?oh=14d3385cf665d072aba1485eac630b1e&oe=59B61DD6
TIA

[Bri MT]

Would appreciate help with the following:
5. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21557295_1321052091353644_1671431094_o.jpg?oh=0055bf0754276894538f81a3e740e1da&oe=59B53747

F=ma   Therefore, acceleration will be greatest at maximum force (NOT maximum change in force)

[austv99]

Is b referring to the fact that without a split ring, the coil alternates? Since ac induction motors dont use the motor effect?

[austv99]

Can someone help with this question?
TIA

[jamonwindeyer]

Is b referring to the fact that without a split ring, the coil alternates? Since ac induction motors dont use the motor effect?

Hey! So this is for the second one right? Yeah you've got it, so this is for AC motors that use the slip rings, the ones very similar to DC motors - Not induction motors like the first part of the question. Weird, I know!

So you'd identify what the motor effect actually is for a mark, describe the effect is has on the coil in such a motor for a mark, then explain how the alternating current maintains a constant direction of torque for a mark

Can someone help with this question?
TIA

The two cannonballs, despite being launched at different angles, reach the same maximum height in the same amount of time. That is, the horizontal motion of Cannonball Q hasn't affected its vertical motion - The horizontal and vertical components are independent. This is what supports Galileo's analysis!

[beau77bro]

how does magnetic levitation work??

[pikachu975]

how does magnetic levitation work??

Are you referring to the prac or maglev trains?

[beau77bro]

Are you referring to the prac or maglev trains?

"Analyse information to explain why a magnet is able to hover above a superconducting material that has reached the temperature at which it is superconducting" a secondary dot point in the syllabus - it was not covered well in class so i don't really get it.

[pikachu975]

"Analyse information to explain why a magnet is able to hover above a superconducting material that has reached the temperature at which it is superconducting" a secondary dot point in the syllabus - it was not covered well in class so i don't really get it.

Surface currents (cooper pairs) prevent any magnetic flux from penetrating the superconductor known as the Meissner effect. This means an equal and opposite magnetic field is produced to oppose the magnet's magnetic field and to oppose it's weight force downwards, causing it to levitate.

[beau77bro]

Surface currents (cooper pairs) prevent any magnetic flux from penetrating the superconductor known as the Meissner effect. This means an equal and opposite magnetic field is produced to oppose the magnet's magnetic field and to oppose it's weight force downwards, causing it to levitate.

elaborate please? how do cooper pairs prevent the magnetic field? how is the opposing magnetic field produced - i was told eddy currents aren't responsible. also i just had a trial question based upon it rising initially and what causes it - what happens there?

[pikachu975]

elaborate please? how do cooper pairs prevent the magnetic field? how is the opposing magnetic field produced - i was told eddy currents aren't responsible. also i just had a trial question based upon it rising initially and what causes it - what happens there?

It's not eddy currents, it's surface currents i.e. cooper pairs. Also we don't need to know how meissner effect works I think that's beyond HSC it probably involves some quantum theory. What causes it to rise is basically what I said - the cooper pairs (surface currents) prevent flux entering the superconductor known as meissner effect, then they produce a magnetic field equal in magnitude and opposite in direction to balance the magnet's weight force (make sure to include it balances the weight force or opposes it).

[beau77bro]

It's not eddy currents, it's surface currents i.e. cooper pairs. Also we don't need to know how meissner effect works I think that's beyond HSC it probably involves some quantum theory. What causes it to rise is basically what I said - the cooper pairs (surface currents) prevent flux entering the superconductor known as meissner effect, then they produce a magnetic field equal in magnitude and opposite in direction to balance the magnet's weight force (make sure to include it balances the weight force or opposes it).

Is that all we have to know? thanks pikchu. still interested in how it works beyond the course tho, maybe a certain quantum physicist could send some info our way?