HSC Physics Question Thread

[austv99]

Hi,
Could someone explain Planck's Contribution to Bohr's model and postulates?
Also clarify the significance of the hydrogen spectrum and how this significance differs from the significance of the Balmer Series.

[Mymy409]

Can someone help me with this? The answer is C. Thanks!

[winstondarmawan]

Would appreciate help with the following question, part b).
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20706467_1293245284134325_1582034743_n.png?oh=a0683f2b4127bb6b5f1996fafcea0021&oe=598ACF30
TIA

[Bubbly_bluey]

Can someone help me with this? The answer is C. Thanks!

Hi! I remember doing this question and initially choosing D. But i think its C because if you increase the reistance too high then that would generate greater power loss by P=I^2 R. Its just a thought

[Bubbly_bluey]

Would appreciate help with the following question, part b).
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20706467_1293245284134325_1582034743_n.png?oh=a0683f2b4127bb6b5f1996fafcea0021&oe=598ACF30
TIA

Hi! if you recall the formula for orbital velocity you can that the "m" is actually the mass of the moon not the spacecraft. So although the spacecraft has changed its mass it does not affect the orbital velocity.

[kiwiberry]

Can someone help me with this? The answer is C. Thanks!

I think it's because current and resistance are inversely proportional, so if current is high then resistance has to be low. Since P=I²R, having a high current and low resistance will result in higher heat losses and therefore faster heating than having low current and high resistance, so the answer is C

[jamonwindeyer]

Can someone help me with this? The answer is C. Thanks!

I think it's because current and resistance are inversely proportional, so if current is high then resistance has to be low. Since P=I²R, having a high current and low resistance will result in higher heat losses and therefore faster heating than having low current and high resistance, so the answer is C

This question caused a lot of contention - But kiwiberry is on it. If you've got a fixed AC voltage source (say the 230V we are supplied from mains), then \(I=\frac{230}{R}\). We want \(R\) to be low so current can be high, because joule heating is mostly determined by how much current you've got flowing

[kiwiberry]

Hi,
Could someone explain Planck's Contribution to Bohr's model and postulates?
Also clarify the significance of the hydrogen spectrum and how this significance differs from the significance of the Balmer Series.

Planck proposed that energy emitted or absorbed by a black body is quantised, given by E=hf - this was the beginning of the concept of quantised energy and quantum physics. Bohr used Planck's theory to develop his model of the atom, proposing that the electrons exist in quantised energy levels (his 1st postulate). Bohr's second postulate was also based on Planck's theory, stating that electron transitions between these energy levels would result in the absorption/emission of a quantised amount of energy in the form of EMR, given by E=hf. His third postulate states that angular momentum of electrons in these stationary states is quantised and given by L=nh/2pi, a result of quantum physics. Thus Planck's contribution to quantum physics was significant in the development of Bohr's model and postulates.

The Balmer series is the visible part of the hydrogen spectrum. I'm not sure about the difference in their significance, but the Balmer series was significant because it showed that the hydrogen spectrum was not a continuous rainbow but rather lines at specific wavelengths. This suggested that electrons only existed in certain energy levels and influenced Bohr's atomic model and postulates.

[pikachu975]

Even simpler - Slip rings literally just maintain contact. They change absolutely nothing about the output

I thought it changes polarity of the brushes every half turn for an AC generator

[kiwiberry]

I thought it changes polarity of the brushes every half turn for an AC generator

The output will already be AC so the polarity doesn't need to be changed. In DC generators the output has to be changed every half turn to get a DC output, that's what the split-ring commutator does

[pikachu975]

The output will already be AC so the polarity doesn't need to be changed. In DC generators the output has to be changed every half turn to get a DC output, that's what the split-ring commutator does

But in an AC generator how is it already AC?

[winstondarmawan]

But in an AC generator how is it already AC?

Imagine one side of the coil is "positive" and the other is "negative". As flux lines cut through that side of the coil, an EMF is produced. However, when flux lines cut through the other side of the coil, which will happen as it rotates, then EMF will be produced in the opposite direction. Thus, a generator with slip rings produces AC. Split rings correct this.

[jamonwindeyer]

But in an AC generator how is it already AC?

If you think about it, everything about an AC and DC generator is identical EXCEPT the slip rings vs split ring. If the split ring reverses polarity, and that gives DC, then the slip rings wouldn't want to do the same thing right? You'd still be getting DC

[winstondarmawan]

Would appreciate help with the following quesiton:
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20684659_1293396164119237_362264648_o.png?oh=8c6f6d113a92a264238faf7b26106b94&oe=598A9646
TIA!

[blasonduo]

Would appreciate help with the following quesiton:
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20684659_1293396164119237_362264648_o.png?oh=8c6f6d113a92a264238faf7b26106b94&oe=598A9646
TIA!

This is all about Faradays Law and Lenz's law. When there is a change in magnetic flux, a current can be induced, but to stay in the laws of conservation of energy, it will flow in the direction to oppose that change.

When the power is initially turned on, current will flow, and in the plate, will flow in the direction to oppose that change (so clockwise) but then will stop. (I'm Assuming its DC current here) As there is no more change in flux. As the resistance becomes decreased, more current will flow and thus the plate will induce a current in the same direction again, but continuously this time.

Hope this helps

Final stretch before the exam