HSC Physics Question Thread

[jamonwindeyer]

This is true if and only if we're speaking in the context of electromagnetic induction and basically a second coil is beside the first. Otherwise we just have a DC circuit that's being damaged because flicking the switch on and off constantly isn't even a good idea to begin with.

Actually, it does work outside of induction too. Additionally, a switch turned on and off quickly (an input called a variety of names, most appropriately the Rectangular Function) is identical to producing an AC signal at a variety of frequencies, with the amplitude of those signals at some frequency related to the sinc function. So, a DC signal on for a short period is actually like a whole bunch of AC inputs at once  This is actually fine, depending on the circuit.  For lightbulbs for example, since there is a massive inrush current every time it is switched on, it's definitely not a good idea 

Disclaimer: None of that is assessable. Rui, HPL and I tend to go off on tangents a lot.

[smiley2101]

hi guys, I don't understand this graph ie why it starts from zero and why the amplitude is different for the first but the next two are the same?? thank you legends!

[RuiAce]

hi guys, I don't understand this graph ie why it starts from zero and why the amplitude is different for the first but the next two are the same?? thank you legends!

The reason why it starts at 0 can be analysed in multiple ways depending on which you prefer
The generator starts at the point where there is no change in flux. (Recall that induced EMF is proportional to the CHANGE in flux, not the flux itself.)
The generator starts where the flux passing through it is maximum, hence the change in flux is 0.
The generator is allowing the maximum amount of field lines through the coil. (If instead, it were cutting the field lines, then you would start at the maximum.)

You should be well aware of the typical setup of a generator and when the EMF is max. The analogies are similar to that of motors, and I know that physics in focus has good diagrams for the motor and when torque is maximised.
If the motor undergoes TWO revolutions instead of ONE in the SAME time, then we have effectively increased the SPEED that the motor is being turned at. This creates two implications
- Because the motor is being turned faster (two-fold) now, the period of the graph should decrease (inversely, so 1/2 here). This is simply a direct consequence of the definition of the period of a wave - the time taken for one full oscillation through a point.
- As the motor is being turned faster, the change in flux it experiences increases. The change in flux has to be directly proportional to the speed of which it is being changed at. If the change in flux increases, so must the magnitude of the EMF, thus the amplitude also gets doubled here.

Actually, it does work outside of induction too. Additionally, a switch turned on and off quickly (an input called a variety of names, most appropriately the Rectangular Function) is identical to producing an AC signal at a variety of frequencies, with the amplitude of those signals at some frequency related to the sinc function. So, a DC signal on for a short period is actually like a whole bunch of AC inputs at once  This is actually fine, depending on the circuit.  For lightbulbs for example, since there is a massive inrush current every time it is switched on, it's definitely not a good idea 

Disclaimer: None of that is assessable. Rui, HPL and I tend to go off on tangents a lot.

Ah, fair enough lols

[smiley2101]

What is the difference between gpe =mgh and The other formula and since a change gpe = work done, why is work done BY gravitational field not counted as work done when there is an obvious change in GPE

[RuiAce]

What is the difference between gpe =mgh and The other formula and since a change gpe = work done, why is work done BY gravitational field not counted as work done when there is an obvious change in GPE

For starters, the formula E_p=mgh assumes that we are always on the Earth's surface. The formula E=-GmM/d works for essentially every single planet and wherever you are on the surface.

Whilst the error isn't massive, the formula E_p=mgh is doomed to be inaccurate to some noticeable extent. This formula already treats h with respect to the Earth's "surface" instead of its centre, but the reality is that the Earth simply is not a perfectly spherical body. The 'spheroid' shape of the Earth means that depending on which part of the surface you are on the Earth, there is slight deviation in what mgh apparently computes.

The formula E_p=-GmM/d caters for all of these problems perfectly

[RuiAce]

What is the difference between gpe =mgh and The other formula and since a change gpe = work done, why is work done BY gravitational field not counted as work done when there is an obvious change in GPE

Also, this second part of the question made no sense to me. What exactly are you getting at?

[emarmma]

Hey guys,
how the repulsion force between two magnets can be affected if it was calculated on the North pole and on the equato

[RuiAce]

Hey guys,
how the repulsion force between two magnets can be affected if it was calculated on the North pole and on the equato

This is simply because the Earth has its own magnetic field, and at the poles the effect of the Earth's magnetic field is much greater there.

So when you try to calculate a force resulting from a magnetic interaction, the Earth's magnetic field impedes it a bit more at the poles than at the Equator

[smiley2101]

why did hertz have a small gap between his transmitter apparatus when generating radio waves? what does the gap do? why did he have it?

[smiley2101]

why was an induction coil used to generate voltage in the hertz experiment

[jakesilove]

why was an induction coil used to generate voltage in the hertz experiment

Hey!

I'm not really going to answer your question; mainly because I don't know. However, you definitely don't need to know either. Hertz's experiments don't need to be understood in great depth; just get the importance of his 'discoveries', and how that impacted on future scientific exploration. Maybe also try to be a bit clearer with your questioning, just so that we can answer your specific question! Physics is full of difficult concepts, so if you could identify specifically what you're looking for help with we can best assist you.

Jake

[emm22]

Hi! Could you please explain how Planck's Law solved the Ultraviolet Catastrophe issue?

[Happy Physics Land]

Hi! Could you please explain how Planck's Law solved the Ultraviolet Catastrophe issue?

Hey emm!

Sure thing! Firstly let's just recall Max Planck's first postulate about the quantisation of energy levels in an atom. He stated that energies exist in discrete amounts, not just any random amount, and that in order for an electron to move from one energy level to another, a well-defined amount of energy has to be absorbed or emitted by the electron. Now let's think about the blackbody radiation curve. We know that ultraviolet radiations, X-rays and gamma rays all have low wavelength and hence high frequency, as described by the blackbody curve. According to Planck's equation E=hf, when wavelength becomes increasingly small, frequency would become increasingly large and hence energy would also becoming infinitely large. Such phenomenal energy change would involve a significantly large leap from one energy level to another. However, because such change in energy level is impossible in any atom, high frequency radiations such as UV cannot be emitted and thus for X-rays, UV rays and gamma rays, the intensity is 0. This hence provides an explanation for ultraviolet catastrophe.

[Happy Physics Land]

why was an induction coil used to generate voltage in the hertz experiment

Hey Smiley!

Recall the purpose of a transformer in Motors and Generators. The purpose of induction coils is approximately the same as that of a transformer. Basically we want to create a spark (an accelerator of charge) across the air. But air is an insulator and we would need a very high voltage in order to achieve a spark. Our home supply voltage is around 240 V which isn't high enough to cause a discharge to occur. Thus we need an induction coil to provide short high voltage surges to the electrodes at the gap of the transmitting antenna, causing electrons to have enough energy to accelerate through the air, ionising air molecules along the way, facilitating more electrons to travel through the air. With the induction coil, such high voltage cannot be achieved to cause the acceleration of electrons, which is the fundamental mechanism for the production of an electromagnetic wave.

[emarmma]

This is simply because the Earth has its own magnetic field, and at the poles the effect of the Earth's magnetic field is much greater there.

So when you try to calculate a force resulting from a magnetic interaction, the Earth's magnetic field impedes it a bit more at the poles than at the Equator

Thank you