VCE Physics Exam - 13/11/19 - Discussion/Questions/Solutions

[MubMurshed]

How is it kinetic energy? During a crash, you try to decrease impact force which aims to increase time. Since this is related to impulse(which is change in momentum), I picked momentum should be absorbed.

I would’ve thought that in order to crumble the material, work has to be done as the energy of the crash goes partly into the strain potential energy due to crumbling. Therefore energy was absorbed.

[HolHen]

I realised in the last minute left that I accidentally missed an entire page around 12 marks! Was hoping for 40+ raw so how many marks on the other questions do you think I could afford to lose to get a 40? 

[Adapt]

I would’ve thought that in order to crumble the material, work has to be done as the energy of the crash goes partly into the strain potential energy due to crumbling. Therefore energy was absorbed.

I chose momentum but after searching it up and your explanation I’d say it’s energy.

[Sine]

I realised in the last minute left that I accidentally missed an entire page around 12 marks! Was hoping for 40+ raw so how many marks on the other questions do you think I could afford to lose to get a 40? 

Not many I think. Maybe 3-8 depending on the exam difficulty. Sorry to hear that

[randomrandom2000]

How did you guys do the electron/x-ray diffraction question? I was absolutely clueless on that!

[Adapt]

How did you guys do the electron/x-ray diffraction question? I was absolutely clueless on that!

Was a kinda hard question, you just had to use the energy of the electrons to find the momentum, then find the wavelength from that, then change to frequency? Did it ask for frequency or wavelength i don't remember.

[Erutepa]

How did you guys do the electron/x-ray diffraction question? I was absolutely clueless on that!

The first part was mainly explaining that electrons can have wavelike properties and can have a dobrolie wavelength similar to the wavelength of X-rays. Since diffraction is proportional to wavelenth/slitsize, thus xrays and electons with similar wavelength can produce similar diffraction patterns

The second part was calculating the velocity of the electron from its kinetic energy, using that to calculate the momentum, which you can then use to calculate the wavelength of the electrons. Knowing the wavelength you can determine the frequency of the electrons which will be the same as the frequency of the Xrays.

[Sai Ramishetty]

Was a kinda hard question, you just had to use the energy of the electrons to find the momentum, then find the wavelength from that, then change to frequency? Did it ask for frequency or wavelength i don't remember.

Is the energy of electrons the same thing as the kinetic energy of electrons?

[Sai Ramishetty]

The first part was mainly explaining that electrons can have wavelike properties and can have a dobrolie wavelength similar to the wavelength of X-rays. Since diffraction is proportional to wavelenth/slitsize, thus xrays and electons with similar wavelength can produce similar diffraction patterns

The second part was calculating the velocity of the electron from its kinetic energy, using that to calculate the momentum, which you can then use to calculate the wavelength of the electrons. Knowing the wavelength you can determine the frequency of the electrons which will be the same as the frequency of the Xrays.

Did u end up with an answer something like 1.3x10*19 Hz

[Ryan_DS]

How is it kinetic energy? During a crash, you try to decrease impact force which aims to increase time. Since this is related to impulse(which is change in momentum), I picked momentum should be absorbed.

I think it's kinetic energy. Like the change in velocity is constant for both you and the car since you both go from x-0, with the car obviously having more momentum cause of it's mass, it's just you experience it differently. Even if the car softened the blow with airbags, you're still going from x-0, so you're right to say the force will be less because of the impact time. But it doesn't exactly absorb the momentum, like it isn't taking some of your momentum and somehow putting it into the surrounding objects. You are still after the collision. Your kinetic energy however, instead of being used to deform your bones is deforming the car, which makes more sense.

Or I might just be wrong entirely  . Let's see what the solutions are.

[peds01]

Did u end up with an answer something like 1.3x10*19 Hz

10^19 seems familiar so yeah probably

[Erutepa]

Is the energy of electrons the same thing as the kinetic energy of electrons?

They gave you kinetic energy (I am pretty sure) and that is what is used to calculate velocity/momentum

Did u end up with an answer something like 1.3x10*19 Hz

I unfortunately am horrible at remembering numbers so I have little clue. It seems right - but I am just taking a stab haha

[Ryan_DS]

How did you guys do the electron/x-ray diffraction question? I was absolutely clueless on that!

I first converted the ev to joules by multiplying it by 1.6*10^-19 (charge of electron), then used the kinetic energy formula to find the velocity. Then, I used that velocity to find the momentum by multiplying by the mass of an electron then used the debroglie wavelength formula to find the wavelength. Then, since x-rays travel at C, I used C=f*lambda to find the frequency. The only commonality the x-ray and electron have are their debroglie wavelengths I think.

Maybe there's a shorter way but I'm sure this would work.

[3086]

Did u end up with an answer something like 1.3x10*19 Hz

Yeah I got that answer too so hopefully we both get some marks!!

[3086]

This is how to do it: (Frequency of X-rays using the kinetic energy of photons)

1. E_k=0.5mv²
V=sqr root (2*3000*1.6*10¹⁹/9.11*10^-31)
V-3.2*10⁷

2. lamda=h/mv
lamda= 6.63*10^-34/9.11*10^-31*3.2*10⁷
lamda (de borglie wavelength)=2.2*10^-11

3. C=lamda*F
F=C/lamda
F=3.0*10⁸/2.2*10^-11

4. F= 1.3*10¹⁹

The steps provided also show the areas where marks will be accounted for

Also a question: If you wrote the correct answer but only showed the first step would you get full marks?