VCE Physics Question Thread!

[lzxnl]

How come when finding the work done using a F/d graph, you use the area under the graph and not simply the force*distance (from the graph)?

Because work = force * distance when the force is constant in magnitude and direction only. If it changes in magnitude, you have to do an integral to sum up all the differential work elements dW = F dx and this integral is the area under the aforementioned graph.

[Floatzel98]

In the Light and Matter area of study, is it possible for a question to come up asking us to find the photo-current produced by the photoelectric effect experiment? Because a question like that came up on my SAC which stumped me for a while because I hadn't seen a question like it in any of the practice I had done. Just wondering if this is something that is likely to come up in the exam at all?

[silverpixeli]

In the Light and Matter area of study, is it possible for a question to come up asking us to find the photo-current produced by the photoelectric effect experiment? Because a question like that came up on my SAC which stumped me for a while because I hadn't seen a question like it in any of the practice I had done. Just wondering if this is something that is likely to come up in the exam at all?

never seen it what info did it give you?

[Floatzel98]

never seen it what info did it give you?

It was worded like ' If all the light of the laser was absorbed/used (something like that) then what would the photocurrent be'.

We were given the Power output of the laser and the wavelength of the light.

I tried finding the number of electrons emitted per second () and then using to find the current.

Firstly I tried finding the Kinetic energy of the light (we also had the work function of the metal) and then finding a voltage from it by , but what I was doing didn't really seem to make much sense.

Also is there an allowed value of hc (planck's constant x speed of light) we can use for workings and to put in our calculators because it takes a lot of time typing everything in your calculator constantly.

[silverpixeli]

It was worded like ' If all the light of the laser was absorbed/used (something like that) then what would the photocurrent be'.

We were given the Power output of the laser and the wavelength of the light.

I tried finding the number of electrons emitted per second () and then using to find the current.

Firstly I tried finding the Kinetic energy of the light (we also had the work function of the metal) and then finding a voltage from it by , but what I was doing didn't really seem to make much sense.

Also is there an allowed value of hc (planck's constant x speed of light) we can use for workings and to put in our calculators because it takes a lot of time typing everything in your calculator constantly.

For the constants I would try to store h and c individually in your scientific calculator (h in both units, along with charge and mass of electron, and G)

Type the number, find the 'sto' (store) feature on your calc, and then press the button with whatever capital letter you want to store it under (or google storing constants for your specific calculator model to find instructions)

for ideas, I think i had:

A: gravitational constant
B: charge on electron
C: speed of light
D: mass of electron
X: planck's constant (eVs)
Y: planck's constant (Js)
M: nothing

after you do that, you can just press alpha>letter and put these letters in your formulas rather than typing out the big numbers



ANYWAY in terms of the actual question, i wouldn't expect it on a real exam but you were on the right track

number of photons that hit the metal = number of photoelectrons emitted (assuming everything is absorbed, and that individual photons are above the threshhold frequency)

number of photons:
and times n for total power, so



is equal to number of photoelectrons:
fortunately t's cancel


if i havent made mistakes that should work, use whatever value of h and e is consistent with the units P is given in to make sure it works out

[dankfrank420]

How did we know which of plancks' constants to use (eV or J/s)

[Floatzel98]

How did we know which of plancks' constants to use (eV or J/s)

Depends on what you need to find and what is given. You can use either as long as the units are constant with everything else you are using. It's the same as having your speed in m/s or km/h. If you are given speeds in km/h and need an answer in m/s you can opt to convert everything first or leave everything in km/h till the end. You don't want to have an energy in eV while using plancks' constant in J/s.

[silverpixeli]

Depends on what you need to find and what is given. You can use either as long as the units are constant with everything else you are using. It's the same as having your speed in m/s or km/h. If you are given speeds in km/h and need an answer in m/s you can opt to convert everything first or leave everything in km/h till the end. You don't want to have an energy in eV while using plancks' constant in J/s.

Also if the formula has momentum in it you should put everything in SI units (planck's constant in Js and energy in J if it's there too) because momentum in units like eV is meaningless for our purposes.

[Floatzel98]

So the only thing I still don't really understand in the course is modulation and transmission of waves (in electronics). I don't really understand amplitude modulation and frequency modulation (which I don't think is in the course though?). A lot of the questions about carrier waves transferring analog information talk about the carrier wave being in time variation of the intensity or something. I'm just not really understanding it at all.

Any kind of explanation would be great. Thanks

[dankfrank420]

Depends on what you need to find and what is given. You can use either as long as the units are constant with everything else you are using. It's the same as having your speed in m/s or km/h. If you are given speeds in km/h and need an answer in m/s you can opt to convert everything first or leave everything in km/h till the end. You don't want to have an energy in eV while using plancks' constant in J/s.

Ah ok

I read this after I did my exam, luckily I used your logic. Thanks guys!

[Floatzel98]

So the only thing I still don't really understand in the course is modulation and transmission of waves (in electronics). I don't really understand amplitude modulation and frequency modulation (which I don't think is in the course though?). A lot of the questions about carrier waves transferring analog information talk about the carrier wave being in time variation of the intensity or something. I'm just not really understanding it at all.

Any kind of explanation would be great. Thanks

Any help? Tried my best to understand but still a bit unsure.

[odeaa]

Any help? Tried my best to understand but still a bit unsure.

I'll give you a quick run down tomorrow, as my computer has decided to shit itself (I'm on my phone atm)

[zsteve]

Just found out I'm a bit confused with this scenario:
A spring hangs from a ceiling, and a mass is attached to it, causing it to extend.
Say we have a 1 kg mass, and k = 10 N/m so the extension is 1 m.
Thus spring potential energy = 0.5kx^2 = 5 J
However change in GPE = mg(delta h) = 10 J
So some of the GPE has been lost from the system? Where has it gone?

[silverpixeli]

Just found out I'm a bit confused with this scenario:
A spring hangs from a ceiling, and a mass is attached to it, causing it to extend.
Say we have a 1 kg mass, and k = 10 N/m so the extension is 1 m.
Thus spring potential energy = 0.5kx^2 = 5 J
However change in GPE = mg(delta h) = 10 J
So some of the GPE has been lost from the system? Where has it gone?

When you attach a mass to a spring and lower it until it balances at equilibrium, you need to take that energy out yourself! (it comes out into your hand as you lower the mass slowly)

If you didn't take this extra 5J of energy out, and you just attached the mass and let it go as a closed system, the extra energy would stay there and the mass would bounce up and down, oscillating.

[Floatzel98]

I know this is super simple, but I'm getting really confused looking at these solutions for a prac exam. If you are in an elevator going down at a constant speed and then you slow down to a stop, what is the direction of the acceleration while you are stopping? Hopefully that is worded okay enough so it's not confusing.

Thanks

EDIT: Another question, do magnetic field lines ever cross if they come from the same magnet. Like for example, do the magnetic field lines of a simple bar magnet ever cross each other?

EDIT 2: For the attached image, can someone answer the question. The solutions aren't specific. They just talk about Lenz's law, not specifically why the current flows from A to B. How do we know when we can just be vague when answering the question? Are we not actually able to find out the direction of the current on our own?