VCE Physics Question Thread!

[timton]

Not exactly sure what you're asking, I think you mean a step up/down transformer which is connected to a generator. Also, by 'the same' do you mean e.g. 1:100 for the step up and 100:1 for the step down?

If that's the case, then there's not really any advantage to this, it just means that when you step up the power coming out of the generator to transmit it, and then step it down later, the stepping up and the stepping down exactly cancel each other out and the ratio of voltage to current after transmission is the same as the ratio coming out of the generator, before transmission.

My bad, was meant to write transformer instead of generator.
The question specifically asks "why would the turn ratio be chosen to be the same for both the step up and step down transformer pair?"
You are not given additional information about the number of turn in the coil (question later on) but you are asked why they are the same.

[odeaa]

My bad, was meant to write transformer instead of generator.
The question specifically asks "why would the turn ratio be chosen to be the same for both the step up and step down transformer pair?"
You are not given additional information about the number of turn in the coil (question later on) but you are asked why they are the same.

It's a bit of a weirdly worded question if you ask me. I can't see any reason why they would be the same ratio, usually a question like that would ask you why they need to step the voltage up/down. The only thing I can think of is for safety (if not to minimise power loss) because you aren't directly connected to the power source

[silverpixeli]

My bad, was meant to write transformer instead of generator.
The question specifically asks "why would the turn ratio be chosen to be the same for both the step up and step down transformer pair?"
You are not given additional information about the number of turn in the coil (question later on) but you are asked why they are the same.

yeah i'm not sure on 'why' you would do that specifically, maybe they are looking for something along the lines of 'even though stepping up the voltage and then stepping it down by the same amount causes no change between the generator voltage and the voltage at the house, it allows the transmission to take place at a lower current, which means the power lost in transmission is lower (according to P = I^2 R)'
or something like that.

[dankfrank420]

Re: transformers

As I understand it, you increase the voltage by increasing the turns on the secondary coil (to "step it up"). Since P = V*I and P is constant, therefore the current is decreased as voltage is increased. That's all good.

But isn't power also defined as v^2/r as well as defined as I^2*r? Wouldn't increasing the voltage also increase the power loss?

[silverpixeli]

Re: transformers

As I understand it, you increase the voltage by increasing the turns on the secondary coil (to "step it up"). Since P = V*I and P is constant, therefore the current is decreased as voltage is increased. That's all good.

But isn't power also defined as v^2/r as well as defined as I^2*r? Wouldn't increasing the voltage also increase the power loss?

Yes, P = V^2/R is a valid formula, but to calculate the power loss across the wires using this you would need to use the voltage that is across the wires (which is probably not the voltage in the second part of the transformer because some is also across the houses or the transformer at the other end)
The current in the secondary coil, the wires and in the houses is the same because they are all in series, so this formula is usually easier to use

[dankfrank420]

Yes, P = V^2/R is a valid formula, but to calculate the power loss across the wires using this you would need to use the voltage that is across the wires (which is probably not the voltage in the second part of the transformer because some is also across the houses or the transformer at the other end)
The current in the secondary coil, the wires and in the houses is the same because they are all in series, so this formula is usually easier to use

Ah, cheers.

[Kel9901]

My bad, was meant to write transformer instead of generator.
The question specifically asks "why would the turn ratio be chosen to be the same for both the step up and step down transformer pair?"
You are not given additional information about the number of turn in the coil (question later on) but you are asked why they are the same.

I'm guessing because the voltage produced by the generator is itself the desired voltage, and keeping the step up/down ratios the same also keeps the voltage (nearly) the same.

[Floatzel98]

Hey guys. I'm just having a bit of trouble understanding how fringe spacing can be increased/decreased. How does increasing the wavelength and distance to the screen increase the spacing and reducing the slits increase it? I just can't seem to get my head around how it happens.

Also i don't understand the paragraph on Extent of diffraction from my book. It says that the extent of diffraction wavelength / size of slit = . What does this mean exactly?

One more question that I didn't understand: When white light is passed through double slits, colored fringes can be observed. What is responsible for this phenomenon?

[Zealous]

Hey guys. I'm just having a bit of trouble understanding how fringe spacing can be increased/decreased. How does increasing the wavelength and distance to the screen increase the spacing and reducing the slits increase it? I just can't seem to get my head around how it happens.

Also i don't understand the paragraph on Extent of diffraction from my book. It says that the extent of diffraction wavelength / size of slit = . What does this mean exactly?

One more question that I didn't understand: When white light is passed through double slits, colored fringes can be observed. What is responsible for this phenomenon?

These are the ways I like to think about it. May or may not be truly accurate but it worked for me.

Wavelength: Imagine an 800nm wave of light and a 300nm wave of light. If they are both trying to fit through a slit which is 400nm, which one is going to have more trouble getting through the gap? The 800nm will have much more trouble, and therefore will experience greater diffraction which will spread out the band spacing.

Distance: This one is trickier to understand. Have a look at this diagram here:



If you imagine one arrow being a maximum (bright fringe), moving the rectangle further back will result in only 3 of the fringes appearing on the rectangle, so they will spread apart compared to 5 fringes over the same area when the rectangle is much closer.

Reducing Slit: Well, it's the same as the wavelength scenario, but instead of different wavelengths changing, its different slit sizes. If we have light of 500nm wave length, is it going to have an easier time passing through a gap which is 600nm or 400nm? The smaller the slit size, the more impedence it will have with the slit and more diffraction will occur.

Extent of Diffraction: They're taking the ratio of wavelength over slit size. If the wavelength and the slit size are exactly the same, the fraction will equal 1. If the wavelength is bigger than the size of the slit, the fraction will be > 1 and if the wavelength is smaller than the slit the fraction will be < 1. You can use this to get a rough idea as to the extent of the diffraction.

White Light: White light is made up of light of many different wavelengths. When it passes through the slits, the different wavelengths that make up the white light will be diffracted to different degrees, which will cause the red light to spread differently from the green light, which leads to colored fringes.

[Floatzel98]

Thanks Zealous! It all kind of makes a bit more sense now. Then again I literally just started it today so hopefully once I've slept on it and actually practiced it a bit it becomes more obvious. The diagram helped a lot as well. Thanks

Also with the photoelectric effect quickly, is the photo current actually the 'photons' of light from the light or is it just electrons from the metal plate that create that current? I'm also kind of lost on how this predicts light as a particle and not a wave. I can't seem to understand why a wave doesn't work  in this situation. Is the fact that there isnt a delay as a particle one of the points? Why isnt there a delay for?

[Zealous]

Also with the photoelectric effect quickly, is the photo current actually the 'photons' of light from the light or is it just electrons from the metal plate that create that current? I'm also kind of lost on how this predicts light as a particle and not a wave. I can't seem to understand why a wave doesn't work  in this situation. Is the fact that there isnt a delay as a particle one of the points? Why isnt there a delay for?

It's not the photons which create the current - it's the electrons in the metal. The photons provide enough energy to the electrons so that they can "escape" the metal.

The particle model explains this because if the energy of the photon (E=hf) is less than the work function of the metal, each electron will not have enough energy to escape the metal. If we used the wave model, we'd expect energy to accumulate in the metal as a continuous flow, then the electrons will eject when they have the energy.

Think of the particle model as a one to one transaction, the photon will either transfer enough energy or the electron won't leave. Think of the wave model as a continuous transfer of energy, even if it has an extremely low energy, eventually the electron will gain enough energy over time to leave.

Because the electron flow is instantaneous, it supports the particle model the best. There are other things I could talk about such as the amplitude and intensity but I won't go into that since you've just started.

This video explains it really well: https://www.youtube.com/watch?v=GiM21YCvWh4

[lzxnl]

Hey guys. I'm just having a bit of trouble understanding how fringe spacing can be increased/decreased. How does increasing the wavelength and distance to the screen increase the spacing and reducing the slits increase it? I just can't seem to get my head around how it happens.

Also i don't understand the paragraph on Extent of diffraction from my book. It says that the extent of diffraction wavelength / size of slit = . What does this mean exactly?

One more question that I didn't understand: When white light is passed through double slits, colored fringes can be observed. What is responsible for this phenomenon?

Welcome to VCE physics, where concepts are presented in a hand-wavy manner and you're expected to understand everything perfectly for the exam. Kidding. You have a cheat sheet for that. The concepts are still hand-wavy though.

A much better interpretation of 'extent of diffraction' is 'width of the central maximum'. You see, diffraction is actually just interference (and Huygen's principle but forget that for now), so you have a central bright band and other less bright bands. It just so happens that the central bright band is gazillions brighter than the other bands (or more intense for the case of sound) so we normally consider the extent of diffraction to be the width of the central bright band.

If we define the width of the central bright band as the distance between the two perfectly dark spots (aka total destructive interference), it can be shown, by splitting the slit into two separate slits and considering this now as a two-slit interference experiment, that the angle made between a line drawn from the slit to the dark spot and a line drawn from the slit perpendicular to the slit, theta, satisfies the equation d sin theta = wavelength where d is the slit width
AKA sin theta = wavelength / slit width
If this quantity approaches 1, theta approaches 90 degrees, and 90 degrees corresponds to essentially no dark band (draw out a diagram). If the quantity gets bigger than 1, you NEVER have destructive interference and that's what VCAA means by 'complete diffraction'. It's complete in that the diffracted wave has significant amplitude everywhere.

[Cosec]

I just did a Neap practice exam, and seemed to get a bit confused with the transformer/power distribution questions,
In a quesiton like this, do you just assume the value they provided is RMS?

[Floatzel98]

I'm pretty sure it is safe to assume that AC Voltage and Current is RMS unless stated otherwise.

[knightrider]

How would you do this question?

A rope is allowed to move freely over a ‘frictionless’ pulley backstage of a theatre. A 30 kg sandbag,
which is at rest on the ground, is attached at one end. A 50 kg work-experience student, standing on
a ladder, grabs onto the other end of the rope to lower himself.

What is the tension in the rope?