HSC Physics Question Thread

[kiwiberry]

Hey! So the electric field/potential difference set up by the depletion zone is directed from the N type to the P type, due to the excess positive charge in the N-type and the excess negative charge in the P-type. This sets up an electric field which pushes positive charges/holes towards the P-type, and negative charges/electrons towards the N-type. It doesn't really matter where it happens (just imagine it happening somewhere in the depletion zone), but when a photon of appropriate frequency strikes the diode, it frees an electron (thus also forming a hole). This is called an electron-hole pair. The electric field pushes electrons into the N-type, and holes towards the P-type - This constitutes the flow of current.

This video does a great job of explaining it

This is a highly simplified version of what actually happens, mind you. Solar cells aren't actually just sticking a P and an N type semiconductor together and shining a light on it. But this is a perfectly acceptable description for HSC Physics

Ah thanks so much Jamon

[seventeenboi]

Hi
how do you explain the output (emf vs time) graph of a dc or ac motor in an explicit/detailed manner???
This is what I have in my notes so far:
AC:
Constant connection between rotating coil and external circuit due to split rings
As polarity of induced emf changes with every half-turn of the coil
Voltage varies like a sine wave
Current changes direction   
 
DC:
Every half turn, a reversal of connection between coil and external circuit occurs due to split rings
As polarity of induced emf changes with every half turn
Voltage in external circuit fluctuates between 0 and maximum allowing current to flow in a constant direction

ALSO, for the bolded/ underlined parts "As polarity of induced emf changes with every half turn" is this applicable to both AC and DC generators? I'm not sure if i typed up my notes incorrectly :/
thanks!!

[pikachu975]

Hi
how do you explain the output (emf vs time) graph of a dc or ac motor in an explicit/detailed manner???
This is what I have in my notes so far:
AC:
Constant connection between rotating coil and external circuit due to split rings
As polarity of induced emf changes with every half-turn of the coil
Voltage varies like a sine wave
Current changes direction   
 
DC:
Every half turn, a reversal of connection between coil and external circuit occurs due to split rings
As polarity of induced emf changes with every half turn
Voltage in external circuit fluctuates between 0 and maximum allowing current to flow in a constant direction

ALSO, for the bolded/ underlined parts "As polarity of induced emf changes with every half turn" is this applicable to both AC and DC generators? I'm not sure if i typed up my notes incorrectly :/
thanks!!

For DC it uses split ring commutator not split rings. Also the polarities don't change that's just AC.

Firstly a motor doesn't have a purpose of inducing EMF, motors use supplied EMF to generate mechanical energy.

For a motor the current supplied just changes direction every half turn due to the split ring commutator

[seventeenboi]

For DC it uses split ring commutator not split rings. Also the polarities don't change that's just AC.

Firstly a motor doesn't have a purpose of inducing EMF, motors use supplied EMF to generate mechanical energy.

For a motor the current supplied just changes direction every half turn due to the split ring commutator

oops sorry! yeup change my 'motor' to a 'generator' HAHAHHA sorry ;-;;

[jamonwindeyer]

oops sorry! yeup change my 'motor' to a 'generator' HAHAHHA sorry ;-;;

Woop! Aha pretty much everything pikachu stays relevant, make sure you know:

- DC generators use split ring commutators, AC generators use slip rings (this results in the differences in output current patterns you describe)
- By default, the polarity of induced emf does swap every half turn. However, in DC, the split ring commutator maintains constant current direction

[Jyrgal]

hello, im pretty confused on the concept of voltage between electrical plates. Lets say one plate has 0V and the other has 100V. In my way of thinking, this means that when I place 1 coloumb of negative electric charge near the 100V plate, the charge will possess 100J of potential energy since 100V=100J/1c. This means the charge should flow from the 100V to 0V plates, meaning the 0V should be the positive terminal and 100V should be the negative. Is this correct?

[jamonwindeyer]

hello, im pretty confused on the concept of voltage between electrical plates. Lets say one plate has 0V and the other has 100V. In my way of thinking, this means that when I place 1 coloumb of negative electric charge near the 100V plate, the charge will possess 100J of potential energy since 100V=100J/1c. This means the charge should flow from the 100V to 0V plates, meaning the 0V should be the positive terminal and 100V should be the negative. Is this correct?

Hey! You are close, except if it is a negative charge, it would move towards the 100V plate (the plate with higher potential). If you swapped to a positive charge, then you'd be 100% on the money

[Jyrgal]

Hey! You are close, except if it is a negative charge, it would move towards the 100V plate (the plate with higher potential). If you swapped to a positive charge, then you'd be 100% on the money

Ohhhh so electric potential for voltage relates with positive charge not negative charge. Thanks for clearing that up! hehe

[jamonwindeyer]

Ohhhh so electric potential for voltage relates with positive charge not negative charge. Thanks for clearing that up! hehe

Yeah exactly! Basically, wherever the higher voltage is, that is the place where you can think of a lot of positive charge being. So, positive charges will want to get away from it, negative charges will want to head towards it

Glad I could help

[Bubbly_bluey]

Hi! If a question says to talk about application of photoelectric effect in solar cells,is that asking how the solar cell actually works(p-n junctions and electrons moving around to make electricity) or it is talking about applications as in solar panels and solar powered devices?
Thanks

[jamonwindeyer]

Hi! If a question says to talk about application of photoelectric effect in solar cells,is that asking how the solar cell actually works(p-n junctions and electrons moving around to make electricity) or it is talking about applications as in solar panels and solar powered devices?
Thanks

The former - You don't need to know much at all about how solar cells are used!

[Bubbly_bluey]

The former - You don't need to know much at all about how solar cells are used!

Thanks! Also,  when a photon enters the solar cell, does it strike the n-type to release the excess electron to flow back to the hole in p-type?

[jamonwindeyer]

Yep! It can technically strike anywhere near the depletion zone, but most explanations at this level tend to default to the N-type, and that is totally fine

[Bubbly_bluey]

hi again! So I'm starting to get confuse: is a Solid State Device a fancy name for a transistor? (are they the same thing?) I have to  talk about the invention of the transistor
Thanks

[jakesilove]

hi again! So I'm starting to get confuse: is a Solid State Device a fancy name for a transistor? (are they the same thing?) I have to  talk about the invention of the transistor
Thanks

Yep! Well, technically, a solid state device describes a broad range of electronic items, including transistors, but for the sake of the HSC they are one and the same