VCE Physics Question Thread!

[silverpixeli]

Just had a question, when we are looking at pictures like these (attached), is the magnetic field one created from the current carrying wire or has a current carrying wire been placed into a magnetic field? I was just a bit confused as to how you can actually cause a magnetic field to not be perpendicular to the wire (As in the 3rd and 4th diagrams). Is it just maybe interacting with another magnetic field and that changes the direction of the original one?

And with motors and generators, the difference is one moves with a current through it and then one moves/gets moved to make a current? How do you increase current produced from a generator and how high can it get? .Why don't cars make use of those generator principles? I might be correct to say I'm probably not the first person to suggest that though....

Also, with magnetic flux, how exactly is a current and EMF induced induced? Does the magnetic field like use the force and bring electrons up from the ground and put them into the wire? Also, since the rate of change of flux is equal to the induced EMF, how do you get there? Can you derive or something? I know it is probably more complicated but I feel it involes differentiation somehow since it is the rate of change of the flux.

With V(RMS) or I(RMS), is it just supposed to be the positive side of the graph because it is DC or is is positive and negative values of it?

Thanks

all good questions i am on my phone right now but i will attempt to start explaining

one, its an external field, the interaction between this external field and the one i didnt draw (caused by wire) is what causes the force

two, yes thats right, and cars only need motion and they get a lot of torque from a combustion engine that injects fuel and turns a piston turning the wheels
cars also generally charge their battery by using these pistons to turn a generator too

the production of emf is just one of the cool things about electromagnetism, youll have to consult someone like lzxnl who payed attention in first year physics and is probably studying electromag this semester to fill you in on more details and maybe the derivation
or im sure you could find it online, though maybe a bit above the reding level of a vce student, just look up faraday's law

vrms and irms are the 'root mean square' of the ac signals, they are arrived at by squaring the sine wave and finding the average value of this and then sqrt the result (like standard deviation from methods probability)
this takes care of the negatives (obviously the regular average of a plain old sine wave is just 0) because when you're talking power you dont usually care about direction

[lzxnl]

Flux = BA is a definition of magnetic flux. Which only actually holds true for the very specific scenario where the magnetic field is uniform and is pointing parallel to the surface normal.
As for Faraday's law, its proper statement is indeed a derivative; the emf (aka a closed line integral of the electric field about some curve; a fancy way of defining emf) is equal to the negative total time derivative of the magnetic flux. This comes from a law that you sort of can't prove (the Maxwell-Faraday equation which relates the electric field generated to the negative partial derivative of the magnetic field) you need a foundation for every field of physics and this is one of them for electromagnetism. How do you prove F = ma in mechanics? You can't. That's essentially a definition of force.

If you want an explanation of why an EMF is induced, I'm not exactly too sure on that myself but it probably has to do with the fact that according to special relativity, electric and magnetic fields are aspects of the same phenomenon. This last point is easy to explain though.

Imagine two people, A and B. A is moving with a test particle P at velocity v, so A is in P's rest frame. B is in the lab frame, seeing P move at v. Let's put a magnetic field now in the lab frame. Assume P is moving at a constant velocity. Two observers that are both in inertial frames (non-accelerating) have to see the same forces acting. In B's frame, P is moving at speed v in a magnetic field -> there is a magnetic force on P. A must also see some force acting on P, but A is moving with P, so A sees P as stationary. The force acting on P is thus not magnetic, but electric. A must see an electric field instead of a magnetic field. This electric field is a special type of electric field for reasons I won't go into here.

But yeah, magnetism and electricity are closely intertwined.

[Floatzel98]

Flux = BA is a definition of magnetic flux. Which only actually holds true for the very specific scenario where the magnetic field is uniform and is pointing parallel to the surface normal.
As for Faraday's law, its proper statement is indeed a derivative; the emf (aka a closed line integral of the electric field about some curve; a fancy way of defining emf) is equal to the negative total time derivative of the magnetic flux. This comes from a law that you sort of can't prove (the Maxwell-Faraday equation which relates the electric field generated to the negative partial derivative of the magnetic field) you need a foundation for every field of physics and this is one of them for electromagnetism. How do you prove F = ma in mechanics? You can't. That's essentially a definition of force.

If you want an explanation of why an EMF is induced, I'm not exactly too sure on that myself but it probably has to do with the fact that according to special relativity, electric and magnetic fields are aspects of the same phenomenon. This last point is easy to explain though.

Imagine two people, A and B. A is moving with a test particle P at velocity v, so A is in P's rest frame. B is in the lab frame, seeing P move at v. Let's put a magnetic field now in the lab frame. Assume P is moving at a constant velocity. Two observers that are both in inertial frames (non-accelerating) have to see the same forces acting. In B's frame, P is moving at speed v in a magnetic field -> there is a magnetic force on P. A must also see some force acting on P, but A is moving with P, so A sees P as stationary. The force acting on P is thus not magnetic, but electric. A must see an electric field instead of a magnetic field. This electric field is a special type of electric field for reasons I won't go into here.

But yeah, magnetism and electricity are closely intertwined.

Thanks guys. Do you guys have any other resources/videos on special relativity, because i have been reading up on the basics of it for a while like the detailed study in the physics textbook, I've read a Brief History of Time and a couple other books but i don't understand it too much. I know it's a hard topic to get your head around though. Thanks

[Cosec]

Couple of questions with regards Unit 4.

1. When answers questions with regard RMS. Such as the power output when the voltage and current are given as RMS values. What is the expect answer? As in, should they be used as RMS values or converted. What happens if your given a voltage in RMS and a current in peak. To you give your answer as a RMS or peak value for say power output of a device.

2. With regards worded questions, i went to the TSFX lecture and the lecturer mentioned that in the exam one should not make refrence to the right hand grip or slap rules when answering worded questions. How should one explain them instead? Can anyone provide a perfect response to such a question as an example.

[knightrider]

Using the image attached.

How would you do these questions.

 When does the bus first start gaining ground on the bicycle?

At what time does the bus overtake the bicycle?

 How far has the bicycle travelled before the bus catches it?

[Floatzel98]

Using the image attached.

How would you do these questions.

 When does the bus first start gaining ground on the bicycle?

At what time does the bus overtake the bicycle?

 How far has the bicycle travelled before the bus catches it?

1) The bus will start  gaining ground just after the velocities are equal. Even if it is still behind the bike, at the point when their velocities are equal i will now start lowering the distance between them because it is travelling towards it faster. Velocities are equal at t = 4s.

2) Well firstly you should try to remember that this is a velocity time graph and the area under it will be the displacement. You literally just have to count the squares to find when the distance is equal and this should happen at t = 10s if i counted right.

3)So you know the time when their distance is the same so you just have to find the area under the curve at that time (the actual distance for that time). So at 10s there are 10 boxes. 1 box = 2 seconds and 4 m/s so 10 x 2 x 4 = 80m

I think i did that right.

[knightrider]

1) The bus will start  gaining ground just after the velocities are equal. Even if it is still behind the bike, at the point when their velocities are equal i will now start lowering the distance between them because it is travelling towards it faster. Velocities are equal at t = 4s.

2) Well firstly you should try to remember that this is a velocity time graph and the area under it will be the displacement. You literally just have to count the squares to find when the distance is equal and this should happen at t = 10s if i counted right.

3)So you know the time when their distance is the same so you just have to find the area under the curve at that time (the actual distance for that time). So at 10s there are 10 boxes. 1 box = 2 seconds and 4 m/s so 10 x 2 x 4 = 80m

I think i did that right.

Thanks so much Floatzel98 
Really helped and yes you are right.   

[knightrider]

For this multiple choice question which option is right and why?

A stone is dropped vertically into a lake. Which one of the following statements best describes the motion
of the stone at the instant it enters the water?
A Its velocity and acceleration are both downwards.
B It has an upwards velocity and a downwards acceleration.
C Its velocity and acceleration are both upwards.
D It has a downwards velocity and an upwards acceleration.

[Floatzel98]

For this multiple choice question which option is right and why?

A stone is dropped vertically into a lake. Which one of the following statements best describes the motion
of the stone at the instant it enters the water?
A Its velocity and acceleration are both downwards.
B It has an upwards velocity and a downwards acceleration.
C Its velocity and acceleration are both upwards.
D It has a downwards velocity and an upwards acceleration.

Well i think it's safe to say that it's velocity is still going down as soon as it hits the water. It hasn't stopped or floated upwards or anything because it just entered the water. So we can cancel out B and C. I think the logic here is that because the stone slows down immediately after it hits the water it decelerates which means that there is now a greater upwards force on it than it gained from falling. So the acceleration is now much greater in the opposite direction. Leaving it to be D i think. 

[knightrider]

Well i think it's safe to say that it's velocity is still going down as soon as it hits the water. It hasn't stopped or floated upwards or anything because it just entered the water. So we can cancel out B and C. I think the logic here is that because the stone slows down immediately after it hits the water it decelerates which means that there is now a greater upwards force on it than it gained from falling. So the acceleration is now much greater in the opposite direction. Leaving it to be D i think.

Thanks so much Floatzel98 

[Zealous]

Couple of questions with regards Unit 4.

1. When answers questions with regard RMS. Such as the power output when the voltage and current are given as RMS values. What is the expect answer? As in, should they be used as RMS values or converted. What happens if your given a voltage in RMS and a current in peak. To you give your answer as a RMS or peak value for say power output of a device.

2. With regards worded questions, i went to the TSFX lecture and the lecturer mentioned that in the exam one should not make refrence to the right hand grip or slap rules when answering worded questions. How should one explain them instead? Can anyone provide a perfect response to such a question as an example.

1. Keep it all in RMS when you are working out the power output. You can find the RMS average current and voltage then take the product of them to find the RMS power output.

2. I personally think it's okay to refer to the rules, as long as you've properly explained why you have used the rule. So if you are using the RH grip rule, you might explain that the current in a wire causes a change in magnetic field, and therefore the magnetic field will have a certain direction as dictated by the right rule. Or if you are using the slap rule, you might explain that a current carrying wire will experience a force in a magnetic field and this force will act in a direction dictated by the slap rule. That's personally how I did things - there may be better ways of doing it but I dont think it's necessary.

Well i think it's safe to say that it's velocity is still going down as soon as it hits the water. It hasn't stopped or floated upwards or anything because it just entered the water. So we can cancel out B and C. I think the logic here is that because the stone slows down immediately after it hits the water it decelerates which means that there is now a greater upwards force on it than it gained from falling. So the acceleration is now much greater in the opposite direction. Leaving it to be D i think. 

Great explanation!

[Floatzel98]

With the attached picture. I'm having trouble with the direction of the current in question 4 and 6. I don't really understand the explanation of the answer from the book. Can someone try to explain it for me.

Also with question 5, is D wrong because they change the direction of the magnetic field before they actually move the coil? It's not happening while it is getting pulled out. If it was all happening simultaneously, it would induce a greater EMF though, right?

Thanks

[odeaa]

For q4 and 6, I used to struggle with those as well but I finally got my head around it (I think, havent got my SAC back yet lol)
Initially, there is a downwards north flux. When the coil is removed, we have a change in the north flux (I just call it that, but the polarity is important) in the upwards direction (because the flux is decreasing). To oppose this, the coil induces a current that will create a downwards force, which is clockwise when viewed from above, and I think you know how to find the magnitude of the current judging from your question

As for question 5, D is incorrect because while there is a change in flux when they change the direction of the magnet, this change is only temporary and the system kinda becomes used to it (its like using a DC battery for a transformer- there is only a temporary change in flux). When the coil is removed, the same emf is generated in the coil as when the magnet was in the original orientation, just in the opposite direction.

I dont think they would ever ask you what would happen if it was being changed simueltaneously, because the only way I can think of doing it is to graph the flux and then derive it to find the emf, which is outside the scope of the course (I could be wrong though)

[Floatzel98]

For q4 and 6, I used to struggle with those as well but I finally got my head around it (I think, havent got my SAC back yet lol)
Initially, there is a downwards north flux. When the coil is removed, we have a change in the north flux (I just call it that, but the polarity is important) in the upwards direction (because the flux is decreasing). To oppose this, the coil induces a current that will create a downwards force, which is clockwise when viewed from above, and I think you know how to find the magnitude of the current judging from your question

As for question 5, D is incorrect because while there is a change in flux when they change the direction of the magnet, this change is only temporary and the system kinda becomes used to it (its like using a DC battery for a transformer- there is only a temporary change in flux). When the coil is removed, the same emf is generated in the coil as when the magnet was in the original orientation, just in the opposite direction.

I dont think they would ever ask you what would happen if it was being changed simueltaneously, because the only way I can think of doing it is to graph the flux and then derive it to find the emf, which is outside the scope of the course (I could be wrong though)

Thanks for the help. I'm still wrapping my head around it all so i have a few questions still. I understand everything up to the part in bold. I understand the direction of the changing flux, i just don't think i understand the opposing directions properly. Why is it a downwards force. i thought the magnetic field direction was the important part, it needs to oppose the changing flux?

[odeaa]

Thanks for the help. I'm still wrapping my head around it all so i have a few questions still. I understand everything up to the part in bold. I understand the direction of the changing flux, i just don't think i understand the opposing directions properly. Why is it a downwards force. i thought the magnetic field direction was the important part, it needs to oppose the changing flux?

Thats the part I was confused with as well, I'll try to explain it a bit better
Because the initial direction of the flux was downwards (assume when I'm talking about direction, I'm always talking about direction of North, because this is whats important), when we remove the loop we have a decreasing downwards flux, or an increasing upwards flux (just depends which way you think about it, but it is very important that the direction is included). To oppose this, because the system always wants to oppose the change in flux, we need an increasing downwards force to oppose the increasing upwards force (or decreasing downwards force, again just depends how you visualise it). Because of the right hand grip rule, to create a downwards force, we need a current going clockwise.

Hope that makes more sense!