VCE Physics Question Thread!

[Floatzel98]

Hey guys, i finally started UNIT 4 AOS 1 and I am a bit confused on somethings.

Firstly, i think my main problem is not understanding how a magnet works. Like what is flowing around a magnetic field? Electrons? What makes them move around exactly? What actually gives a magnet it's magnetism? More in the sense of a permanent magnet? How does this relate to the poles of a magnet. Is it what is inside a magnet that makes it magnetic, or does it relate more to it's magnetic field? What is different between the two poles and how can atoms/electrons(??) in a magnet form different poles. Does it stop and end at a certain point in the middle? I guess one of my questions is, what exactly is the field made out of and/or does anything flow in the field? How is this different/the same as a gravitational field?

I think this might tie into what we learn later, but my textbook says that a piece of iron will have magnetism induced in it when placed in an external magnetic field. Is this specific to Iron? Will it create it's own magnetic field or just use the other one, and how exactly does any of that work? My book explains that it happens but says it is somehow induced the piece of iron to become a magnet?

What is the difference between the grip and palm rule? I feel like they are doing the same thing. Well i understand the grip rule; your thumb shows the direction of current in a wire and your fingers will show the direction of the magnetic field. While the palm rule does the same then but then also shows the force? My book says that it gives the direction of the force on a current carrying wire placed in an external field? Does this mean it always involve >2 magnets pulling each other?

This one might be a bit hard for me to explain, but in a simple rectangular magnet, we always see that the fields always come from the two poles. And i know these are just illustrations, but how many field 'lines can actually come out of a magnet? Are there actually space in between all of them or do they fill up all the space it can? How much distance is between each field 'line'? Do all the field 'lines' come back to the south pole of the same magnet? Sometimes it looks as if there is a straight line coming out of the top of the magnet, where does this go. Does it just stop? Also, probably just bounded by book illustrations, but do all the field lines go out in all directions. Does it form an 'oval' shape around the entire magnet? And then would a magnetic field around a length of wire form a cylindrical shape around the wire?  This might not make sense because i might be totally confused and wrong.

In my book there is an equation for the strength of an electric field written as . What exactly is the constant K? Is it referring to, like if the magnet was is space or in air or underwater?

Lastly, i got up to the force on a wire carrying a charge in a magnetic field. Probably still relating to the palm rule question, where and what is this force exactly? Secondly I'm a bit confused with the equation. We have which I understand, but then we 'derive' it to . I understand that the B force needs to be perpendicular to the current from the pictures in my textbook, but i don't understand how we find B perpendicular. I also got confused with this in Torque when i did the detailed study.

Spoiler

Is this picture a correct way of doing it? Can we just use cos as well? Sometimes i feel i can just use cos of an angle and it makes it easier.

Really lastly, my book has a part talking about how we express the relationship between the 3 vectors Force, Length and Field though vector cross multiplication. It says that 'if we write F = Il x B this is taken to mean that the magnitude of vector F is the product IlBsin(theta)'. I know this might be a bit more maths now, but how does the cross product work to get this, and is this the same way they find Torque as well?

I don't expect anyone to answer every question, but thanks to anyone that can help at all. I did try to find some of my answers elsewhere before looking here, but i didn't really find anything that helped.

[odeaa]

Hey guys, i finally started UNIT 4 AOS 1 and I am a bit confused on somethings.

Firstly, i think my main problem is not understanding how a magnet works. Like what is flowing around a magnetic field? Electrons? What makes them move around exactly? What actually gives a magnet it's magnetism? More in the sense of a permanent magnet? How does this relate to the poles of a magnet. Is it what is inside a magnet that makes it magnetic, or does it relate more to it's magnetic field? What is different between the two poles and how can atoms/electrons(??) in a magnet form different poles. Does it stop and end at a certain point in the middle? I guess one of my questions is, what exactly is the field made out of and/or does anything flow in the field? How is this different/the same as a gravitational field?

I think this might tie into what we learn later, but my textbook says that a piece of iron will have magnetism induced in it when placed in an external magnetic field. Is this specific to Iron? Will it create it's own magnetic field or just use the other one, and how exactly does any of that work? My book explains that it happens but says it is somehow induced the piece of iron to become a magnet?

What is the difference between the grip and palm rule? I feel like they are doing the same thing. Well i understand the grip rule; your thumb shows the direction of current in a wire and your fingers will show the direction of the magnetic field. While the palm rule does the same then but then also shows the force? My book says that it gives the direction of the force on a current carrying wire placed in an external field? Does this mean it always involve >2 magnets pulling each other?

This one might be a bit hard for me to explain, but in a simple rectangular magnet, we always see that the fields always come from the two poles. And i know these are just illustrations, but how many field 'lines can actually come out of a magnet? Are there actually space in between all of them or do they fill up all the space it can? How much distance is between each field 'line'? Do all the field 'lines' come back to the south pole of the same magnet? Sometimes it looks as if there is a straight line coming out of the top of the magnet, where does this go. Does it just stop? Also, probably just bounded by book illustrations, but do all the field lines go out in all directions. Does it form an 'oval' shape around the entire magnet? And then would a magnetic field around a length of wire form a cylindrical shape around the wire?  This might not make sense because i might be totally confused and wrong.

In my book there is an equation for the strength of an electric field written as . What exactly is the constant K? Is it referring to, like if the magnet was is space or in air or underwater?

Lastly, i got up to the force on a wire carrying a charge in a magnetic field. Probably still relating to the palm rule question, where and what is this force exactly? Secondly I'm a bit confused with the equation. We have which I understand, but then we 'derive' it to . I understand that the B force needs to be perpendicular to the current from the pictures in my textbook, but i don't understand how we find B perpendicular. I also got confused with this in Torque when i did the detailed study.

Spoiler

Is this picture a correct way of doing it? Can we just use cos as well? Sometimes i feel i can just use cos of an angle and it makes it easier.

Really lastly, my book has a part talking about how we express the relationship between the 3 vectors Force, Length and Field though vector cross multiplication. It says that 'if we write F = Il x B this is taken to mean that the magnitude of vector F is the product IlBsin(theta)'. I know this might be a bit more maths now, but how does the cross product work to get this, and is this the same way they find Torque as well?

I don't expect anyone to answer every question, but thanks to anyone that can help at all. I did try to find some of my answers elsewhere before looking here, but i didn't really find anything that helped.

I know this probably doesnt help, but unfortunately no-one really knows the answer to half those questions ahah
While we have explanantions for almost everything, these are based on assumptions, and the deeper you go in physics the more you realise how little we actually know as 100% true. For the purposes of VCE, we just take all these assumptions to be true
I'll let someone else with more knowledge (probs silverpixeli lol) answer the questions, but just thought I would chime in with my two cents

[lzxnl]

Hey guys, i finally started UNIT 4 AOS 1 and I am a bit confused on somethings.

Firstly, i think my main problem is not understanding how a magnet works. Like what is flowing around a magnetic field? Electrons? What makes them move around exactly? What actually gives a magnet it's magnetism? More in the sense of a permanent magnet? How does this relate to the poles of a magnet. Is it what is inside a magnet that makes it magnetic, or does it relate more to it's magnetic field? What is different between the two poles and how can atoms/electrons(??) in a magnet form different poles. Does it stop and end at a certain point in the middle? I guess one of my questions is, what exactly is the field made out of and/or does anything flow in the field? How is this different/the same as a gravitational field?

I think this might tie into what we learn later, but my textbook says that a piece of iron will have magnetism induced in it when placed in an external magnetic field. Is this specific to Iron? Will it create it's own magnetic field or just use the other one, and how exactly does any of that work? My book explains that it happens but says it is somehow induced the piece of iron to become a magnet?

What is the difference between the grip and palm rule? I feel like they are doing the same thing. Well i understand the grip rule; your thumb shows the direction of current in a wire and your fingers will show the direction of the magnetic field. While the palm rule does the same then but then also shows the force? My book says that it gives the direction of the force on a current carrying wire placed in an external field? Does this mean it always involve >2 magnets pulling each other?

This one might be a bit hard for me to explain, but in a simple rectangular magnet, we always see that the fields always come from the two poles. And i know these are just illustrations, but how many field 'lines can actually come out of a magnet? Are there actually space in between all of them or do they fill up all the space it can? How much distance is between each field 'line'? Do all the field 'lines' come back to the south pole of the same magnet? Sometimes it looks as if there is a straight line coming out of the top of the magnet, where does this go. Does it just stop? Also, probably just bounded by book illustrations, but do all the field lines go out in all directions. Does it form an 'oval' shape around the entire magnet? And then would a magnetic field around a length of wire form a cylindrical shape around the wire?  This might not make sense because i might be totally confused and wrong.

In my book there is an equation for the strength of an electric field written as . What exactly is the constant K? Is it referring to, like if the magnet was is space or in air or underwater?

Lastly, i got up to the force on a wire carrying a charge in a magnetic field. Probably still relating to the palm rule question, where and what is this force exactly? Secondly I'm a bit confused with the equation. We have which I understand, but then we 'derive' it to . I understand that the B force needs to be perpendicular to the current from the pictures in my textbook, but i don't understand how we find B perpendicular. I also got confused with this in Torque when i did the detailed study.

Spoiler

Is this picture a correct way of doing it? Can we just use cos as well? Sometimes i feel i can just use cos of an angle and it makes it easier.

Really lastly, my book has a part talking about how we express the relationship between the 3 vectors Force, Length and Field though vector cross multiplication. It says that 'if we write F = Il x B this is taken to mean that the magnitude of vector F is the product IlBsin(theta)'. I know this might be a bit more maths now, but how does the cross product work to get this, and is this the same way they find Torque as well?

I don't expect anyone to answer every question, but thanks to anyone that can help at all. I did try to find some of my answers elsewhere before looking here, but i didn't really find anything that helped.

Firstly, it's good that you're looking deeper into course material. However, you'll find that the more you look into VCE physics, the more you'll be confused as a better understanding of electromagnetic fields is...not expected of high schoolers.

Now to attempt to answer these questions. Magnetic fields are better thought of as disturbances that charges interact with. It's a bit like a gravitational field in this regard. However, the gravitational field as a vector is directed parallel to the direction of the force it would exert. A magnetic field's direction is the direction of zero torque on a magnetic dipole (i.e. the direction a compass needle would point).
Magnetic fields are, according to relativity, a different form of electric field seen by an observer moving relative to the charge. In other words, electric charges set up electric fields and anyone moving with respect to the electric charge sees a magnetic field. From our perspective, therefore, a moving charge, or a current, generates a magnetic field. Another way of setting up a magnetic field is to possess an intrinsic magnetic moment (think of this like a compass needle; has a direction). Any magnetic substance (I'm not talking about diamagnetism here) has magnetism due to their electronic structure. Paramagnetic materials have unpaired electrons. A good example would be NO2. Things like iron are magnetic because their electrons, who have their own magnetic moments, all line up so you get a macroscopic magnetic moment -> permanent magnet.
So, iron in a magnetic field does not use the other magnetic field. The external magnetic field lines up the electron magnetic moments, giving the iron its own magnetic field which will persist even after the external field is removed

The grip and palm rules are used for different things. One finds the force from a current and magnetic field. One finds the magnetic field from a current and vice versa. They are all related to vector cross products though. And yes, magnetic forces require two magnets, or one magnet in a magnetic field generated by some means.

Field lines are a mathematical construct. They are essentially just a vector field, which is a function whose inputs are x, y and z coordinates and the output is a vector. How many points are there in a parabola? That question is equally meaningless as how many field lines there can be.
As for field line shapes, they must always form closed loops. This is a fundamental law of electromagnetism known as Gauss's law for magnetism.

You are right about the cylindrical symmetry of a magnetic field around a straight wire. They form circles/cylinders (depending on how you want to think about them).

The magnetic field strength equation you have is for the magnetic field around a wire. If it is indeed the field due to a current-carrying wire, then K is equal to the permeability constant mu divided by 2pi. It may change depending on the medium though.

As for the force on a wire, this force can actually be explained in terms of relativity but I won't go into the details. F = IlB only works if the force is exactly perpendicular to the magnetic field. F = il x B (vector cross product) and the sin theta comes from the magnitude of a cross product. Torque is also defined via a cross product so it might be good to read up on those. With cross products and dot products, the angle is ALWAYS between the two vectors. No exception.

The fact that there is a sin theta in the magnitude of the cross product is a definition. You have to just live with it unfortunately

I know this probably doesnt help, but unfortunately no-one really knows the answer to half those questions ahah
While we have explanantions for almost everything, these are based on assumptions, and the deeper you go in physics the more you realise how little we actually know as 100% true. For the purposes of VCE, we just take all these assumptions to be true
I'll let someone else with more knowledge (probs silverpixeli lol) answer the questions, but just thought I would chime in with my two cents

Don't be TOO discouraging for the asker

[odeaa]

Don't be TOO discouraging for the asker

I feel like a dick now lol

[Floatzel98]

Firstly, it's good that you're looking deeper into course material. However, you'll find that the more you look into VCE physics, the more you'll be confused as a better understanding of electromagnetic fields is...not expected of high schoolers.

Now to attempt to answer these questions. Magnetic fields are better thought of as disturbances that charges interact with. It's a bit like a gravitational field in this regard. However, the gravitational field as a vector is directed parallel to the direction of the force it would exert. A magnetic field's direction is the direction of zero torque on a magnetic dipole (i.e. the direction a compass needle would point).
Magnetic fields are, according to relativity, a different form of electric field seen by an observer moving relative to the charge. In other words, electric charges set up electric fields and anyone moving with respect to the electric charge sees a magnetic field. From our perspective, therefore, a moving charge, or a current, generates a magnetic field. Another way of setting up a magnetic field is to possess an intrinsic magnetic moment (think of this like a compass needle; has a direction). Any magnetic substance (I'm not talking about diamagnetism here) has magnetism due to their electronic structure. Paramagnetic materials have unpaired electrons. A good example would be NO2. Things like iron are magnetic because their electrons, who have their own magnetic moments, all line up so you get a macroscopic magnetic moment -> permanent magnet.
So, iron in a magnetic field does not use the other magnetic field. The external magnetic field lines up the electron magnetic moments, giving the iron its own magnetic field which will persist even after the external field is removed

The grip and palm rules are used for different things. One finds the force from a current and magnetic field. One finds the magnetic field from a current and vice versa. They are all related to vector cross products though. And yes, magnetic forces require two magnets, or one magnet in a magnetic field generated by some means.

Field lines are a mathematical construct. They are essentially just a vector field, which is a function whose inputs are x, y and z coordinates and the output is a vector. How many points are there in a parabola? That question is equally meaningless as how many field lines there can be.
As for field line shapes, they must always form closed loops. This is a fundamental law of electromagnetism known as Gauss's law for magnetism.

You are right about the cylindrical symmetry of a magnetic field around a straight wire. They form circles/cylinders (depending on how you want to think about them).

The magnetic field strength equation you have is for the magnetic field around a wire. If it is indeed the field due to a current-carrying wire, then K is equal to the permeability constant mu divided by 2pi. It may change depending on the medium though.

As for the force on a wire, this force can actually be explained in terms of relativity but I won't go into the details. F = IlB only works if the force is exactly perpendicular to the magnetic field. F = il x B (vector cross product) and the sin theta comes from the magnitude of a cross product. Torque is also defined via a cross product so it might be good to read up on those. With cross products and dot products, the angle is ALWAYS between the two vectors. No exception.

The fact that there is a sin theta in the magnitude of the cross product is a definition. You have to just live with it unfortunately

Don't be TOO discouraging for the asker

Thanks so much for that response lzxnl. It really helps. I think i will be able to just move on with the rest of the course now since all that information will tie me over and I  don't need to confuse myself any further.

Also, thanks Odeaa. Most of those questions are just ramblings in my head i feel i need answers too. I just try to connect everything for myself so I can understand it. But obviously a lot of these things are out of VCE, so your right, i shouldn't try to question everything too much. Don't feel bad

Thanks guys

[paper-back]

Why does the EMF graph get cut off when using a split-ring commutator? whereas when using an AC slip ring commutator, it goes the entire length?

[lzxnl]

Why does the EMF graph get cut off when using a split-ring commutator? whereas when using an AC slip ring commutator, it goes the entire length?

The split ring commutator connects the wire loop in the magnetic field to a different battery terminal. Every half a revolution the wire loop's polarity switches. Hence the voltage/current direction changes sharply.

[paper-back]

The split ring commutator connects the wire loop in the magnetic field to a different battery terminal. Every half a revolution the wire loop's polarity switches. Hence the voltage/current direction changes sharply.

But doesn't using AC current in the loop also mean that there is a change in the direction of the current? Why isn't the graph when using slip rings also cut off?

[odeaa]

With the slip rings, the direction of the induced emf in your wires changes from positive (max flux) to negative (inverse of max flux ie when the loop is upside down)
With the commutator, it is always positive, as the direction of the induced emf is always in the same direction, although it still cycles with the flux from maximum to minimum every quarter turn
hope that makes sense, its a hard topic to explain in writing

[paper-back]

When drawing EMF graphs from the magnetic flux graphs, do we draw the negative gradient function of the magnetic flux graph?

E.g. for a sine flux graph, we draw a negative cosine EMF graph and,
for a cosine flux graph, we draw a positive sine EMF graph?

[silverpixeli]

When drawing EMF graphs from the magnetic flux graphs, do we draw the negative gradient function of the magnetic flux graph?

E.g. for a sine flux graph, we draw a negative cosine EMF graph and,
for a cosine flux graph, we draw a positive sine EMF graph?

yep sounds good! this isnt really assessed though (but anything goes on your sac)
to determine the direction that current actually flows in a circuit (and then make sense of which way you have defined positive to be on your graph) you'll need to think through lenz' law for the situation

[paper-back]

Thanks silverpixeli and odeaa

[dankfrank420]

Starting to get into electromagnetism for Physics, and the amount of times that the words "but you don't have to know about that, it's not on the course" is frustrating.

Well, what exactly are we learning about in electromagnetism? Are we barely scratching the surface and not really learning anything at all?

I've been able to do many past questions without actually having a fundamental understanding of what electromagnetism exactly is. Why is this fundamental understanding not even taught or even needed?

[lzxnl]

When drawing EMF graphs from the magnetic flux graphs, do we draw the negative gradient function of the magnetic flux graph?

E.g. for a sine flux graph, we draw a negative cosine EMF graph and,
for a cosine flux graph, we draw a positive sine EMF graph?

Technically, yes. It's meant to be a derivative but welcome to VCE physics...where year 9 maths is all that's required...

But doesn't using AC current in the loop also mean that there is a change in the direction of the current? Why isn't the graph when using slip rings also cut off?

A slip ring commutator does nothing except maintain the electrical connections. A split ring commutator emf graph looks the same as a slip ring commutator graph, except every half revolution the voltage changes sign. Mathematically, this sign change corresponds to a change in sign of a sine function, meaning your sine function (the voltage) is always positive. To represent this by a function, the voltage with a split ring commutator is |a sin bt| where a and b are constants and the voltage with a slip ring commutator is just a sin bt. No mod signs.

Starting to get into electromagnetism for Physics, and the amount of times that the words "but you don't have to know about that, it's not on the course" is frustrating.

Well, what exactly are we learning about in electromagnetism? Are we barely scratching the surface and not really learning anything at all?

I've been able to do many past questions without actually having a fundamental understanding of what electromagnetism exactly is. Why is this fundamental understanding not even taught or even needed?

If you want a fully blown and completely blunt rant, PM me and I'll be more than happy to give you one on how inadequate the VCE physics course is. Yes, you essentially don't learn anything about electromagnetism in VCE physics. In a nutshell, this is essentially all you do in AoS 3. Which I've taught to my physics student in around three lessons. Maybe four.

Magnetic fields of bar magnets, right hand rules for currents/magnetic fields of wires/loops, induction, Lenz's law, transformers, magnetic forces on wires, motors/generators

Have I missed out anything that doesn't fit into one of those categories? It's not a huge list. Here is a short list of important bits of electromag that you should get but aren't getting.
1. What is magnetism
2. What is a field
3. Electric fields (if you're going to learn about magnetic fields, why not electric fields)
4. Energy of things in electric/magnetic fields (i.e. why is a compass needle stable when it points in the same direction as the magnetic field)
5. When things are NOT perpendicular or parallel

I mean VCAA cut out one of the detailed studies so I don't see why they don't put some content in so that students like you aren't completely confused about everything

[dankfrank420]

This topic is more frustrating than all I've learnt so far. It's so confusing that we're missing vital pieces, and the teacher doesn't really help. It's like putting together a jig saw without all the pieces. We're learning about all this specific stuff without going over the fundamental core of the course - whose idea was that?

I finally see where you and pi and all the others are coming from, VCE Physics is a joke.