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#### Vincezor

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##### Vincezor's Unit 4 Physics Questions~
« on: September 24, 2011, 08:13:51 pm »
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It's the holidays, and I've finally somewhat finished the physics course.

So anyway, first question of the day:

Explain, referring to magnetic fields, the principle reason why the current-carrying conductor experiences the magnetic force when placed in the magnetic field.

Maybe it is time to revise some electric power, but I feel as I have "some" idea on how to answer this question, I just don't quite know how...
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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #1 on: September 24, 2011, 08:35:23 pm »
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Explain, referring to magnetic fields, the principle reason why the current-carrying conductor experiences the magnetic force when placed in the magnetic field.
$F=nIlBsin\theta$ summarises the relationship quite nicely. My approach would be to expand off what that equation tells us.

#### paulsterio

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #2 on: September 24, 2011, 09:38:38 pm »
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That formula does summarise it quite nicely, but it's just a mathematical relationship, to explain, we have to look at the concept of what happens when a current is passed through a conductor, it generates its own Magnetic Field around the conductor, thus when it is placed in another field, it is the interaction between the two fields, the magnetic attraction, that produces a force on the wire, pushing it towards one side of the magnetic field, the side it is pushed to is then determined using the Right Hand Slap Rule, the Magnitude of force is given by the equation F = nBILsin(theta)

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #3 on: September 25, 2011, 12:03:42 am »
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I prefer to start off by stating the mathematical representation mainly because I find that easier to remember and that's what will be provided on our formula sheets (well a less descriptive version usually) - useful for mind blanks. There is a field $B$. $Il$ talks about current passing through a length of wire. Current passing through a wire means that a field would be induced. $F$ is a vector, so that's where RH slap rule comes in. $B$. The $sin \theta$ serves as a reminder that the field and the wire cannot be parallel et cetera. I just keep going off that until I've described what I was aiming to describe.

I guess it's a backwards approach to what Paul stated. I imagine it would probably differ from person to person, I guess it's just the way I choose to think about it.

#### paulsterio

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #4 on: September 25, 2011, 12:31:14 am »
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Yeah, I understand that too, and I guess whenever I can't think of anything, I just state the formula and use it to describe the relationships, so at least that way I get some of the marks

If you were interested though, the formula and the sin(theta) part come from the vector cross product. The length of the wire is a scalar quantity, so it doesn't matter, same as the number of wires, n, but the two vector quantities, the current, I, and the field, B, are multipled using a vector cross product (there are two ways of multiplying vectors, the cross product (which you'll do in Specialist) where you get a scalar, and the cross product (which you'll do in First Year Uni Maths), where you get a vector) - and the vector cross product is defined as the Product of the Magnitudes of the two vectors by the sin of the angle between them in the direction perpendicular to both of them.

So the Vector Cross Product explains the formula F = nIBL sin(theta)  as well as the Right-Hand slap rule - finding the direction of the Force (direction perpendicular (orthagonally) to the Current and Field)

That's the maths behind it

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #5 on: September 25, 2011, 10:36:31 am »
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I took a look at the derivations and stuff earlier in the term, it's pretty damn interesting. KhanAcademy has a few videos on cross products and dot products and stuff in their physics section. They went into enough detail to ensure that I could understand what was going on.

There was also a textbook that I was reading that explained the derivations for a lot of physics formulas step by step, can't remember which book it was though.

#### Vincezor

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #6 on: September 26, 2011, 05:01:07 pm »
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Question 2:

I've always though the poles align or something, but to be honest I've forgetten quite alot of Electric power, so this is probably a simple question
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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #7 on: September 26, 2011, 05:49:45 pm »
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Yeah, a magnet when placed in a magnetic field will try to align itself with the field. You can see this happening in a compass or a suspended magnet aligning itself with the Earth's magnetic field. I would put down 'A' as my answer.

If you're having trouble remembering stuff, I would suggest spending a couple of hours or so just re-reading the textbook and also reading iTute notes etc. That should help you refresh your knowledge.

#### paulsterio

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #8 on: September 26, 2011, 06:10:25 pm »
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That question looks familiar, it's from one of the 2011 Exams I remember doing it

The answer is A - magnetic field lines run from North to South, meaning that the left side will be north and the right side will be south, hence the bar magnet's south will want to be near the north side (left)

#### Vincezor

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #9 on: October 07, 2011, 03:56:17 pm »
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I was wondering, can point X also be "reflected across the x axis" and be at that point as well?

I labelled 'X' at that point and am not sure if I'm also correct :S I think I am though!
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#### paulsterio

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #10 on: October 07, 2011, 03:58:19 pm »
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No you can't because it says in the question "distance BX - distance AX" which means that distance AX has to be the smaller quantity (:

#### Vincezor

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #11 on: October 07, 2011, 07:54:10 pm »
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No you can't because it says in the question "distance BX - distance AX" which means that distance AX has to be the smaller quantity (:

Ohhh damn guess I didn't read the question correctly. Sigh it's the minute things that get me D: Gotta learn to read T___T
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#### paulsterio

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##### Re: Vincezor's Unit 4 Physics Questions~
« Reply #12 on: October 07, 2011, 10:05:05 pm »
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Ohhh damn guess I didn't read the question correctly. Sigh it's the minute things that get me D: Gotta learn to read T___T

Yeah, and then once you realise your mistake you think to yourself OMG, I should have got that, it's a hard feeling! D: