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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: matteh on November 08, 2008, 03:51:16 pm

Title: EMF = Blv
Post by: matteh on November 08, 2008, 03:51:16 pm: is this on the course? in the TSFX 2008 they use this, where a plane is traveling through a constant magnetic field at a constant speed, i would have thought a current is only induced for a changing flux?
Title: Re: EMF = Blv
Post by: cara.mel on November 08, 2008, 03:53:44 pm: what's the exact question?

if the area of the loop is changing that will induce a current, that could be it.

edit: TSFX are good at including stuff not on the course. I don't remember the VCAA study design any more but I'm leaning towards no its not on it but I'm not confident in my answer
Title: Re: EMF = Blv
Post by: matteh on November 08, 2008, 03:59:10 pm: "An aeroplane with a wing span of 25 meters is flying over the south magnetic pole located within the Antartic, at 540 km hr^-1. The magnetic field with this region is vertically orientated and the magnetic field strength is 6.0 x 10^-5 T"

--- "What is the magnitude of the emf induced across the wing tips?"

there's a diagram with a plane and field lines underneath it pointing up towards it, here are the solutions by tsfx..

"velocity = 540 = ms−
emf = Bl v = 6.0 × 10−5 × 25 ×150 = 0.23V (0.225V )"
Title: Re: EMF = Blv
Post by: ReVeL on November 08, 2008, 04:02:03 pm: Wtf. Never seen that before.
Title: Re: EMF = Blv
Post by: champorado on November 08, 2008, 04:22:51 pm: Oo, there was a question like that on the 2006 paper.

Too long to write, but basically a wire that forms a square with side length 2cm. It travels through a magnetic field at 4cm/s. Find the maximum induced voltage in the loop if the field strength is 3.7 x 10^-3.

You could either use Blv, which is just a simple .02 x .04 x 3.7 x 10^-3, or you could use n(delta)flux / (delta)time. It's a bit more work, but you get the same answer.

Anyway, when the plane enters the pole's field, the flux does change. Most of the questions on induced voltage I've seen use the latter formula, though. Just write it on your formula sheet, it won't take up that much space.
Title: Re: EMF = Blv
Post by: onlyfknhuman on November 08, 2008, 04:46:06 pm: Yes blv is in the course,

i find tsfx really insightful lol, and that question is a simple substition of numbers. :D

Hopefully we dont get something circuit and transformer based, like that tram question in 04 was it, fckn hated that.
Title: Re: EMF = Blv
Post by: matteh on November 08, 2008, 04:46:33 pm: ah yea i remember that question, didn't use Blv though. This sort of thing relates to an instantaneous sort of induced emf right? because it doesn't specify that the plane exits the field so it wouldn't be constantly producing an emf correct?
Title: Re: EMF = Blv
Post by: onlyfknhuman on November 08, 2008, 04:48:41 pm: Too what i can see, yea its instantaneous.
Title: Re: EMF = Blv
Post by: mark_alec on November 08, 2008, 05:06:21 pm: I'll try to offer some explanations for the emf induced across a conductor of length $l$ , travelling with velocity $v$ through a magnetic field $B$ . This is not required knowledge.

The conductor has charges in it, and by travelling through the magnetic field, a force $qvB$ will cause them to travel to one side of the wire. This charge separation in turn, creates an electric field $E$ which has a force on the charges $qE$ . This will happen until an equilibrium is reached when $qvB = qE$ . There is now an electric field across the conductor, and since $V = Ex =$ , where $x$ is the distance, it is clear that $V = El = vBl$

Alternatively, using a concept of flux. $V = -\frac{d\Phi}{dt} = -B\frac{dA}{dt} = -Bl\frac{dx}{dt} = -Blv$
Title: Re: EMF = Blv
Post by: onlyfknhuman on November 08, 2008, 05:08:38 pm: what the ... i feel stupid
Title: Re: EMF = Blv
Post by: matteh on November 08, 2008, 05:26:52 pm: Ah yeah, how would you determine which end the charge builds up on? I don't see how you can do this with RH grip rule?
Title: Re: EMF = Blv
Post by: onlyfknhuman on November 08, 2008, 05:29:55 pm: current = flow of positive charge.

hence direction of current is the direction were positive charge are.

And obviously the opposite is were the negative charge are.
Title: Re: EMF = Blv
Post by: matteh on November 08, 2008, 05:34:50 pm: yeah but how do you determine that from just the direction of the field?
Title: Re: EMF = Blv
Post by: mark_alec on November 08, 2008, 05:39:35 pm: Quote from: matteh on November 08, 2008, 05:26:52 pm
Ah yeah, how would you determine which end the charge builds up on? I don't see how you can do this with RH grip rule?
Right hand slap rule. The force on a positive charge moving with a velocity in the direction of your thumb, through a magnetic field in the direction of your fingers (when out straight), will be perpendicular to your palm (in the direction you would 'slap'.)
Title: Re: EMF = Blv
Post by: onlyfknhuman on November 08, 2008, 05:39:52 pm: what?

Right hand slap rule => palm = force / fingers = field / thumb = current . charge flows within a conductor . so no idea what you dont know.
Title: Re: EMF = Blv
Post by: onlyfknhuman on November 08, 2008, 05:40:47 pm: Quote from: mark_alec on November 08, 2008, 05:39:35 pm
Quote from: matteh on November 08, 2008, 05:26:52 pm
Ah yeah, how would you determine which end the charge builds up on? I don't see how you can do this with RH grip rule?
Right hand slap rule. The force on a positive charge moving with a velocity in the direction of your thumb, through a magnetic field in the direction of your fingers (when out straight), will be perpendicular to your palm (in the direction you would 'slap'.)

Slap a bitch how i remember it :D

Mark_alec can you check what i wrote in the other thread about diffraction.
Title: Re: EMF = Blv
Post by: matteh on November 08, 2008, 09:00:14 pm: you dont have the force.. all you have is whats stated in the question (my 2nd post)
Title: Re: EMF = Blv
Post by: onlyfknhuman on November 08, 2008, 09:10:38 pm: Quote from: matteh on November 08, 2008, 03:59:10 pm
"An aeroplane with a wing span of 25 meters is flying over the south magnetic pole located within the Antartic, at 540 km hr^-1. The magnetic field with this region is vertically orientated and the magnetic field strength is 6.0 x 10^-5 T"

--- "What is the magnitude of the emf induced across the wing tips?"

there's a diagram with a plane and field lines underneath it pointing up towards it, here are the solutions by tsfx..

"velocity = 540 = ms−
emf = Bl v = 6.0 × 10−5 × 25 ×150 = 0.23V (0.225V )"

omg dude... improvise a bit.

Lenz law, right hand grip rule. as long as you know the direction of current you can find it.

Hence the field of earth initially increases the flux hitting the plane.. hence the plane will respond to counter act this and induce, flux in the opposite direction. from this you can find out the direction of current = direction of postive charge.
Title: Re: EMF = Blv
Post by: NGUB on November 09, 2008, 08:45:19 am: No, i think it's not on the course, i remember doing the same question on tsfx 2006 i think, but i asked my teacher and she said it's not on the course, and they're not supposed to ask it.