TSFX 2005 Phys trial exam

[Ninox]

Hey everyone,
I just need some help with question 4. Electric Power.

An aeroplane with a wing span of 40m flies over the North Magnetic Pole at 120m/s. The Mag. field strength is 5x10^-4 T.
a) What is the induced EMF?

They use the formula F = BLv =2.4 Volts
I don't quite get how this works though.
Could someone please explain this?
Thanks

Good luck with the exam tomorrow

[Galelleo]

Ok.

the wings are basically linear, so you can assume they will be treated as a rod moving through a magnetic field. the formula for that is BLv because...

emf = BA/t .... A= (ina  square lengthxwidth) which are measures of distance... so you have BxLxL/t ... L/t (length on time) is velocity... because if the length of the wingspan is considered length in one direction, its movement forward is considered the other... so we can use velocity.

therefore BL x L/t = BLv.

BLv = 5x10^-4 x 40 x 120 = 2.4
does that answer your question?

[Galelleo]

But you shuold just know the formula E(voltage) = BLv... hurry, go write it on your cheat sheet ! !:P

[Galelleo]

I dont like this question tho, because the rod is moving horizontally, as well as its got width and depth ( hence volume), so you would expect to have to use area

i doubt VCAA would use this sort of example because we tend to focus on 2 dimensions.

[Ninox]

Ok.

the wings are basically linear, so you can assume they will be treated as a rod moving through a magnetic field. the formula for that is BLv because...

emf = BA/t .... A= (ina  square lengthxwidth) which are measures of distance... so you have BxLxL/t ... L/t (length on time) is velocity... because if the length of the wingspan is considered length in one direction, its movement forward is considered the other... so we can use velocity.

therefore BL x L/t = BLv.

BLv = 5x10^-4 x 40 x 120 = 2.4
does that answer your question?

I didn't quite understand your explanation the first time, but I thought about BA/t and I get it. My thinking is that the wings sort of transcribe an area in space. So BA/t in 1 second  = Bx(40x12)/1 = 2.4V
I imagine a rectangle being drawn in 1 second as the plane flies over. And different values of distance and time work too. But I'm happy.