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# need help with a physics electromagnetic question watch

1. I am stuck on 3 questions which are all kind of the same thing I think...I dunno which formula to use.
here are the 3 questions. btw i think once I understand question 1 i'll understand 2 but the method for question 3 is different from the other 2 right?
1) A horizontsal wire of length 1m, places perpendicular to the magnetic meridian falls at a constant sppeed of 30ms^-1 at a place where the horizontal component of Earth's magnetic field is 2.0 x 10^-5T. Calculate the induced e.m.f between the ends of the wire and draw a diagram to show which end is at the higher potential.

2) A vertical car radio aerial is 1m long. Calculate the steady e.m.f induced across it when the car is travelling at 80km h^-1 in an E to W direction. What would be the e.m.f induced if the car was travelling N to S? (Assume horizontal component of earth's magnetic field is 2.0 x 10^-5T).

3) An aeroplane of wingspan 30m flies horizontally at a speed of 1000km h^-1. What is the e/m/f between the wing-tips if the vertical component of the Earth's magnetic field is 4.0 x 10^-5T?

I really need these questions explaining to me as I have no idea on how to start them. Much appreciated if anyone can help me out here.
2. (Original post by hero)
I am stuck on 3 questions which are all kind of the same thing I think...I dunno which formula to use.
here are the 3 questions. btw i think once I understand question 1 i'll understand 2 but the method for question 3 is different from the other 2 right?
1) A horizontsal wire of length 1m, places perpendicular to the magnetic meridian falls at a constant sppeed of 30ms^-1 at a place where the horizontal component of Earth's magnetic field is 2.0 x 10^-5T. Calculate the induced e.m.f between the ends of the wire and draw a diagram to show which end is at the higher potential.

2) A vertical car radio aerial is 1m long. Calculate the steady e.m.f induced across it when the car is travelling at 80km h^-1 in an E to W direction. What would be the e.m.f induced if the car was travelling N to S? (Assume horizontal component of earth's magnetic field is 2.0 x 10^-5T).

3) An aeroplane of wingspan 30m flies horizontally at a speed of 1000km h^-1. What is the e/m/f between the wing-tips if the vertical component of the Earth's magnetic field is 4.0 x 10^-5T?

I really need these questions explaining to me as I have no idea on how to start them. Much appreciated if anyone can help me out here.
ok these use Faradays law which says that the induced emf = -d(phi)/dt ie that the voltage through the wire is due to the rate at which flux lines are cut. so first you need to find phi. this is equal to B (the magnetic flux density)* A the area.

now we know B and A is the length * the velocity. to give you the area swept per second. Which basically answers the question for you because it includes the time (ie you are dividing by 1) so the answer to 1 is:

Emf = - (2*10^-5 * 30*1)/1
= - 6*10^-4 volts.

The minus is only there because the voltage is always so as to oppose the force causing its generation

The bit about potential is due to the fact that it is perpendicular & so the bit of wire nearer to the magnetic source is at a higher potential. I'll draw you a diagram in a minute.
3. thank you i understand it now
4. Ok so I hope the diagram helps. Question 3 is exactly the same by the way. Just take the wingtips as the 2 ends of the wire & its exactly the same:

Emf = - magnetic flux density * length * velocity
5. thank you that was really helpful, these questions arent as hard as they seem now
thanks again
6. (Original post by hero)
thank you that was really helpful, these questions arent as hard as they seem now
thanks again
you are welcome. Im just doing EM so Im hot on it. I much prefer doing yours to mine

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Updated: February 7, 2005
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