Average EMF (1 Viewer)

[ils4826]

A closely wound rectangular coil of 85.0 turns has dimensions of 30.0 cm by 47.0 cm. The plane of the coil is rotated from a position where it makes an angle of 31.0 degrees with a magnetic field of 1.20T to a position perpendicular to the field. The rotation takes 9.00×10−2s.

This is what I attempted:
I found the area by multiplying A=L*W (multiplied cm then converted to m). Change in flux = B*A. then EMF=-(change in flux)/time. But I doesn't seem to work. Please Help!

 

[alcalder]

A closely wound rectangular coil of 85.0 turns has dimensions of 30.0 cm by 47.0 cm. The plane of the coil is rotated from a position where it makes an angle of 31.0 degrees with a magnetic field of 1.20T to a position perpendicular to the field. The rotation takes 9.00×10−2s.

This is what I attempted:
I found the area by multiplying A=L*W (multiplied cm then converted to m). Change in flux = B*A. then EMF=-(change in flux)/time. But I doesn't seem to work. Please Help!

Convert measurements in cm to metres before doing anything else. Always convert anything in non SI units before using them.

Don't forget there are 85 turns and hence A = nLW

Flux at start = nBA cos(angle between B and the plain of area A)

= 85 x 1.2 x 0.3 x 0.47 x cos 31 = 12.32778 Wb

Flux at end = 85 x 1.2 x 0.3 x 0.47 x cos 90 = 14.382 Wb

Therefore:

Change in flux = Flux at end - Flux at start = 2.05422 Wb

EMF = - (Change in Flux)/time
EMF = - 2.05422/9.00×10⁻²
EMF = - 22.8 V (to 3 sig fig)

Gosh I hope that's right otherwise that would be very embarassing.

 

[xiao1985]

re^

i agree with everything cept cos 31 part... in this case i believe it is right to use sin 31, as you are calculating the part of coil which is perpendicular to the field

 

[jyu]

Convert measurements in cm to metres before doing anything else. Always convert anything in non SI units before using them.

Don't forget there are 85 turns and hence A = nLW

Flux at start = nBA cos(angle between B and the plain of area A)

= 85 x 1.2 x 0.3 x 0.47 x cos 31 = 12.32778 Wb

Flux at end = 85 x 1.2 x 0.3 x 0.47 x cos 90 = 14.382 Wb

Therefore:

Change in flux = Flux at end - Flux at start = 2.05422 Wb

EMF = - (Change in Flux)/time
EMF = - 2.05422/9.00×10⁻²
EMF = - 22.8 V (to 3 sig fig)

Gosh I hope that's right otherwise that would be very embarassing.

I can spot three misses from the calculation.

(1) Flux at start = 85 x 1.2 x 0.3 x 0.47 x cos 59

(2) Flux at end = 85 x 1.2 x 0.3 x 0.47 x cos 0

(3) Another possibility: Flux at end = 85 x 1.2 x 0.3 x 0.47 x cos 180

 

[xiao1985]

I can spot three misses from the calculation.

(1) Flux at start = 85 x 1.2 x 0.3 x 0.47 x cos 59

(2) Flux at end = 85 x 1.2 x 0.3 x 0.47 x cos 0

(3) Another possibility: Flux at end = 85 x 1.2 x 0.3 x 0.47 x cos 180

totaly agrees

though whether it's cos0 or cos180 only change the direction at which the currect flows...

 

[jyu]

though whether it's cos0 or cos180 only change the direction at which the currect flows...

This statement is false. Why?

 

[xiao1985]

This statement is false. Why?

my bad... i wasn't thinking thoroughly when typing that up...

it seems it really depends on whether it (the coils) rotates through an obtuse angle or acute angle...