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Applied Electricity Paper 2 , Nov/Dec. 2008  
Questions:   1 2 3 4 5 6   Main
General Comments
Weakness/Remedies
Strength










































Question 2

  1. Explain the following terms:
  2. magnetic field strength;
  3. magnetic flux density.

 

  1. A coil of 240 turns is wound uniformly on a wooden ring of mean circumference 60cm and a uniform cross-sectional area of 500 mm2.

If a current of 5A flows through the coil, calculate the:

  1. magnetizing force;
  2. flux density.

[Take µo=4 x 10-7H/m]

_____________________________________________________________________________________________________
Observation

The expected answers are as follows:

  1. (i)  Magnetic field strength is the magnetomotive force per unit length.    Its  symbol is H.

  H =mmf/ l
(ii) Magnetic flux density is the flux magnetic per unit area
Flux density, B = / A

  1. (i) Magnetic field strength, H =mmf/ l= NI/l

                                                                   

                                                  H = 1200 = 2000AT/m
                                                           0.6
(ii) Flux density, B = µoH
                             B = (4 x 10-7 x 2000)T = 0.002512 T = 2.51mT

The question was on Electromagnetic Field. It was attempted by many candidates. The performance was less than average.

Many of the candidates who attempted this question did well in the explanation of the magnetic flux density but missed a(i) and the entire (b) parts.

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