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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

(a) Define the following terms:
    1. magnetic flux density;
    2. magnetic flux.

 

(b) An e. m. f. of 1.35V is induced in a conductor of effective length 0.25 cm.  The conductor moves at right angles through a magnetic field at a velocity of  5ms-1.  Calculate the magnetic flux density of the field.

_____________________________________________________________________________________________________
OBSERVATION

The expected answers were:
(a)(i)     Magnetic flux density is the amount of magnetic flux per unit area of a
            section, perpendicular to the direction of flux.
                        B =  Φ
                                A
            where   B  =  magnetic flux density in Tesla (T)
                        Φ  =  magnetic flux in webers (Wb)
                        A  =  area in square metres (m2)

The S. I. unit of magnetic flux density in weber per square metre (Wb/m2)
(ii)        Magnetic flux is the group of magnetic field lines emitted outward from
the north pole of a magnet.  The symbol for magnetic flux is Φ (phi).

The S. I. unit of magnetic flux is the weber (Wb)

(b)        E  =  1.375v
            L  =  0.25cm = 2.5 x 10-3m     
            v   =  5ms-1

            E   = Blv
: .         B   = E
                     Lv

           B    =          1.375
                   hh 
                            2.5 x 10-3 x 5
                      
                          B    =  110 Wb/m2

This question was on magnetic field.  Many candidates could not just give the definitions of B and Φ correctly. 

Many candidates did not convert from centimetres to metres thus getting the final answer wrong.

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