Question 4
(a) Define resistivity of a material.
(b) A magnetic circuit has a flux of 1.671 mWb and a cross-sectional area of 3 mm2.
Calculate the flux density.
(c) Explain different between a permanent magnet and an electromagnet.
Observation
The expected responses were:
- Definition of Resistivity of a material
Resistivity of a material is a measure of the extent to which the material opposes the flow of current
OR
It is the resistance of a unit length of material per unit cross sectional area.
- Flux
= 1.671 mWb = 1.671 x 10 -3
Area A = 3 mm2 = 3 x ( 10 -3) 2 = 3 x 10 -6
B = 
= 
= 
= 557 Wb/m2 or 557 T
- Difference between Permanent Magnet and Electromagnet
Permanent magnets are materials in which the magnetic fields are generated by their internal structure while an electromagnet is a material that generates its magnetic field when current flows through a coil associated with it.
Question 4 required candidates to define the resistivity of a material and also calculate the flux density of a magnetic circuit. While majority of the candidates were said to have responded well to the question in general, it was however reported that the major difficulty many candidates had with this question was with the calculation of the flux density of a magnetic circuit.