Tutorial 1
Magnetism and electromagnetism
1. State Ampere’s law. How do you use the law to define a flux line and flux density?
2. State Ampere’s circuit law. How is it used in magnetic circuit analysis?
3. What is the magnetic force which creates magnetic flux density? What name is used for it?
4. What is permeability and relative permeability? What are the magnetic quantities it relates?
5. Explain what is magnetomotive force and compare it with electromotive force?
6. In a magnetic circuit, the core has an air gap. Why is the reluctance of the air gap much higher than the
rest of the circuit?
7. Write the expression for energy stored magnetic field. Convert the expression to energy density form.
8. State the dot convention for mutually coupled coil in terms of the flux direction and also in terms of emf
induced if the flux is varying.
9. Distinguish between leakage flux and fringing flux. How are these accounted for empirically?
10. For the couple circuit of the figure 1 below place the dot on each coil.
Figure 1
11. In the magnetic circuit of figure 2 below the coil F2 is supplying 500 AT in the direction indicated. Find
the AT (in magnitude and direction) that the coil F1 must provide to produce a flux of 4 mWb in the air
gap in the central limb from A to B. the relative permeability of the core is 4500.
Figure 2
12. In the magnetic circuit shown in Figure 3 below the area of cross-section of the central limb is 12 cm2
and that of each outer limb (A to B) is 6 cm2. A coil current 0.5 A produces 0.5 mWb in the air-gap. Find
the relative permeability of the core material.
Figure 3
13. For the magnetic circuit of Figure 4 below,
a) Calculate the energy stored in the core and in the air-gap for a coil current of 4 A. what will these
value be if µr=∞?
b) Calculate the excitation current and induced emf in the coil to produce a flux of 0.4sin314t mWb in
the air gap.
c) Calculate the inductance of the coil. What will be its value for µr=∞? Figure 4