[Materials Science & Engineering 2] – 4th Assignment
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1. A parallel-plate capacitor using a dielectric material having an εr of 2.5 has a plate spacing
of 1 mm (0.04 in.). If another material having a dielectric constant of 4.0 is used and the
capacitance is to be unchanged, what must be the new spacing between the plates?
2. The polarization P of a dielectric material positioned within a parallel-plate capacitor is to
be 1.0 x 10-6 C/m2 . (a) What must be the dielectric constant if an electric field of 5 x 104 V/m
is applied? (b) What will be the dielectric displacement D?
3. The magnetic flux density within a bar of some material is 0.435 tesla at an H field of 3.44
x 105 A/m. Compute the following for this material: (a) the magnetic permeability, and (b) the
magnetic susceptibility. (c) What type(s) of magnetism would you suggest is(are) being
displayed by this material? Why?
4. Confirm that there are 2.2 Bohr magnetons associated with each iron atom, given that the
saturation magnetization is 1.70 x 106 A/m, that iron has a BCC crystal structure, and that the
unit cell edge length is 0.2866 nm.
5. Estimate (a) the saturation magnetization, and (b) the saturation flux density of nickel ferrite
[(NiFe2O4)8], which has a unit cell edge length of 0.8337 nm.
6. Briefly explain why the magnitude of the saturation magnetization decreases with increasing
temperature for ferromagnetic materials, and why ferromagnetic behavior ceases above the
Curie temperature.
7. An iron bar magnet having a coercivity of 4000 A/m is to be demagnetized. If the bar is
inserted within a cylindrical wire coil 0.15 m long and having 100 turns, what electric current
is required to generate the necessary magnetic field?
8. For a superconducting material at a temperature T below the critical temperature TC, the
critical field HC (T), depends on temperature according to the relationship
where HC(0) is the critical field at 0 K.
(a) Using the data in Table 20.7, calculate the critical magnetic fields for tin at 1.5 and 2.5 K.
(From the table, for Sn, TC = 3.72 K and BC(0) = 0.0305 tesla.)
(b) To what temperature must tin be cooled in a magnetic field of 20,000 A/m for it to be
superconductive