FAKULTI KEJURUTERAAN MEKANIKALUNIVERSITI TEKNIKAL MALAYSIA MELAKA

SAINS BAHANBMCB 2423TUTORIAL 4

Physical properties of Materials

1. A wire 0.40 cm in diameter must carry a 25 A current.(a)If the maximum power dissipation along the wire is 0.025 W/cm,

what is the minimum allowable electrical conductivity of the wire (give answer in SI units)?

(b)What is the current density in the wire?

2. An iron wire is to conduct a 6.5 A current with a maximum voltage drop of 0.005 V/cm. What must be the minimum diameter of the wire in meters at (20°C)?

3. Calculate the electrical resistivity (in ohm-meters) of a silver wire 15 m long and 0.030 m in diameter at 160°C. [e(Fe at 0°C) = 9.0 × 10-6 · cm.]

4. At what temperature will an iron wire have the same electrical resistivity as an aluminum one has at 35°C?

5. Why are the valence-electron energy levels broadened into bands in a solid block of a good conducting metal such as sodium?

6. Why is the 3s electron energy band in a block of sodium only half-filled?

7. How does the energy-band model explain the poor electrical conductivity of an insulator such as pure diamond?

8. A photon in a ZnO semiconductor drops from an impurity level at 2.30 eV to its valence band. What is the wavelength of the radiation given off by the transition? If the radiation is visible, what is its color?

9. Explain why cut diamonds sparkle. Why is PbO sometimes added to make decorative glasses?

10. Explain why gold is yellow in color and silver is “silvery.”

11. Calculate the reflectivity of ordinary light from a smooth, flat upper surface of (a) borosilicate glass (n = 1.47)(b) polyethylene (n = 1.53).

12. Ordinary incident light strikes the flat surface of a transparent material with a linear absorption coefficient cm-1. If the plate of the material is 0.80 cm thick, calculate the fraction of light absorbed by the plate.

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FAKULTI KEJURUTERAAN MEKANIKALUNIVERSITI TEKNIKAL MALAYSIA MELAKA

SAINS BAHANBMCB 2423TUTORIAL 4

Physical properties of Materials

13 Calculate the minimum wavelength of the radiation that can be absorbed by the following materials: (a)GaP, (b) GaSb, (c) InP.

14. How can a magnetic field around a magnetized iron bar be revealed?

15. Define magnetic permeability and relative magnetic permeability.

16. Describe two mechanisms involving electrons by which magnetic fields are created.

17. Define diamagnetism. What is the order of magnitude for the magnetic susceptibility of diamagnetic materials at 20°C?

18. Define paramagnetism. What is the order of magnitude for the magnetic susceptibility of paramagnetic materials at 20°C?

19. Calculate a theoretical value for the saturation magnetization of pure gadolinium below 16°C assuming all seven unpaired 4f electrons contribute to the magnetization. (Gd is HCP with a = 0.364 nm and c = 0.578 nm.)

20. Cobalt has a saturation magnetization of 1.42 × 106 A/m. What is its average magnetic moment in Bohr magnetons per atom?

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