ENGRI111_Mid_04_solutions

ENGRI111_Mid_04_solutions - Page 1 of 8 ENGRI 111 MIDTERM...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Page 1 of 8 ENGRI 111 MIDTERM EXAM Wednesday October 6, 2004 in class Time Length: 50 minutes, Total Points: 100 _______________________________________________________________________ Suggestions for how to structure your answers: write simple and concise answers incomplete sentences are ok some answers only require a single word or number some answers only require a well labeled diagram Q1: 25 points a) When many atoms are put together to form a solid, the electrons reside in bands. Describe qualitatively the band structures of a metal and an insulator and any differences between these band structures. Use words and schematic diagrams. (11 points) In the band structure of a metal, the valence band is not entirely full (3 points). Observing that there can be a conduction band in a metal and that the conduction band is empty earns 1 bonus point. A completely accurate sketch is 2.5 points and does not need to include the conduction band—if the conduction band is included in the sketch and it is indicated that it is empty, 1 bonus point is awarded if the written answer did not include this already. In the band structure of an insulator, the valence band is full (2 points). The conduction band is nearly empty (1 point) (if the answer describes a temperature at which there are or there are not electrons in the conduction band, this is also acceptable). A completely accurate sketch is 2.5 points. If the answer notes in addition that the band gap between the conduction band and the valence band is usually greater than 3 volts in an insulator, then 1 bonus point is awarded. Partially Filled Valence Band Empty Conduction band Filled Valence Band Empty Conduction band Band Gap Band Gap
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Page 2 of 8 ENGRI 111 MIDTERM EXAM Wednesday October 6, 2004 in class Time Length: 50 minutes, Total Points: 100 _______________________________________________________________________ b) CdSe has a bandgap of 1.8 eV i) What color is CdSe? Justify your answer. (5 points) With a bandgap of 1.8eV, CdSe would absorb energy from visible light at energies greater than 1.8 eV, hence leaving some red light at 1.7 eV not being absorbed. Hence the color would be reddish. (2.5 points for getting the color correctly, 2.5 points for a correct explanation) ii) How will the color of a nanoparticle of CdSe differ from the color of normal CdSe? Justify your answer. (3 points) In a nanoparticle, the band gap is larger because there are fewer atomic interactions and less spreading of the bands. If the band gap gets larger, then not only would red light not be absorbed, but yellow or green light would not be absorbed, depending on how much the band gap increased. Hence the color would become more yellow or green or even blue (2 points for correlating the band gap change with a color change, 1 point for saying that the bigger the band gap increase the more the trend toward blue). c) Sodium (Na) and potassium (K) are both Group I elements.
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This homework help was uploaded on 02/03/2008 for the course ENGRI 1110 taught by Professor Giannelis during the Fall '07 term at Cornell.

Page1 / 8

ENGRI111_Mid_04_solutions - Page 1 of 8 ENGRI 111 MIDTERM...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online