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問題的種類

自然科學 大學

求解答案是否正確

Multiple-choice questions: (70 points) write the letter of statement that is not true: 1. A.B.U The Drude Model: (a) It is a classical approach using concepts from the kinetic theory of gases. (b) A relaxation-time approximation is used where the probability of collision is l/t. (c) No forces act on the electrons in between collisions. (d) It predicts accurately the electronic heat capacity of metals. 2. The Sommerfeld Model: (a) The crystal potential is replaced by an average Coulomb potential. (b) The separation of states is so close that a continuum model can be applied. (c) The electrons are highly localized due to a strong crystal potential. (Q) The energy of the highest occupied state at 0 K is the Fermi energy. 3. Density of states in k-space, N(k): (a) N(k) is determined by the boundary value conditions. (b) For lattices in any dimensions, N(k) is constant in k-space. (C) N(k) depends on the direction and magnitude of k. (d) It is directly proportional to the volume of the crystal. E影響, c 4. The Fermi surface of a metal: (a) is responsible for many of the transport properties of the metal, (b) depends on the nature of the chemical bonds, (c) is always a sphere, (d) can involve mixing of electron orbitals. ✓ 5. The conductivity of a material () increases with the mobility of its carriers, (b) increases with the density of carriers, (C) it is inversely proportional to the energy gap. cogu changes with temperature. 6. BID Plasmons: (a) They are an important characteristic of the Sommerfeld electron gas model. (b) Plasmons occur spontaneously in metals. (c) They are related to charge density oscillations. (d) The Plasmon energy is typically of the order of 10 eV. The Fermi energy Ef in intrinsic semiconductors 8) is usually close to y E, at OK (b) it ACiD. can increase or decrease with temperature depending on the ratio of hole and electron effective masses, (c) it represents the point at which the probability of occupation is 12, even though there may not been any states with that energy, (d) its value relative to the electronic bands must remain unaffected in the presence of defects or interfaces.

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英文 高中

求詳解,感恩

OW dyainst factories that cause pollution. 閱讀測驗3 But the universal of our climate message of crisis wareness hing a fever pitch across the with increasing urgency, a pparent every day. For the Vandana Shiva will never forget a lesson she learned at the age of 13. Her parents insisted that she should wear clothing made only of homespun cotton. One day, Vandana, having returned from a boarding school to her home, demanded a nylon dress, the fashion followed by her rich friends. Her mother agreed. “If that is what you want, of course you shall have it,” she said. “But remember, your nylon dress will help a rich man buy a bigger car. And the cotton that you wear will buy a poor family at least one meal.” Did she buy the nylon dress? Of course not! Maybe this experience led to her resolution to fight for social justice in her later years. Now 60, ever visiting Taiwan in 2010, Vandana's main concern these days is how to preserve Indian traditional crops. This is severely attacked by many global companies. They encourage farmers to grow so-called high-yielding crops; many farmers, therefore, turn to high-tech methods and depend on bioengineered seeds, chemical fertilizers and pesticides. As a result, hundreds of traditional crops are disappearing. Contrary to expectation, high-tech agriculture can not rescue Indians from starvation. Bioengineered seeds are expensive and cannot adapt to local conditions. Hundreds of Indian farmers have committed suicide during the past five he world ironment. ns in 1970. years because of failed harvests. But there is hope. Many farmers are returning to traditional methods promoted by Navdanya, an organization based in New Delhi that Vandana helped found 20 years ago. Navdanya encourages farmers to produce native varieties of crops that can be grown with natural fertilizer and no artificial chemicals. It also helps local farmers form their own self-supporting organization and seed bank. nean? +明一 生能但奇 63

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