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1、Warming-up I. Can you point out what kind of resistor components they are in the following pictures? II. Do you know what the following abbreviations or symbols stand for? LDR LED LCR III. Talk about Current in English. And the following patterns you can use: Do you know the symbol R? Could you tell
2、 me how many kinds of resistors there are? And what are they?,Unit 4 Resistance,下一页,返回,Could you tell me the unit of resistance? What is the symbol of resistance? The unit / symbol of resistance is Text Comprehension Resistance In electricity, resistance is the opposition to current flow in a body w
3、hen there is a potential difference between the ends of the body. In order to compare the resistance of different substances, it is necessary to consider the resistance offered by a sample of fixed dimensions. It is usual to choose a cube as the sample body, and the dimensions of the cube will depen
4、d on the system of units we employ. The resistance between opposite face of this sample cube is called resistivity, which is the electric field per unit current density. The greater the resistivity, the greater the field needed to establish a given current density, or the smaller the current density
5、 caused by a given field.,Unit 4 Resistance,上一页,下一页,返回,A “perfect” conductor would have zero resistivity, and a “perfect” insulator would have infinite resistivity. Metals and alloys having the lowest resistivities are the best conductors. Experiments show that the resistance of a wire is directly p
6、roportional to its length, inversely proportional to its crosssectional area, and directly proportional to the resistivity of the substance forming the wire. Resistance components are as follows: heating elements, resistors, variable resistors, thermistors and lightdependent resistors(LDRs). If the
7、resistors are connected in series The greater the resistance, the larger the opposition to current, the more the potential drop. Therefore, the resistors can be used to control the current and voltage in the circuit, thats to say, to decrease voltage, divide voltage or current, and restrict the curr
8、ent, and so on. The resistors have values measured in ohms. A resistor of 100is a much greater obstacle of current than a resistor of 10. variable resistors have values that can be altered so it is possible to adjust the current flow in a circuit.,Unit 4 Resistance,上一页,下一页,返回,Vocabulary 1. oppositio
9、n n. 相对，对抗 9. establish v. 建立，形成 2. compare v. 比较，对照 10. variable adj.可变的，可调的 3. substance n. 物质 11. resistor n. 电阻器 4. sample n. 样品，标本 12. thermistor n. 热敏电阻 5. dimension n. 尺寸，量纲 13. restrict v. 限制，节流 6. cube n. 立方体 14. obstacle n. 障碍，阻碍 7. density n. 密度，强度 15. alter v. 改变，变更 8. resistivity n. 电阻率
10、 16. adjust v. 调节，调整 Phrases and expressions 1.depend on 依赖，取决于 2.cross-sectional area 横截面积 3.heating elements 发热元件,Unit 4 Resistance,上一页,下一页,返回,4.lightdependent resistors 光敏电阻 5.connect in series 串联 6.as follows 如下 Notes 1. A “perfect” conductor would have zero resistivity. 译成：理想的导体具有零电阻率。或者：理想的导体电
11、阻率为零。此处 “perfect”译成“理想的”而不要译成“完美的”。 2. “having the lowest resistivities”该短语是现在分词短语作后置定语。 3. “the resistors can be used to control the current and voltage in the circuit”全句译为 “ 电阻器可用来控制电路中的电流和电压，即可用来降压、分压、分流、限流等等。”,Unit 4 Resistance,上一页,下一页,返回,I Choose the right answer from A, B, and C according to t
12、he text. 【 】1.The unit of the resistance is _. A. volt B. ampere C. ohm 【 】2. The current density depends on _. A. resistance B. resistivity C. current 【 】3. A conductor has _ resistivity than an insulator. A. greater B. smaller C. Not mentioned. 【 】4. The resistance of a wire is related to its _. A
13、. length B. cross-sectional area C. both A and B 【 】5. The value of _ can be altered to adjust the current in a circuit. A thermistors B. variable resistors C. LDR,Unit 4 Resistance,上一页,下一页,返回,II True or false. 1. The resistance is opposite to the current in a circuit. 2. The smaller the resistivity
14、, the smaller the current density caused by a given field. 3. The resistance of a wire is inversely proportional to the resistivity of the substance forming the wire. 4. The resistors can be used to divide voltage or current. 5. The resistors have values in the unit of ohm. III Translate the followi
15、ng sentences. 1. In electricity, resistance is the opposition to current flow in a body when there is a potential difference between the ends of the body. 2. The greater the resistivity, the greater the field needed to establish a given current density.,Unit 4 Resistance,上一页,下一页,返回,3. Experiments sh
16、ow that the resistance of a wire is directly proportional to its length, inversely proportional to its crosssectional area, and directly proportional to the resistivity of the substance forming the wire. 4. The greater the resistance, the larger the opposition to current, the more the potential drop. 5. the resistors can be used to control the current and voltage in the circuit, t
