tailieunhanh - PHYSICS 3 (ELECTRICITY AND MAGNETISM) - CHAPTER 3

CURRENT AND RESISTANCE, DIRECT CURRENT CIRCUITS Dòng điện trong một dây dẫn được xác định bởi i = DQ dt [A] () Có DQ là số tiền của (tích cực) phí đi trong thời gian dt thông qua một bề mặt giả thuyết rằng vết cắt trên dây dẫn. Theo quy ước, hướng của dòng điện được thực hiện như hướng mà nhà khai thác điện tích dương sẽ di chuyển. Đơn vị SI của dòng điện là ampe (A): 1A = 1C / s. | Electricity and Magnetism 30 Chapter 3 CURRENT AND RESISTANCE DIRECT CURRENT CIRCUITS Electric Current 1 The electric current in a conductor is defined by i A 3J here dq is the amount of positive charge that passes in time dt through a hypothetical surface that cuts across the conductor. By convention the direction of electric current is taken as the direction in which positive charge carriers would move. The SI unit of electric current is ampere A 1A 1C s. 2 The current i a scalar is related to the current density J a vector by i J JdA where dA is a vector perpendicular to a surface element of area dA and the integral is taken over any surface cutting across the conductor. J has the same direction as the velocity of the moving charges if they are positive and the opposite direction if they are negative. A Model for Electrical Conduction When a conductor does not have a current through it its conduction electrons move randomly with no net motion in any direction. When the conductor has a current through it these electrons still move randomly but now they tend to drift with a drift speed vd in the direction opposite that of the applied electric field that causes the current. The drift speed is tiny compared with the speeds in the random motion. For example in the copper conductors of house-hold wiring electron drift speed are perhaps 10-5 or 10-4 m s where as the random-motion speeds are around 106 m s. . a b . . 1 c Fig. Random motion of an electron from A to F the electron collides with an atom at B C D E a without electric field. b in presence of an electric field E the electron drifts rightward. c superposition of figure a and figure b. Electricity and Magnetism 31 Consider a wire of length L cross-sectional area A number of carriers free electrons per unit volume n. The total charge of the wire is q - nAL e C e x 10-19 C . Since the free electrons drift along the wire with speed vd in the direction opposite that of the current i the .