tailieunhanh - PHYSICS 3 (ELECTRICITY AND MAGNETISM) - CHAPTER 4

MAGNETISM Các lĩnh vực từ nguồn quen thuộc nhất của từ trường là một thanh nam châm. Một đầu của thanh nam châm được gọi là cực Bắc và cực Nam. Nếu chúng ta đặt một số la bàn gần một thanh nam châm, kim tiêm sẽ sắp xếp theo hướng của từ trường, như trong hình. . Quan sát có thể được giải thích như sau: Một la bàn từ tính bao gồm một thanh nam châm nhỏ có thể xoay tự do về một điểm trục đi qua trung tâm của nam châm. Khi la bàn được đặt. | Electricity and Magnetism 38 Chapter 4 MAGNETISM The Magnetic field The most familiar source of magnetic fields is a bar magnet. One end of the bar magnet is called the North pole and the other the South pole. If we place some compasses near a bar magnet the needles will align themselves along the direction of the magnetic field as shown in Fig. . The observation can be explained as follows A magnetic compass consists of a tiny bar magnet that can rotate freely about a pivot point passing through the center of the magnet. When a compass is placed near a bar magnet which produces an external magnetic field it experiences a torque which tends to align the north pole of the compass with the external magnetic field. Fig. Fig. Magnetic field of a bar magnet When two magnets or magnetic objects are close to each other there is a force that attracts the poles together. When two magnetic objects have like poles facing each other the magnetic force pushes them apart Fig. . Magnets also strongly attract ferromagnetic materials such as iron nickel and cobalt. Fig. Like poles repel opposite poles attract Fig. Magnetic field lines Magnetic field lines emanate primarily from the north pole of a magnet and curve around to the south pole. Electricity and Magnetism 39 The Earth s magnetic field behaves as if there were a bar magnet in it Fig. . Note that the south pole of the magnet is located in the northern hemisphere. Fig. Magnetic field of the Earth Fig. The iron filings suggest the magnetic field line of a bar magnet Motion of a Charged Particle in a Uniform Magnetic Field 1 The Lorentz force Consider a test particle with charge q moving through the magnetic field B with the velocity v. The Lorentz force is v Fb qv x B The SI unit for B is the Tesla T 1T 1 N Am 104 Gauss. The right-hand rule gives the direction of a vector resulting from the cross product of two other vectors. To find the direction of the resulting vector sweep .