Magnetism General Physics Instructor: Xiao, Yong ( 肖湧 ) ， Wang Kai( 王凯 ) TA: Li, Yueyan （李跃岩） Recitation TA: Zhai, Chenyu （翟宸宇）

Magnets and Magnetic Fields The history of magnetism begins thousands of years ago. In a region of Asia Minor known as Magnesia, rocks were found that could attract each other. These rocks were called "magnets" after their place of discovery.

Magnets and Magnetic Fields Any magnet, whether it is in the shape of a bar or a horseshoe, has two ends or faces, called poles, which is where the magnetic effect is strongest. A compass needle is simply a bar magnet which is supported at its center of gravity so that it can rotate freely. The pole of a freely suspended magnet that points toward geographic north is called the north pole of the magnet. The other pole points toward the south and is called the south pole.

Magnets and Magnetic Fields When two magnets are brought near one another, each exerts a force on the other. The force can be either attractive or repulsive. If the north pole of one bar magnet is brought near the north pole of a second magnet, or the south pole with the south pole, the force is repulsive. When a north pole is brought near the south pole of another magnet, the force is attractive.

Magnets and Magnetic Fields We have all observed a magnet attract paper clips, nails, and other objects made of iron. Only iron and a few other materials, such as cobalt, nickel, gadolinium, and some of their oxides and alloys, show strong magnetic effects. They are said to be ferromagnetic.

Magnets and Magnetic Fields In the last course, we used the concept of an electric field surrounding an electric charge. In a similar way, we can picture a magnetic field surrounding a magnet. Just as we drew electric field lines, we can also draw magnetic field lines.

Electric Currents Produce Magnetic Fields But in 1820, Hans Christian Oersted ( ) found that when a compass needle is placed near a wire, the needle deflects as soon as the two ends of the wire are connected to the terminals of a battery and the wire carries an electric current. As we have seen, a compass needle is deflected by a magnetic field. So Oersted's experiment showed that an electric current produces a magnetic field.

Force on an Electric Current in a Magnetic Field Suppose a straight wire is placed in the magnetic field between the poles of a horseshoe magnet as shown in the figure. Experiments show that the direction of the force is always perpendicular to the direction of the current and the magnetic field, B.

Force on an Electric Current in a Magnetic Field The direction of the force is given by right-hand rule. Orient your right hand until your outstretched fingers can point in the direction of the conventional current I, and when you bend your fingers they point in the direction of the magnetic field lines, B. Then your outstretched thumb will point in the direction of the force F on the wire.

Force on an Electric Current in a Magnetic Field

Force on an Electric Charge Moving in a Magnetic Field

Magnetic Dipole Moment

Homework Force on an Electric Current in a M agnetic Field P727/1,2,8 Force on an Electric Charge Movin g in a Magnetic Field P727/13,21,22 Magnetic Dipole Moment P729/35