VIEWS: 9 PAGES: 24 POSTED ON: 7/18/2010 Public Domain
Magnets • Magnetism is closely related to electricity • Magnets are materials in which some electric charges are in orderly orbits • We shall learn about how moving charges create magnetism Magnets Every magnet has two special ends called poles. We designate these as the north pole and the south pole for historical reasons. Like poles repel and unlike poles attract. Magnets If we saw a magnet in half, we create a new north pole and a new south pole. Poles always occur in pairs. No magnetic monopoles have ever been observed. Magnetic Fields • When we dealt with electric charges, it was convenient to define the interaction between charges as taking place due to each charge creating an electric field. • Similarly, we can define the concept of a magnetic field established by the two poles of a magnet. Magnetic Fields • The field lines originate at a north pole and terminate at a south pole Magnetic Fields • As before, the number of lines per unit area is proportional to the strength of the field • The direction of the field is tangent to a line at any point in space. • We will define the strength of a magnetic field in terms of the torque the field applies to a compass needle. Magnetic Fields The magnetic field strength B is proportional to the torque applied to the compass needle. We will define it precisely later. Earth’s Magnetic Field The field doesn’t point to true north. The difference is called the declination. Also, the field is not exactly tangent to the earth’s surface at all points. The deviation is called the angle of dip. Earth’s Magnetic Field Can be lots of confusion. The magnetic pole in the Arctic is really a south magnetic pole. The magnetic pole in the Antarctic is really a north magnetic pole. Note how the compass needle is labeled. Magnetic Fields • The simplest magnetic field occurs between the poles of two very large magnet pole faces. Near the center the field will be simple and straight. Magnetism and Currents • It was found long ago that a stationary electric field and a stationary magnetic field do not interact. • Oersted found that if you pass a current through a wire, a magnetic field is created around the wire!!! • The charges have to be moving!!! Magnetism and Currents The magnetic field lines are circles which close on themselves. Note that there is no north pole or south pole. The field lines simply are arranged as circles around the wire. We can find the direction by using the right-hand rule. Right Hand Rule Point the thumb of your right hand in the direction of the conventional current (+ to -) and your fingers will curl in the direction of the magnetic field lines. Circular Loop of Wire If you have a circular loop of wire carrying a current, the magnetic field lines run perpendicular to the plane of the loop. We can use another right- hand rule to determine the direction. Circular Loop of Wire Definition of B • We know that a current-carrying wire exerts a force on a compass needle (a magnet) • By Newton’s Third Law, the compass needle must exert a force on a current- carrying wire • Experiment confirms this to be true Definition of B The current flows from front to back. The magnetic field goes from left to right. The foce is downward!!! Isn’t this a strange result. Test by reversing the current. Definition of B Now the current goes from back to front, while the magnetic field still goes from left to right. Now the force is in the opposite direction, that is down to up! We need a rule to help us remember. Definition of B Definition of B • Each component is perpendicular to the other • I perpendicular to B and to F • B perpendicular to I and to F • F perpendicular to B and to I • WEIRD!!! • Just remember the right hand rule Definition of B • Now that we have the directions straight, we have to get the quantitative relationship • As you might guess from all the direction business, this is a little messy • Start with a picture Definition of B Here is a current-carrying wire in a magnetic field. The force on the wire, using the right hand rule, is into the screen. Force depends directly on length of wire and size of current. Also depends on the angle between the current and the field. When angle is zero, the force is zero. When angle is 90o the force is a maximum. F = I x length x B x sine(theta) Definition of B F I Bsin • The unit for B is the tesla • :You may encounter the unit gauss • One gauss is 10-4 tesla Definition of B Can precisely measure the field with the loop shown here. Forces on the vertical portions of the wire cancel out. The force on the left side wire is to the left. The force on the right side wire is to the right. Leaves only the force on the horizontal wire which is down! Hang from a balance.