ch21_lb

= = =Chapter 21: Magnetism=

21-1: Magnets and Magnetic Fields

 * Magnets** have two poles: A north pole and a south pole. Iron objects are strong attracted to both of these poles.
 * A compass, constructed standardly of a magnetic needle, or bar magnet, that swings freely on a horizontal plane, points north because of the earth's magnetic pull.
 * Like poles repel each other while opposite poles attract. This is similar to charged objects because when two similarly charged objects approach each other, they repel, and when two differently charged objects, they attract.
 * Magnetic poles are different from electric charges because electric charges can be isolated while magnetic poles always occur in pairs. No matter how often a permanent magnet is cut, each piece will always have a north and south pole. Electric charges can be separate and alone, positive and negative.
 * Magnets are similar to electric charges in that materials like rubber and wool can become charged by rubbing together. Like this, an unmagnetized iron bar can become a permanent magnet by contact with another permanent magnet.
 * Magnetic Field**: A region in which a magnetic force can be detected.
 * A magnetic dipole, or a pair of magnetic poles, create a magnetic field.
 * **Magnetic Field Strength** (//B//) is measured in tesla (//T//). This is related to the force on a moving charge.
 * The direction of the magnetic field at any location is defined as the direction in which the north pole of a compass needle points at that location.
 * The field lines of a bar magnet diagram, as shown above, show that the north pole is going toward the south pole. All of the arrows point directly or indirectly to the south pole.
 * Bar magnets have a north-seeking and a south-seeking pole. If such a magnet were used in a compass, it would point to the geographic North Pole of the earth. This shows us that the geographic North Pole corresponds to the magnetic south pole, and vice versa.
 * The difference between true North and the north that a compass points to is referred to as magnetic declination. This varies from different places on the earth. It depends on the magnetic field affecting the certain area in correlation with the magnetic poles of the earth.
 * When drawing the direction of a magnetic field, an arrow signifies some direction on the plane of a piece of paper, x's signify an magnetic field going into the paper, and circles signify a magnetic field coming out from the paper.

21-2: Electromagnetism and Magnetic Domains
A **Current Carrying Wire** has a magnetic field that follows a circular path following the right hand rule: If you graph the wire with your right hand, your thumb points in the direction of the current and your fingers wrap the way the circles of current do around the wire. A **Current Carrying Loop** also follows the right hand rule, but has an increased magnetic field strength.
 * The coiling causes an increased strength in the center of the coil because the magnetic field becomes more concentrated there as the circles of current curl up through the center.
 * The repitition of many coils on top of each other creates a solenoid.
 * Solenoid**: a long, helically wound coil of insulated wire.
 * With this tense stacking of coils and building magnetic field inside of the coils, a solenoid is often called an electromagent.
 * The exterior magnetic field is much weaker than the interior.
 * The magnetic field inside the coils increases with the current and is proportional to the number of coils per unit length.
 * Magnetic Domain**: a magnetic region composed of a group of atoms that share a common direction.
 * In an unmagnetized substance, the atoms of a domain are randomly oriented. Those that are randomly oriented will shift with a newly introduced magnetic domain.
 * Domains that are already aligned will grow at the expense of surrounding domains and become even more strongly aligned.
 * If a substance is easily magnetized, it will easily become unmagnetized when the magnetic field leaves. if a substance is not easily magnetized, it will remain the same when the field is removed.

21-3: Magnetic Force
Charges that move through a magnetic field experience a magnetic force: The magnetic force reaches its maximum value when the charge moves perpendicular to the magnetic field. It decreases its value at other angles, and it reaches zero when it becomes aligned with the field lines. The magnetic field strength's relationship to charge, velocity, and the magnetic force can be shown with this formula: math B = \frac {F_{magnetic}}{qv} math The direction of the magnetic force can be found with the right hand rule: A charged particle moving perpendicularly through a magnetic field will move in a circular path, again following the right hand rule, the current going in the direction of the thumb.
 * 1) You're right hand fingers point in the direction of the magnetic field (B).
 * 2) You're thumb points in the direction of the charge's velocity (v).
 * 3) You're palm, perpendicular to your fingers and thumb, is the direction of the magnetic force.
 * 4) If the charge is negative rather than positive, go opposite the direction of your palm, so with the back of your hand.

Magnetic Force on a Current Carrying Conductor: A length of wire //(l)// in an external magnetic field (//B//), carries a current (//I)// and undergoes a force of math F_{magnetic} = BIl math This formula applies when the magnetic field and the current are at 90 degrees, or perpendicular, from each other.

Two parallel conducting wires exert an attracting force on each other. the magnetic fields the two forces create are perpendicular to each other, and so there is a magnetic force between the two. The right hand rule proves their attraction to each other. Having the two wires go in opposite directions causes a repulsion between them.

Magnetic fields created by solenoids and other conductors can be used for producing power and sound by the intensity of their magnetic fields.

Pictures from: [] First Picture