ch36_fp

**36.1 Magnetic** **Poles**
 * haris and frankChapter 36: Magnetism **
 * Intro to Magnetism: **
 * The term for //magnetism// comes from rocks called //lodestones//. The Chinese were the first to use the magnets as compasses for sailing and thanks to a French physicist Charles Coulomb we now know that lodestones contain iron ore, which has been named magnetite.
 * In the early nineteenth century, people believed that electricity and magnetism were unrelated. In 1820, Hans Christian Oersted discovered a relationship between the two while demonstrating electric currents in front of a class of students. Electric current was passed in a wire near a magnetic compass and the entire class noticed the deflection of the compass needle.
 * **Magnetic poles** are located at the ends of magnets. A magnetic pole is just the region that produces magnetic forces. The earth acts like a magnet, the two poles are the north and south poles. How the earth actually works like a magnet will be addressed later on.
 * Every magnet consists of two poles, a north and south seeking pole. A north seeking pole seeks the north pole of the earth, and a south seeking pole does the opposite. This is how compasses work, they are just suspended magnets that are attracted to the two poles in the earth.
 * Like poles repel, opposite poles attract. This just means that you can’t bring a south pole next to another south pole and expect it to attract because they repel. The same goes for two north poles next to each other. However, if you bring a north pole next to a south pole, they will attract.
 * You can never truly have one magnet because if a magnet breaks, both pieces of the magnet will inherit two poles. So if you break one magnet in two, you have two separate magnets and four poles, two on each.
 * 36.2 Magnetic Fields **
 * The space around a magnet in which a magnetic force is exerted or exists is a **magnetic field.** To find the magnetic field around a magnet, all you have to do is put a piece of paper over the magnet and pour iron filings all on and around the magnet. You will see the shape of the field through the iron filings as they outline the magnetic field lines. In the presence of one magnet, the magnetic field lines will curve from one pole to the other. However if two magnets are present, the magnetic lines will show that every North Pole goes to a South Pole.
 * 36.3 Nature of a Magnetic Field **
 * Electricity and Magnetism are very closely related. There is always and electric field surrounding an electric charge, well that same charge will also be surrounded by a magnetic field only if the electric charge is moving.
 * Now you might begin to wonder how a stationary magnet has an electric charge moving in it. This is because the magnet is composed of atoms that are in constant motion around nuclei, thus constituting a tiny current and producing a magnetic field. Every spinning electron generates a very small magnetic field, and electrons that spin in the same direction combine to make a stronger magnetic field. Opposite spinning electrons cancel each other out. This is why not everything you see is a magnet. In most materials the fields cancel each other out.
 * 36.4 Magnetic Domains **
 * The magnetic field of individual iron atoms is so strong that interactions among adjacent iron atoms cause large clusters of them to line up with each other. These clusters of aligned atoms are called **magnetic domains.** The only difference between say an ordinary piece of iron and an iron magnet is the alignment of their magnetic domains.
 * In an iron nail, the domains are randomly oriented. However, when a magnet is brought nearby a few things happen. There is a growth in the size of domains oriented in the direction of the magnetic field. This growth is only at the expense of domains that are not aligned. The other effect that happens, is that the domains rotate as they are brought into the alignment. When the nail is removed from the magnet an ordinary thermal motion causes either most or all of the domains to return to a random arrangement.
 * To create a magnet and to get the domains to stay aligned, iron pieces or alloys are placed in strong magnetic fields. Softer iron is easier to magnetize than steel. You can also tap the iron to nudge stubborn domains into alignment.
 * 36.5 Electric Currents and Magnetic Fields **
 * We know that moving charges produce magnetic fields. Well when there are many charges in motion (an electric current) we also have a magnetic field. To demonstrate this, you simply put compasses around a wire. When there is no current present, the compasses will align with the earth’s magnetic field. However, if you pass a current through the wire, the compasses will align with the wire and the current it produces.
 * A wire carrying a current with many loops in it is an **electromagnet.** When you bend the wire that has the current present, the magnetic field lines within will bunch up inside that loop. If you bend it two times, overlapping the first, the concentration is even double. The intensity of the magnetic field is controlled by the number of loops present. Sometimes iron is placed inside the coil of an electromagnet. The magnetic domains in the iron are induced into alignment, increasing the intensity of the field.
 * 36.6 Magnetic Forces on Moving Charged Particles **
 * Charged particles at rest won’t interact with a static magnetic field. The charges have to move inside the magnetic field and then their magnetic characters become evident. The particle experiences a deflecting force. It creates the largest force when it moves in a direction perpendicular to the magnetic field lines. The force is zero when they move parallel to the field lines. The direction of the force is always perpendicular to both the magnetic field lines and the velocity of the charged particle. So a moving charge is only deflected when it crosses magnetic field lines but not when they travel perpendicular.
 * The force that acts on a moving charged particle does not act in a direction between the sources of interaction, but instead acts perpendicular to both the magnetic field and the electron velocity.
 * 36.7 Magnetic Forces on Current-Carrying Wires **
 * Currents, just like particles, will also be deflected by magnetic field lines. If for some reason the current in the wire would be reversed, the force would be reversed on the wire as well. The force exerted on the current is perpendicular to both field lines and current, and is always a sideways force.
 * 36.8 Meters to Motors **
 * A galvanometer, named after it’s founder Luigi Galvani, is used to measure the current through a circuit. The unit used in describing the amount of current is named amperes and the galvanometer takes the name of an ammeter. It can also be calibrated to measure volts, so it can also be a voltmeter. Through the galvanometer, you can modify it to make an electric motor. The difference here is the current is made to change direction every time the coil makes a half revolution. After it’s been forced to rotate one half revolution, it overshoots just in time for the current to reverse, whereupon it is forced to continue another half revolution, and so on in cyclic fashion to produce continuous rotation.
 * In a DC motor, a permanent magnet is used to produce a magnetic field in a region where a rectangular loop of wire is mounted so that it can turn about an axis as shown. When a current passes through the loop, it flows in opposite directions in the upper and lower sides of the loop. If the upper portion of the loop is forced to the left, then the lower portion is forced to the right, sa if it were a galvanometer. However unlike a galvanometer, the current is reversed during each half revolution by means of stationary contacts on the shaft. The parts of the wire that brush against these contacts are called brushes.
 * When you make a larger motor, whether it be ac or dc, it’s usually made by replacing the permanent magnet by an electromagnet that is energized by the power source. Many loops of wire are wound about an iron cylinder, called an armature.