Chapter 14 Magnetism

Properties of Magnets You can investigate the properties of magnets by bringing two magnets together. The ends of the magnets attract each other and stick together. The ends of the magnets repel each other and the magnets move apart.

Magnetic Poles Magnetic poles: One of the two ends of a magnet where magnetic force is strongest. When a magnet spins freely it always points the same direction The end of a magnet that points north is called the north pole, and the end pointing south is the south pole. All magnets have a north and a south pole.

When two north poles or two south poles are brought near each other, they repel. But if the north and south magnetic poles are brought near each other they will attract. Magnetic poles can’t be separated from each other. If you break a magnet in half each piece is still a magnet with a north and south pole.

Magnetic Field: The area of magnetic force surrounding a magnet. The magnetic fields is strongest at the poles of a magnet, but exists around the entire magnet. Magnetic field lines exists from one pole to the other.

The number of field lines in any given region indicates the relative strength of the field Although the magnetic field is invisible you can see its effect around a magnet by placing a piece of paper on top of a magnet and then sprinkling iron fillings over the paper If you were to place a magnetic material, such as iron, near the magnet it would be most attracted to either the north or south pole

Iron would also be attracted by the magnetic field around the magnet What do you think happens if you place the entire magnet in a dish of iron filings

Magnetic fields

Magnetic Materials The electrons of all atoms spin as they move about the nucleus A spinning electron produces a magnetic field with both a north and south pole In most materials, the magnetic fields of individual atoms cancel each other, so the materials aren’t magnetic In certain materials this isn’t the case

The poles line up in the same direction in microscopic magnetic regions, called magnetic domains When all the domains are arranged with their poles in the same direction, the iron bar becomes a permanent magnet When the domains are arranged randomly, the iron bar is not magnetized

Earth As A Magnet If you hang a magnet by a string, the north seeking pole will always point north because the earth itself is a huge magnet An instrument that takes advantage of the earths magnetic field is the compass A compass has a magnetized needle in it that turns freely The north and south pole of the earth’s axis are referred to as geographic north pole sometimes called true north

Evidence suggests that the earths magnetic field is caused by the movement of molten metals near the earths core Measurements show that the earths magnetic poles change position over time Changes in the flow of the molten metals inside the earth may cause the magnetic poles to move

Magnetic Effects The most visible effect of the earths magnetic field is a colorful light display, called an aurora An aurora hangs like a curtain of light stretching over the polar regions of the earth Collisions between the charged particles and other particles in the upper atmosphere create glowing lights

Aurora

Aurora

Aurora

The color of aurora depends on the kind of atoms in the atmosphere Magnetic storms interfere with compass needles and radio and television waves. Magnetic storms occur when solar flares produce charged particles that become trapped in the earth’s magnetic field.

Earth’s magnetic field affects living things. They have magnetic particles inside their bodies These particles help organisms using the magnetic field to find their way.

Electromagnetism In 1820, Christian Oersted, a Danish physicist made an observation that when a compass was brought near electric current, the compass needle no longer pointed north. It turned 90 degrees. The compass needle turned in the opposite direction when he reversed the current.

He hypothesized that when an electric current flowed through it, the wire acted like a magnet. Somehow electricity could produce magnetism.

                                                                                    

                                                                        

Electromagnets Oersted’s discovery is responsible for the invention of new tools based on the principles of electromagnetism. Electromagnet: a magnet made of a soft-iron core surrounded by a coil of wire through which an electric current passed. The strength depends on the number of turns in the coil, the amount of current, and the size of the iron core.

The greater number of turns a coil has, the stronger the magnetic field can produce. The greater the size of the soft-iron core, the stronger the magnet is. When a magnet is turned on an electric current flows through the wire coil, creating a magnetic field around the coil. The magnetic domains in the soft-iron core align with the magnetic field of the coil.

The soft-iron core becomes magnetized. One end of the soft-iron core is a north pole, and the other end is the south pole. The magnetic field of the magnetized soft-iron core combines with the magnetic field of the wire coil. The combined magnetic fields create a very strong magnet.

Pure iron is referred to as soft iron. An electromagnet exerts a magnetic force that can make things move.

Electric Motors An electromagnet, called an armature, is placed in the magnetic field of permanent magnet. When current flows through the electromagnet, its poles repel the like poles of the permanent magnets. When the direction of the current changes, the poles on the electromagnet reverse, and the electromagnet spins

The commutator is split metal ring that acts as as a switch The communicator reverses the current in the electromagnet Electric current enters the electromagnet through brushes that touch the spinning communtator rings

Current Meters The response of magnetic forces between an electromagnet and a permanent magnet is used in various kinds of meters

Current Meters The two springs connected to the rod through the electromagnet control the pointer of the galvanometer When an electric current passes the electromagnet, the poles of the electromagnet respond to the poles of the permanent magnet

Electromagnetic induction: The process of inducing a current by moving a magnetic field through a wire coil without touching it. This occurs any time motion takes place between the wire and the magnetic field. A weak current is produced when the movement of the wire is slow. A strong current is produced when the movement is fast.

Generators Devices for converting mechanical energy to electrical energy. Spin a coil of wire through a magnetic field Will make a current flow through wire Make alternating current as the go past the different poles of the magnet.

Transformer Changes the voltage of alternating current Power comes at high voltage because the power company loses less energy. A step-down transformer lowers the voltage to 120V or 240V for your house Uses two coils of wire and a soft iron core Primary coil in Secondary coil out

Step-down Transformer Has more coils on primary than secondary Decreases voltage

Step-Up Transformer Has more coils on secondary than primary Increases voltage

Transformers As the alternating current changes direction So does the magnetic field Which makes the alternating current in the other coil