Chapter 8 Magnetism & Its Uses

Sec 1: Magnetism Over 2000 years ago Greeks first discovered magnetism in a mineral they called magnetic. Magnetism—properties and interactions with magnets Magnets attract metal objects.

Magnetic Force & Magnetic Fields Magnetic force—the interaction between magnets. The strength of this force increases as magnets move closer together. Magnetic field—the area surrounding a magnet that exerts the magnetic force. Iron and other magnets react to the magnetic force when placed in the magnetic field. Magnetic field can be represented by lines of force, or magnetic field lines.

Magnetic field lines of a bar magnet

Magnetic Poles Magnetic field lines are closest to each other at the ends of the bar magnet These are magnetic poles—where the magnetic force exerted by the magnet is the strongest. All magnets have a north and south pole. Bar magnet—poles are at opposite ends Horseshoe magnet—the poles are the two ends (like a bent bar magnet)

Interacting Magnets Magnets either attract or repel each other. When 2 north poles or 2 south poles come close, you can feel a force preventing the magnets from touching. North poles always attract south poles. When 2 magnets come close, their magnetic fields combine.

A Compass Needle A compass contains a needle (a small bar magnet) that is free to rotate. When placed near a bar magnet, the north pole of the needle will turn towards the south pole of the magnet. A compass needle will point towards the North Pole of Earth. Earth acts as like a gigantic bar magnet, and its magnetic field extends into space.

Earth’s Magnetic Poles The north pole of a magnet is the end of the magnet that points towards geographic north. The north pole of a magnet attracts a south magnetic pole. Earth’s south magnetic pole is near the geographic north pole. It is located in Canada about 1500km from the geographic north pole.

Magnetic Materials A magnet will not attract all metal objects. Only a few elements can be made into permanent magnets Iron, cobalt, & nickel Objects made from these metals are not always magnetic. Put an iron nail next to a refrigerator, let go, and it falls to the floor. You can make these metals behave like magnets temporarily.

Permanent Magnets Iron, cobalt, or nickel can be made into permanent magnets by placing them in a strong magnetic field. This creates a magnetic field inside the material It can retain magnetic properties for a long time. Permanent magnets can lose their magnetism if heated or dropped. When a magnet is broken, each piece still has a north and south pole! Even the smallest pieces of a magnet have a north and south pole.

Sec 2: Electricity & Magnetism Electric currents cause magnetic fields. The magnetic field around a current-carrying wire forms a circular pattern around the wire. The strength of the field depends on the amount of current flowing in the wire.

Electromagnets Electromagnet—a temporary magnet made by placing a piece of iron inside a current-carrying coil of wire. When current flows through the loop of wire, magnetic field lines form around the wire. The more loops of wire, the stronger the magnetic field.

Properties of Electromagnets Electromagnets are temporary because they are only magnetic when current is flowing through the wire. Electromagnets convert electrical energy into mechanical energy to do work.

Stereo Speakers The mechanical energy produced from an electromagnet vibrates parts of a speaker to produce sound.

Electric Motors Electric motor—a device that changes electrical energy into mechanical energy. Contains an electromagnet that is free to rotate between poles of a permanent magnet connected to the battery. When current flows through the electromagnet, a magnetic field is produced in the coil.