Chapter 36 Magnetism 17 Sep20 Magnetic Forces General

Chapter 36 Magnetism 17 -Sep-20

Magnetic Forces General observations regarding magnets: Iron (and a few other metals) are ferromagnetic, which means they can become magnetized. Magnets attract ferromagnetic metals. Two magnets can either attract or repel each other depending on poles. 17 -Sep-20

Magnetic Poles Two types of magnetic poles: North (N) and South (S) As with electric charges, like poles (N&N, S&S) repel and opposites (N&S) attract. Unlike electric charges, cannot have just a North or just a South pole Magnetic poles are not electric charges 17 -Sep-20 Cow magnets N S Magnets N&S on sides Compass needle is magnet

Demo: Magnetic Fields Magnetic field points from South to North. Iron filing act as tiny compass needles, outlining magnetic field lines. Magnet Iron filings in clear oil 17 -Sep-20 Inside the magnet, field lines go North to South

Ferromagnetic Metals Ferromagnetic metals have similar atomic structure. Spin of the electron in these metals produces a net magnetic field 17 -Sep-20 Iron, Cobalt, Nickel

Demo: Magnetism & Money Most US coins are not made of ferromagnetic materials but many other countries use iron steel in their currency. Some pennies were made of steel during World War II Some Euro coins contain steel Iron is in the ink used in US paper currency to avoid counterfeiting. Buffalo nickels are 25% nickel metal, which is ferromagnetic

Magnetic Domains The magnetic field of an single iron atom is so strong that interactions among adjacent atoms cause large clusters of atoms, called magnetic domains, to line up with one another. A microscopic view of magnetic domains in a crystal of iron. Each domain consists of billions of aligned iron atoms. The blue arrows pointing in different directions tell us that these domains are not aligned. 17 -Sep-20

Magnetizing Iron Magnetic domains can be induced to align by an external magnetic field. S N S 17 -Sep-20 Strong Magnet N N

Demo: Magnetizing Iron Magnetic domains in iron nails are induced to align by proximity of the strong magnet Each nail becomes itself a magnet, which in turn magnetizes the nail below it, forming a chain. When the strong magnet is removed, most of the domains unalign and nail lose most of their magnetization. 17 -Sep-20

Demo: Demagnetizing Iron Magnetic domains can be scrambled by heating the iron, striking it with great force, or other disruptions of alignment. Magnetized Test tube of un-magnetized iron filling SHAKE S S N Magnetizer 17 -Sep-20 Demagnetized N

Demo: Demagnetizing Iron nail is attracted to the large magnet due to alignment of domains in the nail. Heat the nail to a high temperature and the domains become randomized so the nail is no longer attracted to the magnet. 17 -Sep-20

Demo: Electric Currents & Magnetic Fields An electric current produces a magnetic field. Electric Current Magnetic Field lines point in the northward direction; never reach a “North Pole” 17 -Sep-20

Electromagnets S N Electric current in a coil of wire creates a magnetic field similar to a bar magnet. 17 -Sep-20 Current passing through loops of coiled wire

Demo: Electromagnets Electromagnet created by passing current through a coil of wire. Electromagnet is stronger when an iron bar is inserted within the coil. 17 -Sep-20 N Iron Bar Note: Do this in lab too. Wire Coil S Connect to battery or power supply Physics 1 (Garcia) SJSU Used in lab

Check Yourself When an object is charged with static electricity, is that object also magnetized? But isn’t iron is magnetized when the electrons are aligned. If iron has electrons, then why isn’t it charged? 17 -Sep-20

Loudspeakers Loudspeaker has a membrane but oscillations are created by variations in electrical current, which cause an electromagnet to be pulled towards and away from a second, permanent magnet. These oscillations cause the membrane of the loudspeaker to vibrate with the same frequency as the oscillations in the electrical current. Headphones work essentially the same way, they’re just smaller.

17 -Sep-20 N The configuration of the Earth's magnetic field resembles a strong bar magnet located near the center of the Earth. The magnetic South pole is near the geographic North pole (so North pole of compass attracted towards Northern direction). There isn’t an giant underground magnetized chunk of iron; Earth’s interior is simply too hot. S Earth’s Magnetic Field

Origin of Earth’s Magnetic Field Earth is an electromagnet with electric currents deep below the surface. Moving charges, looping around within the molten part of the Earth, create the magnetic field. These currents are likely the result of thermal convection rising from the central core combined with the rotation of the Earth about its axis. 17 -Sep-20

Demo: Magnetic Force & Current Moving charges in an electric current experience a force due to magnetic field. 17 -Sep-20

Electric Meters Since magnetic force on a wire depends on the current, can use this effect to design a meter to measure current (an ammeter). Current Needle moves Current 17 -Sep-20 ) SJSU

Magnetic Force on Charges Moving electric charges deflected by magnetic fields. 17 -Sep-20

Crooke’s Tube Electron beam in a Crooke’s tube is deflected when a magnet is brought near the tube. 17 -Sep-20

Television Tube Electron beams, deflected by magnetic fields, are used to create TV images. Electromagnets 17 -Sep-20 Physics 1 (Garcia) SJSU