Chapter 24 Magnetism Charged particles in motion have

Chapter 24 Magnetism Charged particles in motion have an electric field and a magnetic field

Oersted’s Discovery

Magnetic Forces Recall From Ch. 22 Coulomb’s Law – force that electrically charge particles exert on each other. Force between electrically charged particles also depends on their motion. Magnetic forces – force due to the motion of the charged particles. (source – motion of charge particles (electrons))

Electromagnetism Electric Forces Magnetic Forces

Magnetic Poles

Horseshoe Magnet Bent bar magnet Similar to electric charges but magnetic poles can not be separated.

Electrons in Constant Motion Two Kinds of Motion Electron Spin (like a top) chief contributor to magnetism Revolve around nucleus

Break a magnet in half, each piece becomes an equally strong magnet.

Magnetic Fields Magnetic field points from South to North. Iron filing act as tiny compass needles, outlining magnetic field lines. Inside the magnet, field lines go North to South

Interactive Figure 24. 2

Fig. 24. 4 Magnetic Fields a) Opposite poles near each other, ends attract b) Like poles near each other (repel)

Compass Needle If the compass gets out of “sync” with the magnetic field a pair of torques – couple twist into alignment

Magnetic Domains Large clusters of atoms caused by a strong magnetic field. A microscopic view of magnetic domains in a crystal of iron. The blue arrows pointing in different directions tell us that these domains are not aligned.

Magnetizing Iron Magnetic domains can be induced to align by an external magnetic field.

Interactive Figure 24. 7

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 un-align and nail lose most of their magnetization.

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”

Side View If you bend the wire magnetic field bunches up inside the loop.

Iron Filings Sprinkled on Paper a) b) c) Current carrying wire Current carrying loop Current carrying coil of loops

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

Superconducting Electromagnets Recall from Ch. 22 Superconductors – no electrical resistance ◦ No limit to the flow of charge ◦ No heating! ◦ Economic Find at: ◦ Fermi National Accelerator Lab ◦ MRI in hospitals

Permanent magnet levitates above a superconductor because its magnetic field can not penetrate the superconducting material.

Magplane Magnetically levitated “Maglev” transportation

Magnetic Force on Charges Moving electric charges deflected by magnetic fields at a right angle.

Video: Magnetic Force on Current. Carrying Wires

Magnetic Force & Current Moving charges in an electric current experience a force due to magnetic field.

Interactive Figure 24. 15

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

Earth’s Magnetic Field The Earth is a magnet!

Origin of Earth’s Magnetic Field Convection currents in the molten parts of the earth may drive electric currents to produce magnetic field Rotational effects of the earth.

Cosmic Rays Shooting gallery of charged particles. ◦ Protons – remnants of the big bang ◦ Alpha particles ◦ Other atomic nuclei (expanding starts) ◦ High energy electrons Cosmic rays deflected by Earth’s magnetic field.

Secondary Comic Rays Secondary cosmic rays do reach the Earth. Greatest at poles, travel along magnetic field lines (not deflected) 5 particles per square inch centimeter hitting your body now! (and always)

Van Allen Radiation Belts Two doughnut shaped rings. Cosmic rays get trapped in the earth’s magnetic field. Inner ring – 3, 200 km Outer ring – 16, 000 km

Aurora Borealis (Northern Lights) Disturbances in Earth’s magnetic field allow some charged particles to dip into earth’s atmosphere. Glow like fluorescent lamps.

Biomagnetism Certain bacteria contain single domain grains of magnetite. They string together to form a compass to detect the dip in earth’s magnetic field to find food. Pigeons – multiple domain magnets in skull. Other animals also, us … Uses in MRI (Magnetic resonance imaging)

Homework Review Questions: 2, 4, 5, 6, 9 -13 all, 15 -20 all, 22, 27, 28 Extra Credit : Project 1 (bring in to class) Exercises: 1, 3, 8, 9, 13, 17, 19, 23, 26, 27, 31, 42 Due 11/10/10