Forces: Fnet causes acceleration. Forces – act at distance • Fg– attractive btw masses. Field • g = Fg/m. Test mass. • Fe – attractive/repulsive between objects w net • E = Fe/q. + Test charge. charge. • Fmag magnetic- attractive/repulsive between objects w net e- spin.. • B ~ currents in wire. Compass needle. • Magnetism What is Magnetism? Magnetism is the force of attraction or repulsion of a material due to the spin of its electrons. Like all forces Fmag can be a Fnet that causes acceleration & is measured in N. Magnetic Fields regions of space around a magnet where magnetic materials feel a force. Magnets create a force field which will deflect a compass needle. • Field lines point in the direction of the deflection of the compass needle. • Through Magnet. The poles of a magnet are where the magnetic effect is the strongest. Each magnet has 2 poles –north, south. Field lines are close where field is strong. Field lines come out of N pole, go into S pole. • • • • Lines never touch or cross Only curves Field lines called lines of flux. Flux density shows field strength. Ex: Make a rough sketch of the horseshoe magnet below. Sketch the field lines near the ends of the magnet. More than 1 magnet – out of N, into S. Earth has a mag. field. The north seeking end of a compass points toward Earth’s N geographic pole. Earth’s north pole acts like south pole of bar magnet. Demo Magnetic Field Lines • magnet on iron bars. What causes magnetism? Atoms have magnetic fields due to excess e- spin. Groups of atoms join so that their magnetic fields are all going in the same direction. These areas of atoms are called “domains” When an unmagnetized substance is placed in a magnetic field, the substance can become magnetized. This happens when the spinning electrons line up in the same direction. An unmagnetized substance looks like this… While a magnetized substance looks like this… Poles of a magnet always come in pairs! How to break a magnet: 1. Drop it 2. Heat it This causes the domains to become random again! Non-magnets with magnetic properties can be polarized in the presence of a magnetic field. N S N S Minute Physics How do magnets work 6.5 minutes. • http://www.youtube.com/watch?v=hFAOX dXZ5TM Hwk Intro Sheet. Relationship of Electricity & Magnetism Similarities of Magnetic (B) to Electric Fields (E) • • • • • • field lines also called lines of flux. Both attract opposite. Repel like. Drop off with d2. Both Polarize objects. Flux density shows intensity of field. Both vector quantities. Differences • • • • • For B field, direction compass needle points. E field direction + test charge would move Magnet feels no force in static E field. Charges feel no force in static B field. Isolated poles do not exist. Isolated charges do. Electromagnetism 57 Min. • http://www.youtube.com/watch?v=bht9AJ1 eNYc • Old Navy film How magnetism produce Electricity 7 min. • http://www.youtube.com/watch?v=uoQelu7 XRjk -Moving charges cause magnetic fields. When a current flows, a magnetic field is generated. *Stationary charges do not cause magnetic fields, but do cause electric fields. Sketching Fields Use arrows to show the direction of compass needle deflection. Field into the page X like tail of arrow. Field out of page … like tip of arrow. Mag Field Direction around Wire. 2nd Hand Rule: Wire loop – thumb in current direction. Fingers in B direction. Solenoid – Coils of wire (can be wrapped around core of iron). 3rd hand rule: Fingers in direction of current. Thumb points to magnetic N. Pole. Solenoids act like bar magnets. Charged particles moving in B fields feel a force!! Force direction determined by hand rule three. Fourth Direction of mag force on q perpendicular to v vector & to B field. For +q place right hand fingers into field, thumb points to v, palm points to mag force. For – q use left hand. Electron in mag field B. Force is proportional to amount of charge on particle. Force on current carrying wire. The current in a wire also feels a force in mag field. Fifth hand rule:–force direction on wire: fingers into field, thumb in current direction, palm points toward force direction. Currents in same direction exert attractive forces. Current in opposite directions, create fields with repulsive forces. By Newton’s 3rd Law, F2 = -F1. F1 F2 I1 I2 The Ampere (A) is a unit of rate of current flow, but is based on the force felt between 2 currentcarrying wires. The Ampere, not the Coulomb, is considered a fundamental unit!! Very weird indeed! For a straight length of wire the induced B field drops off with distance (r). The circles should get… B = moI 2pr mo constant find it. r = dist fr wire m I = current - A B = field strength T Mag Field Strength around a solenoid: B = moNI l N = number turns of wire I = current l = length of wire in field m Force between two wires proportional to current in wires: F = moI1I2l 2pr Ex 4: What is the magnitude & direction of the force btw 5.00 m lengths of wire each carrying 7.5 A of current in the same direction if they are separated by 25.0 cm? 2.25 x 10-4 N toward each other. Mag Field Between Current Carrying Wires. These show neg charge flow.