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Magnetic Fields and Forces © 2010 Pearson Education, Inc. © 2010 Pearson Education, Inc. © 2010 Pearson Education, Inc. Learning Objectives: Investigate basic magnetic phenomena Develop a dipole model of magnetism Explore magnetic forces on moving charges Predict the motion of charged particles in magnetic fields Discuss a simple atomic-level model of ferromagnetism © 2010 Pearson Education, Inc. Experiments Observing to Find a Pattern Visualizing the Region Surrounding a Magnet The Magnetic Field: Definition of the magnitude of the Magnetic Field B = F/qv sin θ SI Units: 1 tesla = 1 T = 1 N / (A∙m) OR 1 gauss = 1 G = 10-4 T (Small magnetic fields are measured in the gauss G unit ) © 2010 Pearson Education, Inc. Magnetic Fields Around Us © 2010 Pearson Education, Inc. Predict: What would happen if you placed the compass over the wire instead of under the wire? 1. Construct a simple circuit. Again, place a compass under one wire. Open and close the circuit several times. Record your observation. 2. Now, place the compass over the wire. Open and close the circuit several times. Record your observation. 3. Reverse the battery and repeat steps 1 and 2. Suggest an explanation for these observations. © 2010 Pearson Education, Inc. Electric Currents Also Create Magnetic Fields A long, straight wire A current loop A solenoid Recall the lab when you used a compass to detect something happening in the wires of a circuit. © 2010 Pearson Education, Inc. The Magnetic Field of a Straight Current-Carrying Wire Magnetic field lines form circles around the wire! © 2010 Pearson Education, Inc. Direction of the Field in a Current-Carrying Wire © 2010 Pearson Education, Inc. Representing Vectors and Currents That Are Perpendicular to the Page © 2010 Pearson Education, Inc. Drawing Field Vectors and Field Lines of a Current-Carrying Wire © 2010 Pearson Education, Inc. The Magnitude of the Field Due to a Long, Straight, Current-Carrying Wire 0 permeability constant 1.257 10 6 T m/A © 2010 Pearson Education, Inc. Summary MAGNETIC FIELD LINES Field is stronger near poles, weakens with distance Spacing of lines indicates magnitude of vector B Field lines continue within the body of a magnet; always form closed loops (so never cross) Direction of magnetic field vector at any point on the line is tangent to the line Direction: leaves magnet at N pole and enters at S pole © 2010 Pearson Education, Inc. MAGNETISM Magnetism has no fundamental magnetic charge (as electrical q does have)! Magnetic fields are created by currents in wires and permanent magnets; these fields in turn exert forces on currents in wires, permanent magnets!! Hans Christian Oersted (Danish, 1820) discovered the connection between electricity and magnetism. © 2010 Pearson Education, Inc. Summary © 2010 Pearson Education, Inc.