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Chapter 20 Magnetism

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Chapter 20 Magnetism Chapter 20 20.1 Magnets and Magnetic Fields Magnets have two ends – poles – called Magnetism north and south. Like poles repel; unlike poles attract. 20.1 Magnets and Magnetic Fields 20.1 Magnets and Magnetic Fields However, if you cut a magnet in half, you don’t Magnetic fields can be visualized using get a north pole and a south pole – you get two magnetic field lines, which are always closed smaller magnets. loops. 20.1 Magnets and Magnetic Fields 20.1 Magnets and Magnetic Fields The Earth’s magnetic field is similar to that of a A uniform magnetic field is constant in bar magnet. magnitude and direction. Note that the Earth’s The field between “North Pole” is really these two wide poles a south magnetic is nearly uniform. pole, as the north ends of magnets are attracted to it. Where is the true north pole? (3:53) 1 20.2 Electric Currents Produce Magnetic Magnetic Field Fields � • Magnetic Field is a vector field B Experiment shows that an electric current • Units of Magnetic Field: Tesla (T) produces a magnetic field. Direction Right determined Hand using Rule #1 small magnets Drawing 3D Fields Can represent B field Out of Into X X X the the X X X Board Board X X X Into the X X X X End X X X Board X X X On X X X X X X X View X X X X Out of Field Out Field Into the of Board Board Board X X X X Into Out of X X X X X the the Board Board X X X X X X X X X X X X X X X X X X X X Out of X X the Everywhere inside the coil, field is X X Board out of board. Outside of coil, some into the X X X board and some out of board. Into X X X the X X X Board 2 20.2 Electric Currents Produce Magnetic 20.3 Force on an Electric Current in a Fields Magnetic Field; Definition of B The direction of the A magnet exerts a force on a current- field is given by a carrying wire. The direction of the force is right-hand rule. given by a right-hand rule. Right Hand Rule #2 Direction of Force Depends on Directions of Current and Magnetic Experimental Observations Field • Force proportional to current in wire • Force proportional to length of wire in magnetic field • Force proportional to strength of magnetic field • Direction of force perpendicular to direction of wire and direction of field Starting Point for Force Formula Experimental Observations • Force proportional to current in wire FILB� This formula only • Force proportional to length of wire in describes magnitude, not magnetic field direction. • Force proportional to strength of magnetic [N ]� [ A ][ m ][ T ] field • Force depends on angle between wire and magnetic field • Direction of force perpendicular to direction of wire and direction of field 3 20.3 Force on an Electric Current in a 20.3 Force on an Electric Current in a Magnetic Field; Definition of B Magnetic Field; Definition of B The force on the wire depends on the Unit of B: the tesla, T. THIS IS THE SI UNIT current, the length of the wire, the magnetic 1 T = 1 N/A·m. field, and its orientation. (20-1) Another unit sometimes used: the gauss (G). This equation defines the magnetic field B. 1 G = 10-4 T. Magnetic field of earth ~0.5G F=I L B sinθ Figure 20-12 Current-carrying wire in a magnetic field Force Maximum When Current Perpendicular to Magnetic Field F=ILB sin(θ) I B θ=90º sinθ=1 F=ILB B θ=0º sinθ=0 F=0 I Drawing 3D Fields Right Hand Rule End X X X X On X X X X View X X X X Field Out Field Into of Board Board 4 For magnetic force on moving particle: The Magnetic Force is only due to magnitude of the force depends on the angle moving charges between v and B Current Moving particle � F� qvBsin(� ) Relate Force on Wire to Force on a The magnitude of the force Single Charge depends on the angle between v and B F� ILBsin(� ) � �Q � � I � v v �t v � For N charges, each having charge q � � B Nq Force I � B B Two versions t Force Zero of same Force equation Let t � time for charge to move distance L Maximum Less � L� vt v � Nq � � � F � � ��vt � B sin(� ) � t � B F� qvBsin(� ) Force Find force on a single charge (N� 1) Zero F� qvB sin(� ) 20.4 Force on Electric Charge Moving in a 20.4 Force on Electric Charge Moving in a Magnetic Field Magnetic Field The force on a moving charge is related to the force on a current: If a charged particle is (20-3) moving perpendicular to a uniform magnetic field, its path will be a Once again, the circle. direction is given by a right-hand rule. 5 20.4 Force on Electric Charge Moving in a 20.4 Force on Electric Charge Moving in a Magnetic Field Magnetic Field Problem solving: Magnetic fields – things to remember 1. The magnetic force is perpendicular to the magnetic field direction. 2. The right-hand rule is useful for determining directions. 3. Equations in this chapter give magnitudes only. The right-hand rule gives the direction. Magnetism and Quantum Physics Magnetism and Special Relativity How do magnets work? 6:25 How Special Relativity Makes Magnets Work 4:18 Magnetic Fields Can Cause A turning 20.10 Applications: Galvanometers, motion Motors, Loudspeakers The forces on opposite sides of a current loop will be equal and opposite. However, if the loop is held fixed but allowed to rotate, the A galvanometer magnetic field can cause the loop to rotate. takes advantage of the torque on a This leads to: current loop to Meters and Motors measure current. This is the core of an ammeter and a speedometer. 6 20.10 Applications: Galvanometers, 20.10 Applications: Galvanometers, Motors, Loudspeakers Motors, Loudspeakers Loudspeakers use the An electric motor principle that a magnet also takes exerts a force on a advantage of the current-carrying wire to torque on a current convert electrical loop, to change signals into mechanical electrical energy to vibrations, producing mechanical energy. sound. Summary of Chapter 20 AC-DC Bulb • Magnets have north and south poles • Like poles repel, unlike attract • Unit of magnetic field: tesla • Electric currents produce magnetic fields • A magnetic field exerts a force on an electric current: Summary of Chapter 20 • A magnetic field exerts a force on a moving charge: Quantum Levitation 5:08 • Magnitude of the field of a long, straight current-carrying wire: 7.
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