Review Sheet 2

# Review Sheet 2 - UM PHY126 Review Sheet 2 PHY126 Review...

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UM PHY126 Review Sheet 2 PHY126 Review Sheet 2 by Dr. Sa-Lin Cheng Bernstein 1. Magnetic field ~ B : Magnetic fields can be produced by magnets and electric currents. The magnetic field at a given point is a vector. The direction of ~ B at any point is indicated by the north pole of a small compass needle placed at that point. Magnetic field sources are essentially dipolar in nature, having a north and south magnetic pole. Units: tesla(T), and gauss. 1 gauss = 10 - 4 tesla. 2. Magnetic field lines : Magnetic field lines originate from the north pole and end on the south pole. They do not start or stop in midspace. The tangent to the magnetic field line at each point is the direction of the magnetic field at that point. No two field lines ever cross. The density in space of magnetic field lines around a particular point is proportional to the strength of the magnetic field at that point. The magnetic field is stronger in regions where the field lines are closer together. The number of lines per unit area passing perpendicularly through a surface is proportional to the magnitude of the magnetic field. 3. Magnetic fields and magnetic forces : A magnet or current creates A moving charge q experiences A current experiences ~ B (T) F = qvBsinθ F = IlBsinθ See Figure 21.9 and Figure 21.18 for RHR-1. The charge must be moving, for there is no magnetic force on a stationary charge. The velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field for there to be a non-zero magnetic force . θ is the angle between ~v (or ~ I ) and ~ B . A current is a collection of moving charges. If the moving charge is negative, the direction of the magnetic force is opposite to that predicted by RHR-1. ~ F is perpendicular to both ~ B and ~v (or ~ I ). ~ F cannot do work and change the kinetic energy of the moving charge. 4. The motion of a charged particle in ~ B : When ~v is perpendicular to ~ B , the charged particle follows on a circular path. r = mv qB . 1/9 February 20, 2007

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UM PHY126 Review Sheet 2 See Figure 21.15. KE = eV = 1 2 mv 2 v = p 2 eV/m , therefore, r = 1 B s 2 V m q , or m = ( qr 2 2 V ) B 2 . 5. The torque on a current-carrying coil τ = NIABsinφ , where φ is the angle between the normal to the plane of the coil and the magnetic field. NIA is the magnetic moment.
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