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Physics 20: Magnetism

Physics 20: Magnetism. Christopher Chui. Magnets and Magnetic Field. A magnet has north pole and south pole Magnetic field lines point from north to south poles Magnetic poles are not found singly Like poles repel and unlike poles attract Magnetic field is denoted by a vector B

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Physics 20: Magnetism

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  1. Physics 20: Magnetism Christopher Chui Physics 20: Magnetism - Christopher Chui

  2. Magnets and Magnetic Field • A magnet has north pole and south pole • Magnetic field lines point from north to south poles • Magnetic poles are not found singly • Like poles repel and unlike poles attract • Magnetic field is denoted by a vector B • The magnitude of B is the torque it makes with B field • The angle between magnetic north and true north is called the magnetic declination • The angle that the Earth’s magnetic field makes with the horizon is referred to as the angle of dip • For compass to point north, the magnetic pole is equivalent to the south end of the Earth’s magnet Physics 20: Magnetism - Christopher Chui

  3. Electric Currents Produce Magnetism • Hans Christian Oersted (1777-1851) found that an electric current produces a magnetic field—right hand rule • A magnet exerts a force on a current-carrying wire • The direction of the force F is always perpendicular to the direction of the current I and also perpendicular to the direction of the magnetic field Bright hand rule • Force on electric current in field B=F = I (length l) sin q • If perpendicular, q = 90o, sin q = 1, then Fmax = I l B • SI unit for B is tesla (T), 1 T = 1 N/A-m = 1 Wb/m2 • cgs unit for B is gauss (G), 1 G = 10-4 T • Force on moving charge in magnetic field, F = qvB sin q • A compass, near a current, may not point North Physics 20: Magnetism - Christopher Chui

  4. Ampere and Coulomb • 1 A is the current flowing in each of 2 long // conductors 1 m apart, which results in a force of exactly 2 x 10-7 N/m of length of each conductor • 1 coulomb is defined exactly as 1 ampere-sec • Andre Marie Ampere (1775-1836) found that the sum of the product of //B and length Dl = moI • For a circular path of radius r  B(2pr) = moI • For a solenoid Bl = mo NI  B = mo nI, where n is the number of loops per unit length • Torque on a current loop = NIA B sin q =(magnetic dipole moment) B sin q Physics 20: Magnetism - Christopher Chui

  5. Some Applications • A galvanometer consists of a coil of wire suspended in the magnetic field of a permanent magnet • A chart recorder, in which a pen graphs a signal such as an ECG on a moving roll of paper, is a galvanometer • An electric motor changes electric energy into mechanical energy • A DC motor uses commutators and brushes to achieve rotation in one direction • An AC motor, with an ac input, works without commutators, using electromagnets—wired coils • A loudspeaker works on the principle that a magnet exerts a force on a current-carrying wire Physics 20: Magnetism - Christopher Chui

  6. The Hall Effect • A current-carrying conductor experiences a sideway force on the charges moving in the conductor • E.H. Hall discovered this effect in 1879 • Hall emf builds up until the electric field on the moving charge is equal and opposite to the magnetic force • In equilibrium, eEH = e vd B  EH = vd B • Hall emf = EH l = vd B l • A mass spectrometer measures masses of atoms by accelerating the atoms through a magnetic field: qE=qvB v = E/B. Using qvB’ = mv2/r m=qBB’r/E Physics 20: Magnetism - Christopher Chui

  7. Ferromagnetism and Domains • Iron can be made into strong magnets—ferromagnetic • A magnet consists of magnetic domains. When aligned in one directionstrong permanent magnets • At Curie temperature, all magnetic domains are randomized. For iron, the Curie temperature = 1043 K • All magnetic fields are caused by electric currentsno single magnetic pole has ever been found • All magnetic field lines form closed loops • A solenoid is an electromagnetright hand rule • Magnetic field curves do not retrace themselves on the same path is called hysteresis • A ferromagnetic material can be demagnetized by reversing the magnetizing current repeatedly while decreasing its magnitude Physics 20: Magnetism - Christopher Chui

  8. Problem Solving for Magnetic Fields • The force experienced by a charged particle moving in a magnetic field is perpendicular to the direction of the magnetic field and to the direction of the velocity of the particle, while the force exerted by an electric field is // to the direction of the field and unaffected by the velocity of the particle • The right-hand rule determines the directions of magnetic field, the force they exert, and the direction of electric current or charged particle velocity • The right-hand rule can be used to find directions of vector quantities Physics 20: Magnetism - Christopher Chui

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