Magnetic Fields and Forces: Currents and Charges, Slides of Physics

Download Magnetic Fields and Forces: Currents and Charges and more Slides Physics in PDF only on Docsity! PHY 2054C – College Physics B Fall 2004 Electricity, Magnetism, Light Optics and Modern Physics Dr. Ingo Wiedenhöver Dr. A. Volya Today: 1) Sources of Magnetism 2) Forces due to Magnetism Intro: Magnetic Fields Magnetic Fields are produced by moving charges (Electric Fields are produced by charges)  Moving single charge  Current in a wire  “Permanent” Magnet: Electrons in the material move in a coordinated manner. + Magnet Coils If we want strong magnetic fields, we have to use many wires, acting together in a coil; The field is very strong inside, weak outside All windings work together to produce a field (inside) Here: Field is independent of radius, position. B≈0 N l I Magnetic Materials Some Materials, Fe, Co, Ni and Gd are called “ferromagnetic”. Each one of their atoms posess a permanent circular current,creating a strong local magnetic field. Usually, those fields are randomly oriented in local domains. However, they will align themselves to an external magnetic field. Thus, they add to the external field, making it a lot stronger, sometimes by a factor 1000 ! B=B0BM B 0 B M Effects of Magnetic Fields To measure a magnetic field, you need a magnet ! (You need a charge to measure the electric field) e.g. a compass needle The compass needle feels a torque that trys to align it with the magnetic field. Field lines point from N to S. Question 1 An electric charge Q is located at a fixed position inside a magnetic field B. It feels 1) no force 2) a force perpendicular to the magnetic field 3) a torque around its axis R q,v Example: Proton in Magnetic Field (Capa#8) What is the magnitude of the magnetic force on a proton traveling at 2.900×106 m/s perpendicular to a uniform magnetic field of 1.4 T ? Force perpendicular to path; This will generate a circular motion with radius R ! Magn. Force = Cetripetal Force F=q v B sin  F=1.6×10−19 C⋅2.9×106 m s ⋅1.4 T=6.5×10−13N q v B=mv 2 R ⇒R=mv q B =0.022m B Torque on a Coil due to Magnetic Field Current loop “coil” on the right; Left and Right wire have different current directions; RHR-> F 1 into page, F 2 out of page If the coil has N windings F 1=−I⋅a B ,F 2=I⋅a B =F 1 b 2 sin F 2 b 2 sin  =I⋅a b B sin  =N I⋅A B sin  between⊥coil face and B