Transcript Magnetism
Chapter 21 James Clerk Maxwell Michael Faraday What is magnetism? …a phenomena in which certain materials exert attractive/repulsive forces on other materials. These materials contain a region where these forces can be detected. (Magnetic fields) Where do magnetic fields occur? Where do magnetic fields occur? How do Magnetic fields work? Field lines move away from the north pole. Field lines move towards the south pole. Magnetic Field of the Earth North magnetic pole located at south geographic pole. South magnetic pole located at north geographic pole. Magnetic Field Strength The strength of a magnetic field (B) is related to the amount of magnetic force (F) that is applied to a moving test charge when it is at a given location in the field. B = Fmagnetic qυ q = test charge magnitude υ = speed of the charge Sample Problem A proton moving east experiences an upward force of 8.8 x 10-19 N due to the Earth’s magnetic field. The field has a strength of 5.5 x 10-5 Tesla’s (T) to the north. Find the speed of the proton. B = Fmagnetic q = 1.60 x 10-19 C B = 5.5 x 10-5 T Fmagnetic = 8.8 x 10-19 N qυ υ = Fmagnetic qB υ= 8.8 x 10-19 N _ = 1.0 x 105 m/s (1.60 x 10-19 C) (5.5 x 10-5 T) Direction of magnetic force The direction of the magnetic force is perpendicular to the plane of the magnetic field and to the direction of the charge. USE RIGHT HAND RULE!!! Magnetic force in current-carrying wires Current carrying wires that are placed in a magnetic field also experience a magnetic force. F I WIRE Magnetic force in current-carrying wires The magnitude of the magnetic force can be written in terms of the current (I) flowing through the length of the wire (L). F = BIL Sample Problem A 10.0 m long powerline carries a current of 20.0 A perpendicular to the Earth’s magnetic field of 5.5 x 10-5 T. What is the magnetic force experienced by the power line? F = BIL B = 5.5 x 10-5 T I = 20.0 A L = 10.0 m F = (5.5 x 10-5 T)(20.0 A)(10.0 m) = 0.011 N Magnetic fields from electricity Charges that are in motion (an electrical current) produce magnetic fields. Magnetic field moves around a wire with a current in a circular fashion. Which direction??? Right Hand Rule 1.Thumb goes in the direction of the current. 2. Fingers wrap around wire in the direction of the magnetic field. Electromagnets Arranging wire in a coil and running a current through produces a magnetic field that is similar to a bar magnet. Electromagnetic Induction It is possible to induce an electric current without the use of a battery or power supply. Coil = Solenoid Electromagnetic Induction Produces an electromotive force (emf) Lenz’s Law An electromagnetic field interacting with a conductor will generate electrical current that induces a counter magnetic field that opposes the magnetic field generating the current.