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Physics ys cs 132: 3 Lecture ectu e 18 8 Elements of Physics II A Agenda d for f T Today d Magnets and the Magnetic Field Magnetic fields caused by charged particles B-field from a currentcurrent-carrying wire Magnetic fields f and forces f B-fields Magnetic forces on moving particles Physics 202: Lecture 8, Pg 1 Magnetism Tape a bar magnet to a piece of cork and allow it to float in a dish of water. It always y turns to align g itself in an approximate northsouth direction. The end of a magnet that points north is called the north-seeking gp pole,, or simply p y the north pole. The end of a magnet that points south is called the south pole. Physics 202: Lecture 8, Pg 2 Magnetism Like poles repel, opposite poles attract Physics 202: Lecture 8, Pg 3 Magnetism Magnets can pick up some objects, j , such as p paper p clips, p , but not all. If an object is attracted to one end of a magnet, magnet it is also attracted to the other end. Most materials, materials including copper (a penny), aluminum, glass, and plastic experience no force plastic, from a magnet. Physics 202: Lecture 8, Pg 4 Magnetism Cutting a bar magnet in half produces two p magnets, g each with a weaker but still complete north pole and a south pole. No matter how small the magnets are cut, even down to microscopic sizes sizes, each piece remains a complete magnet with two poles. Physics 202: Lecture 8, Pg 5 Magnetism 1. 2. 3 3. 4. 5. Magnetism is not the same as electricity. Magnetism is a long range force. All magnets have two poles poles, called north and south poles. Two like poles exert repulsive forces on each other; two opposite poles attract. attract The poles of a bar magnet can be identified by using it as a compass. The north pole tends to rotate t t to t point i t approximately i t l north. th Materials that are attracted to a magnet are called magnetic g materials. The most common magnetic material is iron. Physics 202: Lecture 8, Pg 6 Mapping Magnetic Field Physics 202: Lecture 8, Pg 7 B-field Similar to electric charges a magnet will create a magnetic field in the area around it Due to convention we will call the magnetic field a B-field The force magnets apply on each other can be described as an interaction between a magnet and another magnet’s B-field Physics 202: Lecture 8, Pg 8 B-field lines We will also draw magnetic field lines ( ) The (1) ed direction ec o o of a magnetic ag e c field e d is s tangent a ge to o a magnetic field line at any point (2) The number of lines per area is proportional to the magnitude of the B-field Physics 202: Lecture 8, Pg 9 B-Field Lines Physics 202: Lecture 8, Pg 10 Earth’s Magnetic Field The Earth’s magnetic field resembles that achieved by burying a huge bar magnet deep in the Earth’s interior Physics 202: Lecture 8, Pg 11 B-field of CurrentCurrent-Carrying Wire In 1819 Hans Christian Oersted discovered that an electric current in a wire causes a compass to turn. Physics 202: Lecture 8, Pg 12 B-field of CurrentCurrent-Carrying y g Wire The right-hand rule determines the orientation of the compass needles to the direction of the current. Physics 202: Lecture 8, Pg 13 B-field of CurrentCurrent-Carrying Wire Magnetic field lines are imaginary lines drawn through a region of space so that: th t A tangent to a field line is in the direction of the magnetic field. The field lines are closer t together th where h the th magnetic field strength is larger. Physics 202: Lecture 8, Pg 14 RHR Physics 202: Lecture 8, Pg 15 The Source of the Magnetic Field: Moving Charges The magnetic field of a charged particle q moving with velocity v is given by the Biot-Savart law: Physics 202: Lecture 8, Pg 16 The Magnetic Field The constant 0 in the Biot-Savart law is called the permeability constant: 0 = 4 × 10-7 T m/A = 1.257 × 10-6 T m/A The SI unit of magnetic field strength is the tesla, abbreviated as T: 1 tesla = 1 T = 1 N/A m Physics 202: Lecture 8, Pg 17 Currents Create B-Fields Magnitude: 0I B 2r Current I OUT B r 0 4 10 77 Tm / A • r = distance from wire Direction of b-field from current carrying wire: Curly right hand rule Lines of B Thumb along current I, direction of B along curled fingers. Physics 202: Lecture 8, Pg 18 Give it a try: Compared to the magnetic field at point A, the magnetic field at p point B is A. Half as strong, same direction. B. Half as strong, opposite direction. C. One-quarter as strong, same di ti direction. D. One-quarter as strong, opposite direction. direction E. Can’t compare without knowing I. Physics 202: Lecture 8, Pg 19 Give it a try: What is the direction of the magnetic field at the position of the dot? A. A B. C C. D. E E. Into the screen. screen Out of the screen. Up Up. Down. Left Left. Physics 202: Lecture 8, Pg 20