Physics 202: Lecture 8, Pg 1

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1 Physics 132: Lecture e 18 Elements of Physics II Agenda for Today Magnets and the Magnetic Field Magnetic fields caused by charged particles B-field from a current-carrying carrying wire Magnetic fields and forces B-fields Magnetic forces on moving particles Physics 202: Lecture 8, Pg 1

The end of a magnet that points north is called the north-seeking gp pole,, or simply py

2 Magnetism Tape a bar magnet to a piece of cork and allow it to float in a dish of water. It always turns to align itself in an approximate northsouth direction. The end of a magnet that points north is called the north-seeking gp pole,, or simply py the north pole. The end of a magnet that points south is called the south pole. Physics 202: Lecture 8, Pg 2

3 Magnetism Like poles repel, opposite poles attract Physics 202: Lecture 8, Pg 3

4 Magnetism Magnets can pick up some objects, such as paper p clips, but not all. If an object is attracted to one end of a magnet, it is also attracted to the other end. Most materials, including copper (a penny), aluminum, glass, and plastic, experience no force from a magnet. Physics 202: Lecture 8, Pg 4

5 Magnetism Cutting a bar magnet in half produces two weaker but still complete magnets, each with a north pole and a south pole. No matter how small the magnets are cut, even down to microscopic sizes, each piece remains a complete magnet with two poles. Physics 202: Lecture 8, Pg 5

6 Magnetism 1. Magnetism is not the same as electricity. 2. Magnetism is a long range force. 3. All magnets have two poles, called north and south poles. Two like poles exert repulsive forces on each other; two opposite poles attract. 4. The poles of a bar magnet can be identified by using it as a compass. The north pole tends to rotate t to point approximately north. 5. Materials that are attracted to a magnet are called magnetic materials. The most common magnetic material is iron. Physics 202: Lecture 8, Pg 6

7 Mapping Magnetic Field Physics 202: Lecture 8, Pg 7

8 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

9 B-field lines We will also draw magnetic field lines (1) The direction of a magnetic field ed is tangent to 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

10 B-Field Lines Physics 202: Lecture 8, Pg 10

11 The Earth s magnetic field resembles that achieved by burying a huge bar magnet deep in the Earth s interior Earth s Magnetic Field Physics 202: Lecture 8, Pg 11

12 B-field of Current-Carrying 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

13 B-field of Current-Carrying Carrying 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 Current-Carrying Carrying Wire Magnetic field lines are imaginary lines drawn through a region of space so that: t A tangent to a field line

14 B-field of Current-Carrying Carrying Wire Magnetic field lines are imaginary lines drawn through a region of space so that: t A tangent to a field line is in the direction of the magnetic field. The field lines are closer together th where the magnetic field strength is larger. Physics 202: Lecture 8, Pg 14

15 RHR Physics 202: Lecture 8, Pg 15

16 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

17 The Magnetic Field The constant 0 in the Biot-Savart law is called the permeability constant: 0 = T m/a = 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

18 Currents Create B-Fields Magnitude: I B 0 I 2 r Tm / A 7 B r Current I OUT 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

19 Give it a try: Compared to the magnetic field at point A, the magnetic field at point B is A. Half as strong, same direction. B. Half as strong, opposite direction. C. One-quarter as strong, same direction. D. One-quarter as strong, opposite direction. E. Can t compare without knowing I. Physics 202: Lecture 8, Pg 19

20 Give it a try: What is the direction of the magnetic field at the position of the dot? A. Into the screen. B. Out of the screen. C. Up. D. Down. E. Left. Physics 202: Lecture 8, Pg 20

Elements of Physics II. Agenda for Today

Elements of Physics II. Agenda for Today Physics 132: Lecture e 18 Elements of Physics II Agenda for Today Magnets and the Magnetic Field Magnetic fields caused by charged particles B-field from a current-carrying wire Magnetic fields and forces