Chapter 24: Magnetic Fields

1. Chapter 24: Magnetic Fields

2. Magnets: Permanent and Temporary… • Properties of Magnets • Magnetic poles • Polarized- the quality of having two opposite magnetic poles, one south seeking and one north seeking. • Magnets orient themselves in a north-south direction, opposite sides attract each other. • Magnetic fields around Permanent Magnets • When experimenting with two magnets you notice the forces between magnets, both attraction and repulsion occur. • Magnetic fields- A vector quantity that relates the force exerted on a magnet or current-carrying wire to the strength of the magnet or the size of the current. - Example of a magnetic field

3. What is the first right hand rule? • The method used to determine the direction of the magnetic field around a current carrying wire. • Magnetic field near a coil • An electric current in a single circular loop of wire forms a magnetic field all around the loop. • Applying the right-hand rule to any part of the wire loop, it can be shown that the direction of the field inside the lop is always the same. • Solenoid- A coil of wire with many loops that acts like a magnet when a current is allowed to flow through it. • Electromagnet- A current-carrying coil with a north and south pole that is itself a magnet.

4. What is the second right hand rule? • The method used to determine the direction of the field produced by an electromagnet. • Example: turn to page 563, imagine holding an insulated coil with your right hand. Curl your fingers around the loops in the direction of the conventional (positive) current. Your thumb points toward the N-pole of the electromagnet.

5. Forces caused by Magnetic Fields… • Forces on Currents in Magnetic Fields • The force on a wire in a magnetic field can be demonstrated using the example on page 567. • Example: A battery produces the current in a wire that passes directly between two bar magnets. Recall the magnetic field between two magnets is from the N-pole of one magnet to the S-pole of a second magnet. When there is a current in the wire a force is exerted on the wire. As you can see the force on the wire either pushes it down or pulls it up. • Michael Faraday(1791-1867) discovered that the force on the wire is at right angles to both the direction of the magnetic field and the direction of the current.

6. Faraday’s description of the force on a current carrying wire does not completely describe the direction. • Force can be either up or down • What is third right hand rule? • The method used to determine the direction of the force on a current carrying wire in a magnetic field. • Example: turn to page 567, to use the third right-hand rule, point the fingers of your right hand in the direction of the magnetic field. Point your thumb in the direction of the conventional(positive) current in the wire. The palm of your hand then faces in the direction of the force acting on the wire. • Oersted’s discovery • The direction of the magnetic field in a wire is perpendicular to the flow of electric current in the wire. • This allowed Ampere to demonstrate the forces that current- carrying wires exert on each other.

7. Force on a wire resulting from a magnetic field • it is possible to determine the force of a magnetism that is exerted on a current-carrying wire passing through a magnetic field at right angles to the wire. • Magnitude of the force, on a wire is proportional to three factors: 1. the strength of the field 2. the current in the wire 3. the length of the wire that lies on a magnetic field • Force on a current carrying Wire in a Magnetic Field F=BIL • B, Magnetic field, T, teslas • A magnetic fields having a strength of one teslas causes a force of one Newton to be exerted on a 1-m length of straight wire carrying one ampere of current.

8. Example problem page 569 A straight wire that carries a 5.0-A current is in uniform magnetic field oriented at right angles to the wire. When 0.10m of the wire is in the field, the force on the wire is 0.20N. What is the strength of the magnetic field, B? Known: Unknown: I= 5.0 A B= ? L= 0.10 m F= 0.20 N All are right angles

9. Additional vocabulary to know: • Galvanometer- a device used to measure very small currents. • Electric motor- a device that converts electrical energy to kinetic energy. • Armature- a loop of wire in an electrical motor that is mounted on a shaft or axle. • Domain- A small group, usually 10 to 100 microns, that is formed when the magnetic fields of the electrons in a group of neighboring atoms combine together.

10. Try these on your own: Page 560 1-3, page 563 4-6, page 569 7-9

11. http://www.youtube.com/watch?v=dXpNtPxukiQ