Physics 106 Lesson #19 Magnetism

1. Physics 106 Lesson #19Magnetism Dr. Andrew Tomasch 2405 Randall Lab atomasch@umich.edu

2. Review: EngineeringTrade Studies • When a number of possible engineering solutions exist to meet a given need or goal, engineers perform a “trade study”. • This is a useful approach whenever a choice is to be made between a number of possible options. • The benefits, disadvantages and costs are quantified for each option and decisions are then made by “trading off” advantages against disadvantages for the various options based on a goal such as minimum cost, safety, or maximum reliability.

3. Magnets • All magnets have two poles: North and South • Like poles repel each other; Unlike poles attract each other • Magnetic poles are alwaysfound in pairs • Isolated magnetic poles have never been found

4. Magnetic Forces

5. Originally thought to be independent phenomena Like gravity, both have action-at-a-distance force laws Faraday and others believed in the unity of nature Maxwell proved that they were both aspects of a single phenomenon: Electromagnetism Electricity and Magnetism Michael Faraday James Clerk Maxwell

6. The north pole of a compass needle is a magnetic north pole The direction of the magnetic field at any point in space is the direction indicated by the north pole of a small compass needle (test magnet) placed at that point The Magnetic Field The magnetic field is a vector and denoted by the letter B B

7. Magnetic Field Lines • Magnetic field lines surround magnets • The magnetic field is always tangent to the magnetic field line • The number of lines per unit area (density) is proportional to the magnetic field strength • Outside of the magnet, magnetic field lines point away from north poles and toward south poles B

8. Iron Filings: Visualizing Magnetic Field Lines The magnetic field lines inside a bar magnet run from south pole to north pole.

9. The Earth’s Magnetic Field • The Earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the Earth • Caused by the “dynamo effect”, the rotation of a partially ionized outer core → internal electric current! • The north pole of a compass needle is attracted to the geographic North Pole, which is therefore a magnetic south pole South magnetic pole

10. Magnetic Field Produced by a Long, Straight Wire Demonstration: Oersted Experiment

11. Field Direction for a Straight Wire • The magnetic field due to the current in a long straight wire has circular field lines around the wire • The direction of the field is given by a right hand rule

12. Current flowing in a loop of wire creates a magnetic field The Current loop can be visualized as a “phantom bar magnet” Electromagnets N S = N S

13. N S Which End is the North Pole? • Another Right Hand Rule!

14. The Field of a Solenoid Current Into Page B Inside a solenoid (away from the ends) the magnetic field is constant. Current Out of Page