Ch. 21: Magnetism

1. Ch. 21: Magnetism

2. Magnetism • magnetic poles produce magnetic forces • poles always exist in pairs (N and S) • opposite poles attract, like poles repel • there are no magnetic “monopoles”

3. Magnetic Fields magnetic field lines (B-field) always point from N to S • • • • • • • • • • • • • • • + + + + + + + + + + + + + + +

4. Big and Little Magnets • currents within the mantle • produce the earth’s field • in atoms, orbiting and • spinning electrons produce • tiny magnetic fields • Fe, Ni, and Co are the most • magnetic elements

5. Objectives • Understand and apply the first magnetic “right hand” rule. • Understand and apply the second magnetic “right hand” rule. • Understand practical applications of electromagnets. • Understand and explain the concept of magnetic domains.

6. Electric Current and B-Fields • Hans Christian Oersted (1820) first noticed that • an electric current will deflect a compass needle • firstright hand rule

7. Electric Current and B-Fields • a current in a coil (or solenoid) • produces an electromagnet • second right hand rule B I • How a Speaker Works

8. Magnetic Domains • domains are clusters of billions of • iron atoms with aligned fields • domains will align in a B-field • permanent magnets have been • exposed to very strong fields • heat destroys magnets because • domains become random

9. Objectives • Understand how magnetic force is applied to moving charges. • Apply the third “right hand” rule. • Understand some common applications of magnetic force. • Solve magnetic force problems.

10. Magnetic Force • a charged particle moving perpendicular to a B-field feels a force • 1 Tesla (T) = 1 N/(C · m/s) = N/(A·m) • third right hand rule:

11. Auroras

12. Magnetic Force Problem • A proton moving at 1200 km/s (in the solar wind) runs perpendicular into the earth’s magnetic field (B = 55 mT). How much force is applied to the proton? What is the acceleration of the proton (m = 1.67 x 10-27 kg)?

13. Particle Accelerators

14. Mass Spectrometer • mass spectrometer: an instrument that measures the mass of charged particles • used to identify elements present in a sample

15. Magnetic Force on a Wire • a current-carrying wire in a B-field will feel a force perpendicular to the wire • How much force is applied to a 5-cm long wire carrying 12 A of current when it is placed in a 3 mT magnetic field?

16. Chapter 22: Induction and Alternating Current

17. Magnetic Fields and EMFs • Michael Faraday (1831) and Joseph Henry: • electromagnetic induction: the production of a current caused when a conductor is moved through a magnetic field (or the magnetic field is changed) • emf: electromotive force; an increase in PE per charge (voltage) that pushes charges through a conductor; emf produces a current • Use the 3rd right hand rule to determine direction of current.

18. Lenz’s Law • Lenz’s law: the magnetic field of an induced current opposes the change in the applied magnetic field • energy is conserved due to this “magnetic friction”

19. Faraday’s Law • N = number of loops • A = area • B = magnetic field • t = time • Use this law to calculate the voltage generated by a spinning coil.

20. Applying Faraday’s Law

21. Objectives • Be able to explain how/why a generator works. • Be able to explain how/why an electric motor works. • Understand how different commutators are used to produce/use AC versus DC.

22. Generators and Motors • generator: converts KE to electrical energy (current) • spinning a coil in a B-field causes an AC to form • commutator: determines if AC or DC • armature: multiple-loop coil

23. Electric Motors • motor: a device that converts electric energy (AC or DC) to KE

24. Transformers • transformer: converts AC to higher or lower voltage (step up or step down) • DV2 = DV1N2 / N1 • Electricity is transmitted at high V, low I (due to “I2R loss”) then stepped down • 230kV to 20kV to 120V