Magnetism and You. Fields

Transcription

1 Magnetism and You Fields Areas in which a force acts Lines represent the direction in which the force acts on a body Fields for every force: gravity, electricity, magnetism, etc. Sometimes multiple forces act in the same area this creates complicated field situations. 1

2 Gravitational Field Direction in which the force acts is toward the center of the earth Field lines point toward the center of the earth The lines may represent the strength of the field also Stronger as you get closer Lines in a changing field are called gradients they represent the way the field changes in addition to its direction (lots of information in one little picture) Electric Fields We create them by putting charges on things + or -, and the size varies. They only affect charged objects (We could stand in an electric field and not notice it) Apply forces to charged particles just as gravity applies forces to masses: F=qE (q = charge, E = electric field strength) F=mg (m = mass, g = gravitational field strength) strong electric field will accelerate a particle in the direction of the field that opposes its charge (+ charges go toward the - side of the field) 2

3 Magnetic Fields Given the symbol B (?) Are created two different ways: Alignment of electron spins in substances (ferromagnetism if natural; paramagnetism if created by another B field) Charges moving in an electric field The relationship between electricity and magnetism has been known for 150 years or so All electric devices create magnetic fields Moving magnets are used to generate electricity (dynamo) What does a B field look like? Shape varies, and can only be demonstrated by something subject to B fields (overhead demo) Can be modeled because the theory is so well-known simulation 3

4 Magnetic AND electrical fieldstogether at last! The two are related: electric and magnetic fields are always perpendicular Magnetic fields are created by moving charges Right Hand Rule! Thumb points in the direction of the moving charge (or current flow); curl your fingers toward your palm that s the direction of the magnetic field! Works in reverse too; thumb towards B field, fingers show direction of motion of a charged particle! Charged Particles in E/B Fields Cyclotron motion: when a charged particle has velocity and enters a magnetic field Images Particles in the Earth s magnetic field are subject to cyclotron motion as well 4

5 Prof. James Van Allen discovered these from measurements made by first satellites (Explorer I) Three major belts Proton belt (500-13,000km) Low-energy electron belt (overlaps proton belt) High-energy electron belt (farther out) Van Allen Belts Simulated belts made in a plasma lab. See the lab guy? Van Allens = Cyclotron motion? Yes. Particles bounce around and follow the field lines. When they reach the end of a field line, they reverse direction (the flux is so great they cannot proceed through it) 5

6 Aurorae Note: Aurorae 2, Aurora Borealis (north); Aurora Australis (south) Both explained (sort of) the same way: Charged particles trapped in the Earth s B field are accelerated toward the poles They collide with gases in the upper atmosphere, causing the gas molecules to emit light Red = O at 200 miles Green = O at 60 miles Blue = N * at high altitudes Crimson/purple = N at lower altitudes 6

7 Aurora photographed from ISS 200 mi. above earth Crater UV photo of proton aurora IMAGE satellite 7

8 Aurora Sounds Moving charges create EMF signals, often in the radio part of the spectrum. Radio signals can be turned into sounds (they have the same wavelengths as sound waves) B Fields Protecting the Earth 8

9 Bow Shock Charged particles of the solar wind strike the earth s field lines and are deflected around the earth like wind around a car The majority of particles pass right by the earth Summary Charged particles follow circular paths around field lines in magnetic fields (cyclotron motion right hand rule!) Planetary magnetic fields are the result of circulating liquid metal in the planet s core Planetary magnetic fields shield planetary surfaces from damaging/lethal charged particle radiation (Van Allen belts on Earth) Interaction between PMF and charged particles produces aurorae and radio signals 9