A new force Magnetic force. Today. Force Fields: A disturbance of space. The correspondence of a loop of current and magnet.

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1 Today A new foce Magnetic foce Announcements HW#6 and HW#7 ae both due Wednesday Mach 18th. Thanks to my being WAY behind schedule, you 2nd exam will be a take-home exam! Stay tuned fo details Even if you cut the magnet in half, they still come in pais (N & S) It is a geat mystey why no single poles ( monopoles ) have been seen Moe magnetic and electic fields Fist peek at waves and light ISP209s9 Lectue 14 Cetain objects in natue have magnetic poles They always come in pais, noth and south poles. If you see one, book you ticket to Stockholm to pick up you Nobel Pize! -1- The coespondence of a loop of cuent and magnet ISP209s9 Lectue Foce Fields: A distubance of space Seeing magnetic field lines with ion filings Magnets have an intenal stuctue whee the motion of the electons ceates small egions with cuents. Cuent in a wie ceates a Magnetic field Cuents ceate magnetic fields (see geen aows) ISP209s9 Lectue ISP209s9 Lectue 14-4-

2 How a compass woks ISP209s9 Lectue 14 Maxwell s Equations Unify Electicity/Magnetism Chage makes an electic field. "! E = 4&' 4& 1 #E Moving chage makes a magnetic "%B = J+ field. c Changing magnetic field makes an 1 #B electic field ( Faaday s Law ) "%E = $ Magnets always have a noth and a "!B = 0 south pole See class demo -5- Faaday s Law ==> Electicity geneation ISP209s9 Lectue How to Build a Magnet Maxwell "! E =Eqs. 4&'tell us: 4& 1 #E "%B = J+ c 1 #B "%E = $ "!B = 0 Move a closed loop of wie thu a magnetic field => electic cuent is geneated ISP209s9 Lectue 14 The equations pedict the existence of an wave that tavels with speed c, the speed of light. -7- Moving chage ceates a magnetic field ISP209s9 Lectue 14-8-

3 Magnets in diffeent contexts CC data stip Tansfomes to aise/lowe electic potential in tansmission line and countless othes High speed maglev tains MRI Electic motos Cycloton paticle Acceleato next doo Nothen lights ISP209s9 Lectue Electic and Magnetic Fields If we move a test chage, q, (o magnet) in the vicinity of anothe chage (o magnet) we can make a map of the foce. Define: Electic field E = F/q Electic field is a vecto. Its units ae N/C o /m (volts/mete). Once we know the electic field we can calculate the foce: F=qE F = electic field times chage of the object in the field Which is undelying eality, the foce o the field? ISP209s9 Lectue Samples The Stength of the Electic Field Electic potential SI unit is the olt () Electic field is ate of change of potential! E = "! x Electic field lines point away fom positive chage and towad negative chage. Chage geneates an electic field. The minus sign means that electic fields point fom + to chage. ISP209s9 Lectue ISP209s9 Lectue

4 Sample Poblem A visual pictue Electic Potential What is the magnitude of the electic field at: 0.5 m? 1.5 m? 3.0 m? Positive chage makes a hill +q + Steep slope means a lage foce. The field is 0 /m at 0.5 m and 3.0 m since the slope is zeo. " ( 100! 0 ) E( at 1.5m) = = = 100 " x 2m! 1m ( ) m ISP209s9 Lectue Negative chage makes a valley. ISP209s9 Lectue Electic fields and potential Electic Field Example In equilibium the electic field in a metal conducto is zeo. This means that inside a metal the electic potential is flat, like the flat top of a table. Sitting inside a metal cage is like sitting on top of a lage, flat table. As long as you ae in the cente, thee is no dange of falling off. This is why being in a ca duing a thunde stom is elatively safe. ISP209s9 Lectue Q = 0.5 µc = 0.5x10-6 C What is the magnitude of the electic foce on Q? F = qe F = 0.5E-6C x 40 N/C = 20E-6 N Not asked fo, but the diection is +, to the ight. Electic Field E = -!/!x =-(50-90)/1m= 40 /m + means to the ight in this case ISP209s9 Lectue

5 Flow of Chage - Cuent Types of electic mateials Cuent is the ate of flow of chage. SI units is Ampee = 1 Coulomb/second Batteies ae like pumps that lift chage to a highe potential. The chage flows down the hill to the othe side of the battey. A battey is like a pump. Moving Chage does wok on the way down ISP209s9 Lectue Conducto electons in the conduction ; electons elatively fee to flow (coppe, aluminum, gold, silve) Insulato no electons is the conduction ; electons can not flow (wood, most ubbe, most glass, most plastic) Semiconducto at finite tempeatue, some electons ae in the conduction (used in most electonics; silicon, gemanium) Supeconducto at vey low tempeatue electons pai and can move feely without esistance (Niobium, Titanium, Lead) ISP209s9 Lectue Conducto e.g., coppe, ion, aluminum, silve, gold, Insulato e.g., ubbe, plastic, glass, wood, electons electons Electons hit bumps, but ae fee to oll. enegy Electons ae not fee to oll. enegy ISP209s9 Lectue ISP209s9 Lectue

6 Semiconducto Supeconducto electons enegy electons light LED light emitting diode ISP209s9 Lectue No esistance to flow (also no use of enegy) enegy ISP209s9 Lectue The Electomagnetic Spectum The moden pictue of electomagnetism is that the electic foce is caied by the photon. A photon is a small bundle of enegy. We see photons in the ange of 1.8 e (ed) to 3.1 e (violet) [1 e = 1.6E-19 J] The full ange of diffeent photon enegies is called the electomagnetic spectum. ISP209s9 Lectue Wavelength and Fequency " = 1.0 m peiod = 2.0 s Wavelength Fequency = 1/peiod Distance ove which the wave epeats Numbe of cycles (epeats) pe second. ISP209s9 Lectue

7 The Electomagnetic Spectum Speed = " f " wavelength f Fequency, Hz (1/peiod)(1/s) Fo light Speed c = 3.0E+8m/s Pentice-Hall 2005 Enegy = h f h = 6.625E-34 Js = 4.136E-15 es Planck s constant ISP209s9 Lectue