Electric and Magnetic Fields

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Dorthy Parker
6 years ago
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want to define a magnetic field for magnetic forces.

1 Electric and Magnetic Fields Jst as we want to define an electric field for electrostatic forces, we also want to define a magnetic field for magnetic forces. E (nits of N/C, or V/m) B (nits of Tesla) Fig. 19.2, p.588

2 What if the particle is completely within the magnetic field? It follows a closed circlar path,i.e. a circle What is the radis of the circle? F=qvB=mv 2 /r s since it s the magnetic field that keeps it moving along a circlar path r = mv/(qb) = p/(qb)

3 r = mv/(qb) = p/(qb) How long does it take to travel one complete path? T = 2pr/v = 2pmv qbv The v s cancel ot Ernest Lawrence, in a bar, to anyone who wold listen So the period is the same for a charged particle moving in a magnetic field, so matter how fast it s moving Circlar Paths

4 How does a cyclotron work? Ernest Lawrence Glenn Seaborg It makes se of the fact that (1) the period T is independent of v Robert Oppenheimer and (2) electric fields don t go inside condctors Hey, what do yo think wold happen if I pshed this? Lawrence s 1st cyclotron 4 in diameter - +

5 How does a cyclotron work? Ernest Lawrence Glenn Seaborg It makes se of the fact that (1) the period T is independent of v Robert Oppenheimer and (2) electric fields don t go inside condctors Hey, what do yo think wold happen if I pshed this? Lawrence s 1st cyclotron 4 in diameter - +

6 How does a cyclotron work? It makes se of the fact that (1) the period T is independent of v and (2) electric fields don t go inside condctors Lawrence s 1st cyclotron 4 in diameter - +

7 How does a cyclotron work? It makes se of the fact that (1) the period T is independent of v and (2) electric fields don t go inside condctors Lawrence s 1st cyclotron 4 in diameter + -

8 National Spercondcting Cyclotron Lab Fermilab is a different type of accelerator called a synchrotron

9 Why was this sefl? Cold accelerate to mch higher energies than with a linear type of acceleration DV p becase yo can keep applying the voltage over and over again + DKE=+eDV -

Mass spectrometer Can se to separate isotopes (same nmber of protons in ncles bt different nmber of netrons, and so different masses) r 1 = m 1 v/(qb) r 2 = m 2 v/(qb)

10 Mass spectrometer Can se to separate isotopes (same nmber of protons in ncles bt different nmber of netrons, and so different masses) r 1 = m 1 v/(qb) r 2 = m 2 v/(qb) so if v and q are the same different masses will have different radii can se to screen newborns for genetic disorders s freezor/nshome/index.ht ml

11 What if I have more than one charge, i.e. an electric crrent? Total force = force on each charge carrier X total nmber of carriers Total force = F max = (qv d B)(nAl) bt i=nqv d A F max = Bil n charge carriers per nit volme so what is the direction in this case?

12 Angle, again or more generally, F=Bilsinq, where the direction is again given the right-hand rle

13 Crrents in magnetic fields let s jmp some wires

14 Crrent-carrying loop Sppose I have a rectanglar loop What direction is the force on it? no force force ot of page X force into page no force Fig. 19.8, p.593

15 No net force, bt there is a torqe look down on loop from above =IBb torqe wants to twist the crrent-carrying loop =IBb t max =F 1 a/2 + F 2 a/2 =Biab = BIA t = (N)BIAsinq

talk abot motors again when dring braking or coasting

16 This is the principle of an electric motor electric motor in Honda Insight; also acts as generator We ll talk abot motors again when dring braking or coasting Fig , p.597 we come to magnetic flx and generators

Crrents and magnetic fields So far, we ve stdied electric crrents and we ve stdied magnetic fields There s been no connection between them so far This is

17 Crrents and magnetic fields So far, we ve stdied electric crrents and we ve stdied magnetic fields There s been no connection between them so far This is Hans Christian Oersted (he doesn t have any nits named after him) In 1819 he did a demonstration for his physics class which I will now do Fig. 19.p600, p.600

18 Crrents and Magnetic Fields No crrent, compass needles point North N With a crrent, compass needles point in a circle

19 Crrents and magnetic fields If an electric crrent affects a magnet, then the electric crrent mst be creating a magnetic field itself, right? This is something we can try (demo) So electric charges create electric fields moving electric charges (i.e. crrents) create magnetic fields RHR #2 B field lines are concentric circles abot wire What direction?

20 Crrents and magnetic fields The direction of B is given by RHR #2 The magnitde of B for a long straight wire carrying a crrent I, at a distance r away from the wire is: B = m o I 2pr m o =4p X 10-7 T. m/a remember, we had e o for electric fields

21 Sppose I have a crrent loop What does the magnetic field look like?

Chapter 19. Magnetism. 1. Magnets. 2. Earth s Magnetic Field. 3. Magnetic Force. 4. Magnetic Torque. 5. Motion of Charged Particles. 6.

Chapter 19. Magnetism. 1. Magnets. 2. Earth s Magnetic Field. 3. Magnetic Force. 4. Magnetic Torque. 5. Motion of Charged Particles. 6. Chapter 19 Magnetism 1. Magnets 2. Earth s Magnetic Field 3. Magnetic Force 4. Magnetic Torque 5. Motion of Charged Particles 6. Amperes Law 7. Parallel Conductors 8. Loops and Solenoids 9. Magnetic Domains