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Currents and Magnetism

Physics 1161: Pre Lecture 13. Currents and Magnetism. Textbook Sections 22-4 – 22-7. B. v. I = q/t. +. +. +. +. L = vt. Force of B-field on Current. Force on 1 moving charge: F = q v B sin( q ) Out of the page (RHR). v. q. +. Force on many moving charges:

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Currents and Magnetism

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  1. Physics 1161: PreLecture 13 Currents and Magnetism • Textbook Sections 22-4 – 22-7

  2. B v I = q/t + + + + L = vt Force of B-field on Current • Force on 1 moving charge: • F = q v B sin(q) • Out of the page (RHR) v q + • Force on many moving charges: • F = (q/t)(vt)B sin(q) • = I L B sin(q) • Out of the page!

  3. Torque on Current Loop in B field C F D F • B I B A F C D X A F B Look from here The loop will spin in place! Net force on loop is zero. But the net torque is not!

  4. Torque on Current Loop in B field f C F D F • W B I B A L F C D X A F B L/2 L/2 Force on sections B-C and A-D: F = IBW Torqueon loop is t= 2 x (L/2) F sin(f) = ILWB sin(f) (length x width = area) LW = A !  Torque is t = I A B sin(f)

  5. normal F f D B A between normal and B C B F (areaof loop) Torque on Current Loop Magnitude: t = IAB sinf Direction: Torquetries to line up thenormalwithB! (when normal lines up with B, f=0, so t=0! ) Even if the loop is not rectangular, as long as it is flat: t = I AB sinf. N # of loops

  6. Lines of B Currents Create B Fields Magnitude of B a distance r from (straight) wire: B Here’s a current-carrying wire. CurrentIOUT of page. r • r = distance from wire Right-Hand Rule, 2 Thumb: along I Fingers: curl along B-field lines

  7. Right Hand Rule 2 Fingers give B! I wire

  8. B B Another I towards us F • I towards us F •  Another I away from us Force between current-carrying wires I towards us • Conclusion: Currents in same direction attract! Conclusion: Currents in opposite direction repel! Note: this is different from the Coulomb force between like or unlike charges.

  9. Comparison:Electric Field vs. Magnetic Field Electric Magnetic Source Charges Moving Charges Acts on Charges Moving Charges Force F = Eq F = q v B sin(q) Direction Parallel E Perpendicular to v,B Field Lines Opposites ChargesAttract Currents Repel

  10. Magnetic Fields of Currents • http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html#c1

  11. B Field Inside Solenoids Magnitude of Field anywhere insideof solenoid :B=m0nI n is the number of turns of wire/meter on solenoid. m0= 4p x10-7 T m /A (Note: N is the total number of turns, n = N / L) Right-Hand Rule 3 gives Direction: Fingers – curl around solenoid in direction of I Thumb - points in direction B == Magnetic field lines look like bar magnet! Solenoid has N and S poles!

  12. B Field Inside Solenoids Magnitude of Field anywhere insideof solenoid :B=m0nI n is the number of turns of wire/meter on solenoid. m0= 4p x10-7 T m /A (Note: N is the total number of turns, n = N / L) Right-Hand Rule for loop/solenoid Fingers – curl around coil in direction of conventional (+) current Thumb - points in direction of B along axis Magnetic field lines look like bar magnet! Solenoid has N and S poles!

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