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Chapter 29 Magnetic Fields

Chapter 29 Magnetic Fields. Physics 2212. Differences Between Electric Fields and Magnetic Fields. What creates the field a) Electric - charge created just by sitting b) Magnetic – current ultimately charged at source, but charge must be moving

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Chapter 29 Magnetic Fields

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  1. Chapter 29 Magnetic Fields Physics 2212

  2. Differences Between Electric Fields and Magnetic Fields • What creates the field a) Electric - charge created just by sitting b) Magnetic – current ultimately charged at source, but charge must be moving  right hand current • Field Lines a) Electric – lines start and end b) Magnetic – circular loop lines

  3. Magnetic Versus Electric Field Lines

  4. Magnetic Field Lines • Implies no magnetic charge (no North) • No magnetic charges • Always closed loop surrounding charge • But still makes dipole with same shape

  5. Charges • Both electric and magnetic dipoles: opposites attract • North side of compass orients itself to face South, so that the dipole is opposite • Everywhere on Earth, there is a magnetic field • It produces a force

  6. A Magnet’s Effect • Charges move, producing the magnet’s effects • A force acts on the charges FB = qv × B q – charge v – velocity VECTOR B – magnetic field

  7. This is a CROSS PRODUCT • Tricky • 3 mutually perpendicular vectors

  8. Magnetic fields on Earth point North towards the magnetic South pole • Going into page X • Coming out of the page

  9. Example X X X F B P v X X X FB = qv × B FB = IqIIvIIBI sinθ 0 when parallel Must move perpendicular

  10. Right Hand Rule • V – Thumb • B – Finger • F – Palm • For point charge or current

  11. Current I = dq/dt l I = (dq/dt)l l (dq) = q (dl) l I = q (dl/dt) = qv FB = IqIIvI IBI sinθ F = I lX B

  12. Il **Must be a vector to get force!!

  13. Units • Units of B: Tesla (T) • T = Ns/Cm • A 1T magnet is BIG! A 19T magnet

  14. Magnetic field with current pointed at it: bends path into circle or spiral • Old TV works like this: Cathode Ray • Resistor inside capacitor is heated, free electrons go into the positive end and shoot out hole, causing a beam • Current goes opposite of electrons • Must deflect the beam to other parts of screen  magnetic field • Produce 2D field • 2 sets of charged coils • Side to side produces up and down • Up and down produces side to side

  15. Centripetal Force = q v B mv = qB r For a particle travelling in uniform magnetic field If you know “r”, the particles being bent measure ration, measure mass

  16. m = Br q v curving at different speeds Speed, strength, measure curvature

  17. Mass spectrophotometer – separates ions of different mass • Consistent velocity – add electric field • Take beam, pass into magnetic field that pulls beam up, a capacitor that causes electric field to go straight through Fup = qvBFdown = qE

  18. qvB = qE v = E B Curve up too fast, curve down too slowly, straight = right speed Charge is quantized (have same q)

  19. Loop of Current • No force on top and bottom • Causes torque • Proportional to magnetic field, current and area of the loop perpendicular to the field IA × B

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