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Winter wk 4 – Thus.27.Jan.05

Winter wk 4 – Thus.27.Jan.05. Ch.28: Magnetic fields How to make a magnetic field Magnetic forces and fields Charge/mass ratio Charges circulate around B fields Magnetic force on current-carrying wire Solar applications. Energy Systems, EJZ. How to make a magnetic field.

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Winter wk 4 – Thus.27.Jan.05

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  1. Winter wk 4 – Thus.27.Jan.05 • Ch.28: Magnetic fields • How to make a magnetic field • Magnetic forces and fields • Charge/mass ratio • Charges circulate around B fields • Magnetic force on current-carrying wire • Solar applications Energy Systems, EJZ

  2. How to make a magnetic field Recall your observations with Science Kit CURRENTS cause magnetic fields: Straight wire Solenoid Permanent magnets are due to domains with aligned electron spin

  3. Electromagnetic forces and fields Force on a charge in an electric field is parallel to the E field Magnetic force is PERPENDICULAR to B field Magnetic force is perpendicular to MOTION of q

  4. Practice with magnetic forces p. 756 Q1

  5. Crossed E and B fields Lorentz Force Electric and magnetic forces can be considered independently • How can we align E and B so a moving charge is not deflected? • Solve F=0 for the charge’s speed v. Practice: #6, 7

  6. Thomson’s experiment • Charges are deflected by E field. • Charge motion is deflected by B field. First, how do B fields deflect moving charges?

  7. Deflection of charges by E See Sample Problem 22-5, p.593 Deflection of the charge by E is Recall that F=0 when v=E/B= Solve for m/q=

  8. Deflection of MOVING charge by B Consider a charge q moving with speed v perpendicular to a magnetic field B. Sketch the direction of the magnetic force.

  9. Cyclotron radius Radius of orbit of charge around B line depends on what? F = ma F = qvB a = v/r2 Combine: Solve for r Practice: #16, 79, 84

  10. Charge/mass ratio in playroom • Electron is accelerated through potential Potential energy  Kinetic energy qV = ½ mv2 • Electron is deflected in the magnetic field r=mv/|q|B • Measure V, B, and r. Solve for q/m=

  11. Magnetic force on current F=qvxB, current I = q/t, speed v=L/t Find q(I,L,v) Find F on I Practice:#33 p.759

  12. Solar applications Storms from the Sun: p.13: If a CME travels at 1 million miles per hour, how long does it take to reach Earth? p.16: The 2 May 1994 event dumped 4600 GW-hr of electricity into Earth’s upper atmosphere. How much energy is that in Joules? p.16: If the Earth’s mean magnetic field is B0=0.5 Gauss, and one Tesla=104 Gauss, by what percent does 2000 nanoTesla change Earth’s field? p.54: For the CME of 1 Sept 1859: calculate its speed v, if it took 18 hours to reach Earth.

  13. more Solar applications Storms from the Sun: p.77: If Rsun = 100 REarth, then find the ratio of their volumes, Vsun/VEarth p.77: If m=5 millions tons of mass is converted to energy (E=mc2) each second, calculate the power (P) produced by the Sun. p.82: If the Sun’s mass is M=2x1030 kg, and it keeps losing dm/dt = 5 million tons per second, how long (T) can the Sun last? p.83: If the solar wind pours I=1 million amps into Earths magnetosphere, how much charge (Q) is that per day?

  14. Extra solar applications p.13: Calculate vthermal from Tsolar wind. Compare to vflow. p.16: Derive the altitude for a geosynchronous orbit p.77: If the Sun’s core temperature is about T=107K, calculate the thermal speed vth of protons in the core.

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