Winter wk 3

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Winter wk 3 Thus.20.Jan.05

• Ch.24 Voltage and electric field
• Ch.26 Current and resistance
• Solar applications
• Ch.27 Circuits

Energy Systems, EJZ
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Equipotential surfaces and E fields
Equipotential constant voltage Conductors are
equipotentials, in electrostatics Potential
difference ? Electric field dV/dx -E or,
equivalently, Practice Ch.24 Q5,8
(p.646), P3, 4, 6, 35
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Ch.24 4
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Ch.24 6
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Ch.24 35
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Electrostatics (d/dt0) charges ? fields ?
forces, energy

• Charges make E fields and forces
• charges make scalar potential differences dV
• E can be found from V
• Electric forces move charges
• Electric fields store energy (capacitance)

F q E m a
W qV, C q/V
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Ch.26 Currents and Resistance

• Current rate of flow of charge
• I dq/dt
• Units amps coulombs/sec
• Current density J current/area n e v
• Ch.26 Q1, 2, P.1, 8
• Water flow Electricity flow
• pressure voltage V
• volume/time current I

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Ch.26 Q1, 2, P.1, 8
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Resistance

• Resistance resistivity area/length
• R r A/L
• Which conductor has the greatest resistivity?
• Ch.26 Q3

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Ohms law

• In many substances, for a given resistance R, the
  stronger the driving voltage, the greater the
  current that flows
• Voltage current resistance
• V I R
• Ch.26 Q5, P.17

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Power in electric circuits

• Power rate of energy xfr voltagecurrent
• P V I
• units Watts volts amps
• Recall that work qV. Units J CV
• Solve for V(J,C)
• Then voltsamps ____C/s ______
• If VIR, find P(I,R) P(R,V)
• Ch.26 35, 64

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Ch.27 Circuits

• Battery pumps electricity ? current flows

http//hyperphysics.phy-astr.gsu.edu/hbase/electri
c/watcir.htmlc1
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Voltage emf

• DVoltage potential difference
• Electromotive force e DV dW/dq work done per
  unit charge
• de/dx -E electric field

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Emf e and electric field E

• de/dx -E electric field
• Using the fundamental theorem of calculus, we can
  derive another of Maxwells eqns

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Ch.27 Practice with simple circuits

• Q2
• 5, 14

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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 Earths upper atmosphere. How
  much energy is that in Joules?
• p.16 If the Earths mean magnetic field is
  B00.5 Gauss, and one Tesla104 Gauss, by what
  percent does 2000 nanoTesla change Earths field?
• p.54 For the CME of 1 Sept 1859 calculate its
  speed v, if it took 18 hours to reach Earth.

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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 m5 millions tons of mass is converted
  to energy (Emc2) each second, calculate the
  power (P) produced by the Sun.
• p.82 If the Suns mass is M2x1030 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 I1 million amps
  into Earths magnetosphere, how much charge (Q) is
  that per day?

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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 Suns core temperature is about
  T107K, calculate the thermal speed vth of
  protons in the core.