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The graded exams are being returned today. You will have until the next class on Thursday, Oct 6 to rework the problems (including the multiple choice questions) you got wrong and receive 50% added credit. You are honor bound to work on these alone; this is an exam. I will be going over the answers in class next Tuesday. This will also be your only opportunity to ask for corrections/clarifications on any grading mistakes.
In 1995, the microprocessor unit on the microwave imager on the DMSP F13 spacecraft locked up - occurred ~5 s after spacecraft began to charge up in the auroral zone in an auroral arc. Attributed to high-level charging of spacecraft surface and subsequent discharge.
Spacecraft surfaces are generally covered with thermal blankets - outer layer some dielectric material - typically Kapton or Teflon. Deposition of charge on surface of spacecraft known as surface charging. Incident electrons below about 100 keV penetrate the material to a depth of a few microns, where they form a space charge layer - builds up until breakdown occurs accompanied by material vaporization and ionization. A discharge is initiated - propagates across surface or through the material, removing part of bound charge. Typically occur in holes, seams, cracks, or edges - have been know to seriously damage spacecraft components. Thermal blankets - composed of layers of dieletric material with vapor deposited aluminum (VDA) between each layer. On DMSP, VDA between layers (22) not grounded - serve as plates of a set of 22 parallel-plate capacitors - top plate consists of electrons buried in top few microns of Teflon.
C/A=7.310-9 F/m2 Time to charge outer surface of a parallel plate capacitor to some voltage with respect to spacecraft frame is: Laboratory measurements for Teflon: -discharge at 3 kV in a 20 keV electron beam -secondary electron yield () at 20 keV ~ 0.2 Given measured incident precipitating current density of i = 4.8 A/m2, the time to reach breakdown voltage for conditions experienced by DMSP F13 is 5.7 s - this is the time after the spacecraft began to charge up that the lockup occurred. If the VDA layer on the bottom side of the outer Teflon layer were grounded to the spacecraft frame, the capacitance would have been 3.510-7 F/m2 and the charging time would have been 132 s - no discharge would have occurred.
Charges in Motion – Electric Current Electric Current – a method to deliver energy Very convenient way to transport energy no moving parts (only microscopic charges) Electric currents is in the midst of electronic circuits and living organisms alike Motion of charges in electric fields
Motion in a uniform electric field Deflection by a uniform electric field
Electric Current in Conductors In electrostatic situations – no E-field inside There is no net current. But charges (electrons) still move chaotically, they are not on rest. On the other side, electrons do not move with constant acceleration. Electrons undergo collisions with ions. After each collision, the speed of electron changes randomly. The net effect of E-field – there is slow net motion, superimposed on the random motion
Direction of the Electric Current Current in a flash light ~ 0.5 A In a household A/C unit ~ 10-20 A TV, radio circuits ~ 1mA Computer boards ~ 1nA to 1pA
Current, Drift Velocity, Current Density Current density J, is a vector while total current I is not
Example: An 18-gauge copper wire has nominal diameter of 1.02 mm and carries a constant current of 1.67 A to 200W lamp. The density of free electrons is 8.5*1026 el/m3. Find current density and drift velocity Why, then, as we turn on the switch, light comes immediately from the bulb? E-field acts on all electrons at once (E-field propagates at ~2 108 m/s in copper) Electric current in solution of NaCl is due to both positive Na+ and negative Cl- charges flow
Ohm’s Law Current density J and electric field E are established inside a conductor when a potential difference is applied – Not electrostatics – field exists inside and charges move! In many materials (especially metals) over a range of conditions: J = σE or J = E/r with E-independent conductivity σ=1/r This is Ohm’s law (empirical and restricted) Conductors, Insulators and Semiconductors
I-V curves ohmic (linear) nonohmic (non-linear) Interpreting Resistance Resistivity and Temperature r(T) = r0[1+a(T-T0)]
Electrical Shock “It’s not the voltage but the current.” The current is what actually causes a shock - human body has resistance of ~500,000 with dry skin - ~100 wet! Requires conducting path. Can cause: (1) burning of tissue by heating, (2) muscle contractions, (3) disruption of cardiac rhythms.
Charging on Astronaut Space Suit in Auroral Zone: Potentially hazardous situation • EVA Suit Specified to –40 V • anodized coating arcing occurred at –68V in MSFC test • Possible Sneak-Circuit • 1 mA safety threshold Display and Control Module (DCM) Metal waist and neck rings and other metal portions of the suit make contact with the sweat soaked ventilation garment providing possible conducting path for discharge through astronaut’s thoracic cavity. Safety Tether • Surface of spacesuit could charge to high voltage leading to subsequent discharge. • Discharge to the station through safety tether: • Tether is a metallic cable - connected to astronaut via non-conducting (nylon) housing. • Station maintained at plasma potential - arc path closed when tether gets wrapped around astronaut. Mini Work Station (MWS) Body Restraint Tether (BRT)
