The homework assigned on Tuesday will not be due until Thursday, March 5.

1. The homework assigned on Tuesday will not be due until Thursday, March 5.

2. More charge here Forces Acting on Dielectrics We can either compute force directly (which is quite cumbersome), or use relationship between force and energy Considering parallel-plate capacitor Force acting on the capacitor, is pointed inside, hence, E-field work done is positive and U - decreases x – insertion length Two capacitors in parallel w – width of the plates constant force

3. Electric Current

4. 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

5. Motion in a uniform electric field Deflection by a uniform electric field

6. Application: Cathode Ray Tube

7. 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

8. 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

9. Current, Drift Velocity, Current Density Current density J, is a vector while total current I is not

10. 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 X 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 ionic solution of NaCl is due to both positive Na+ and negative Cl- charges flow

11. 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 ris the E-independent resistivity σ=1/ris the conductivity This is Ohm’s law (empirical and restricted) Conductors, Insulators and Semiconductors r(T) = r0[1+a(T-T0)]

12. Resistance of a straight wire V=IR

13. Water Flow Analogy

14. I-V curves ohmic (linear) nonohmic (non-linear) Interpreting Resistance Resistivity and Temperature r(T) = r0[1+a(T-T0)]

15. 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.

16. 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)

17. Radial current leakage in a coaxial cable