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

Forces Acting on Dielectrics More charge here 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

Electric Current

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

Application: Cathode Ray Tube

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 ~ 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

Electric current in ionic solution of NaCl is due to both positive Na + and negative Cl - charges flow 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 el/m 3. 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 ~ m/s in copper)

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/   is the E-independent resistivity σ=1/  is the conductivity This is Ohm’s law (empirical and restricted) Ohm’s Law Conductors, Insulators and Semiconductors  (T) =  0 [1+  (T-T 0 )]

Resistance of a straight wire V=IR

Water Flow Analogy

Interpreting Resistance ohmic (linear) nonohmic (non-linear) I-V curves Resistivity and Temperature  (T) =  0 [1+  (T-T 0 )]

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. Current (A)Effect 0.001Can be felt 0.005Is painful 0.010Causes spasms 0.015Causes loss of muscle control 0.070Goes through the heart - fatal after more than 1 second

–EVA Suit Specified to –40 V anodized coating arcing occurred at –68V in MSFC test –Possible Sneak-Circuit 1 mA safety threshold Safety Tether Display and Control Module (DCM) Body Restraint Tether (BRT) Mini Work Station (MWS)  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. 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. Charging on Astronaut Space Suit in Auroral Zone: Potentially hazardous situation

Radial current leakage in a coaxial cable