Electrical Shock: Understanding the Dangers and Effects of Current

1. There will be a quiz next Tuesday, March 10. Homework from last week is due Thursday. A new assignment will be given then, due the Tuesday after spring break.

2. 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.

3. 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)

4. Radial current leakage in a coaxial cable

5. Microscopic model for drift velocity and conduction Consider electrons as classical particles – no quantum mechanical properties for now Simplest model – each atom gives one electron to the “pool” of conductive electrons

6. Temperature dependence of resistivity Conductors – quantum mechanics says that at T=0, atoms do not vibrate – no collisions at all (electrons scatter elastically). At T>0 – atoms vibrate, collisions intensify Superconductors – there are certain quantum states where there are only elastic collisions – no energy is transferred to the ions in the crystal Semiconductor have very different electric properties. As T increases, concentration of Free electrons goes up dramatically, decreasing resistivity Most importantly – current strength is not linearly proportional to voltage (diode) Avalanche – uncontrollable stream of electrons, gaining energy as they move through the material.

7. Electromotive Force and Circuits For a conductor to have a steady current, it must be a closed loop path If charge goes around a complete circuit and returns to a starting point – potential energy does not change As charges move through the circuit they loose their potential energy due to resistance

8. “Electromotive force” (emf, ε) is produced by a battery or a generator and acts as a “charge pump”. It moves charges uphill and is equal to the potential difference across such a device under open-circuit conditions (no current). In reality, batteries have some internal resistance. Emf is measured in Volts (so it is not a “force” per say, but potential difference) Sources of emf – batteries, electric generators, solar cells, fuel cells

9. In ideal situation, As the charge flows through the circuit, the potential rise as it passes through the ideal source is equal to potential drop via the resistance, Evolution of the electric potential in the circuit with a load Internal Resistance

10. We measure voltages with voltmeters We measure currents with ammeters An ideal voltmeter would have an infinite resistance An ideal ammeter would have a zero resistance Example: What are voltmeter and ammeter readings?

11. Examples Bulb B is taken away, will the bulb A glow differently? Which bulb glows brighter? Which bulb glows brighter?

12. Potential changes around the circuit Potential gain in the battery Potential drop at all resistances In an old, “used-up” battery emf is nearly the same, but internal resistance increases enormously

13. Electrical energy and power Chemical energy → Electric potential energy → Kinetic energy of charge carriers → Dissipation/Joule heat (heating the resistor through collisions with its atoms) As the charge goes through the resistance the potential energy qV is expended (if both q and V are positive), but charge does not acquire kinetic energy (current is constant). Instead, it converted to heat. The opposite can also happen – if change in potential energy is positive, the charge acquires it - battery

14. Power Output of a Source

15. Power Input to a Source Current flows “backwards” Rate of conversion of electric energy into non-electrical energy Work is being done on, rather than by the top battery (source of non-electrostatic force)