Physics 201- Introduction to Electricity & Magnetism
October 2- Conductors in Electric Fields & more!

 

 

 

 
 

 

  What should you do if you find yourself in a severe thunderstorm while driving through the desert?  Where is the safest place?
 
  • Laying down flat on the ground
  • Standing next to a cactus
  • Standing on top of your car, holding a golf club raised to the sky
  • Sitting inside your car
Why?
 
 
 
 
 
 
  • On average, 73 people die in the US due to lightning strikes and many more are seriously injured each year.
  • Most of the injured and killed are golfers, farmers and fishermen.
  • The temperature of the ionized air: over 25,000 oC.  (This is several times greater than the temperature of the sun's surface.(
  • Typical cloud to ground voltage: over 106 volts. 
  • Peak power: 1011 to 1012 watts. 
  • Average duration: about 30 msec. 
  • Peak current: around 105 amps. 
  • Total energy: about 107 joules.

 
 
 
 
 

What happens to a conductor placed in an electric field?
 
 
 
 
 
 
 
 

Some logic with our past observations will tell us the answer:
 
 

  1. We know that in conductors, the electrons move freely and that like charges repel each other.  So, these electrons want to repel each other and get as far away from each other as possible which means moving to the surface.  (these is even before we add an external electric field.
  2. From this, and Gaussí law we now know that the electric field inside the conductor is zero.
  3. From this last statement and the definition of potential we now know that the potential inside a conductor is zero.  To move a charge around, with zero field, requires no work.
  4. This means that the surface of the conductor is at the same potential.
  5. From this, and what we know about equipotentials (recall this week's lab), we now know that the field, outside the conductor, is always perpendicular to the surface.

 
 
 
 
 

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Jeff Phillips
phillips@lmu.edu
Loyola Marymount University
Fall 2002