"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"
Albert von Szent-Gyorgyi

• The concept of the electric field (E) is useful in determining the effect of a system of charges on a "test" charge brought into the vicinity.

 
• If a "test" charge, q, feels a force F, then the electric field, E, at the location of the "test" charge is given by







• Units of electric field are  Newtons/Coulomb (N/C) or (as we shall see later) Volts/metre (V/m).

 
• If the electric field felt by the "test" charge is due to a point charge Q, located a distance R from q, then its magnitude is given by








• The electric field due to many point charges is given by the vector sum of the fields due to the individual charges



E  =  E₁  + E ₂ +  E ₃ +  .....




In performing this sum the direction of each E  is the same as the force felt by a positive test charge.
 

• The electric field in a region of space can be represented by electric field lines otherwise known as "lines of force".








The direction of the electric field line is the same as that of the force felt by a positive charge.  The density of the field lines provides a measure of the magnitude of the field.  FIeld lines alway begin on positive charges and end on negative charges; they cannot be left 'hanging' in empty space.




The diagram above displays the electric field lines in the vicinity of two equal, but opposite point charges.






• The concept of field lines was invented by Michael Faraday.   They are a convenient construct to describe electromagnetic phenomena, but their true existence is neither proven nor essential.






• There can be no electric field inside a closed conducting object.  Any charge on such an object must reside on the surface.

  Sensitive electrical components can be protected from stray electromagnetic fields by enclosure in a Faraday cage - a closed metal box.

  Lightning creates a very large electric field.  A degree of safety from lightning strikes can be achieved by remaining inside a car.  This is explained, not by the presence of (insulating) rubber tyres, nor completely by the extent to which the vehicle approximates to a closed metal box.  The Faraday cage effect applies for static electric fields - a lightning strike is certainly not static.  The real reason is known as the skin effect .


Since radio waves consist of oscillating electric and magnetic fields, the reason why it is impossible to obtain good reception inside a car without an external antenna is also the skin effect .
 

   Supplementary Problems 

"Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."
Albert Einstein


 

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu