Recall that St. Elmo's fire has been observed on the wing tips of aircraft that are flying near large concentrations of charge within a thunderstorm. ¦The reason this phenomenon occurs on wing tips, as it does at the ends of other sharply pointed projections, has to do with the distribution of charge on the surface of the aircraft.

As an aircraft flies through thunderclouds, the metal surface of the aircraft can become charged.  If electrons are transferred to the aircraft, these excess negative charges will move as far apart as possible due to like charges repelling one another.  On a flat metallic surface, the negative charges spread out evenly so the resulting pattern of electric field due to these charges is also evenly distributed.

 

On a flat metallic surface, the negative charges spread out evenly so the resulting pattern of electric field due to these charges is also evenly distributed. Figure 2 a)

If the same surface is slightly bent, more charges can be added.  Can you think why this would be the case?  The reason has to do with the fact that charges on opposite sides of the curved surface are unable to repel one another as effectively as they can when the surface is completely flat.  On curved surfaces, the electrostatic force that repels each charge away from the others has a component that is no longer parallel to the surface.  This means that the parallel component of the force is less and therefore there is less force acting to separate each individual charge away from the others. The result is that the charges can be packed closer together which means the density of the field lines also increases. Figure 2 b)

To pack more charges on the surface, and get an even denser pattern of field lines, the same surface could be bent into a more pronounced curve.  As the curvature of a given surface increases, so does the capacity of the surface to concentrate charge and produce regions of intense electric fields. Figure 2 c)

 Figure 1 Distribution of charge on different shape conductors


a) Distribution of negative charge on flat surface.  

b) Distribution of negative charge on a curved surface.

c) Distribution of negative charge on a curved surface.


Read Conductors and Electric Field Lines
Learn more about the role of the electrostatic force in explaining the patterns illustrated in the previous diagrams by reading pages 555 to 558 of the text. Note the explanation of the electric field inside a hollow conductor as well as the electric field between two charged parallel plates.


Record the following diagrams in your notebook. Complete each diagram by sketching the electric field lines within each object.





Refer to the " info BIT" in the margin on page 558 of your textbook. Explain the role of the flexible metal shielding in the cable that is used to bring TV signals into a home.

The metal shielding prevents electrical and magnetic interference from influencing the TV signals that are transmitted through the cable. The flexible metal shielding effectively surrounds the conductor, encasing the TV signal with a hollow metal covering. Since electric fields within a hollow conductor are zero, there is no unwanted influence on the TV signals transmitted through the cable.



Electric fields are a key component of radio waves. Other than convertibles, most vehicles on the roadway can be thought of as metal boxes with holes cut in them for windows. Use this information to explain why the antenna for a vehicle's radio must be mounted outside of the vehicle or built into the windshield.

If electric fields are a key component of radio waves, then radio waves cannot penetrate the interior space of a vehicle because electric fields inside a hollow conductor are zero. By placing the antenna outside the vehicle or in the windshield, the radio waves do not have to penetrate the metal shell of the vehicle to be detected.