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Faraday’s "Ice Pail" Experiment
Before describing this historically important experiment, it is worth looking at the operation of one of the early instruments used to detect (and approximately measure) electric charges, the "gold leaf" electroscope.  
     
     
     
In a "modern" (using the term loosely!) version of the instrument, the (expensive and delicate) gold leaf is replaced by a (cheaper) balanced strip of aluminium.    
     
     
     
     
     
     
     
     
If a charged object is held near the electroscope, as shown here, the electroscope itself becomes charged by induction (by the redistribution of free electrons already in the metal parts of the instrument).    
     
The angle through which the aluminium strip rotates, indicates, approximately, the magnitude of the charge.    
     
     
     
   
If an negatively charged object is held near the electroscope, exactly the same thing happens to the aluminium strip but this time because the electrons have moved away from instead of towards the charged object.    
   
In other words, an electroscope cannot normally distinguish between negatively and positively charged objects.    
   
   
   
The electroscope can be made to distinguish between charges by charging the instrument itself, usually by induction.    
     
     
     
Assume that we have charged the electroscope positively, as shown here.    
     
     
     
     
     
     
     
     
If a positively charged object is brought near the electroscope, the charge redistribution is so as to increase the divergence of the aluminium strip.    
     
     
     
     
     
     
When a negatively charged object is brought near the electroscope, the charge redistribution is so as to decrease the divergence of the aluminium strip.    
     
Thus, a charged electroscope can distinguish between negatively and positively charged objects.    
     
     
     
The "Ice Pail" Experiment      
For this experiment, Faraday needed a metal container and chose an "ice pail", hence the title.    
An uncharged metal container is connected to an electroscope and a charged metal sphere is lowered into the container (first two diagrams below).    
The sphere is not allowed to touch the container.    
     
If the sphere is removed from the container at this stage, it is found to be still charged and the electroscope "leaf" goes back to the vertical position (first diagram).     
     
However, Faraday then allowed the sphere to make contact with the inside of the container before being removed (next two diagrams).     
     
     
At the instant the ball touched the inside of the container, it was noticed the position of the "leaf" of the electroscope did not change    
The charge on the sphere had neutralized the induced charge on the inside of the sphere.     
When the sphere was removed from the container the leaf of the electroscope still did not change its position.    
The sphere (tested with another electroscope) was found to have lost all its charge    
     
Faraday concluded that:     
1. an induced charge has the same magnitude as the inducing charge.     
2. the charge on a conductor remains on the outside surface; there can be no net charge inside a hollow conductor (either there are equal quantities of opposite charge or there in no charge).     
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