Electron Current Flow

Greg Zipprich                  Bloom Trail High School
                               Cottage Grove & Sauk Trail
                               Chicago Heights, IL 60411
                               (708) 758-7000

Objectives:

     The student will understand how and why electric current passes through a 
conductor due to a potential difference. 
     The student will discover the proportional relationship between voltage and 
resistance and their effect on the measurement of current flow. 
     The student will discover that, with a constant voltage, the smaller the 
diameter of a conductor, the smaller the current flow. 

Materials needed:

     Five-gallon bucket with globe valve attached to the bottom outside rim, 3/4 
x 18 CPVC pipe w/screw adapter, 1/2 x 18 CPVC pipe w/3/4 screw adapter, 1/4 x 
18 potable water line epoxied to a 3/4 screw adapter, U-tube constructed from 2 
12-oz plastic pop bottles glued into 2 3/4 CPVC 90o ells connected with an 18" 
length of 3/4 CPVC pipe, a 10-foot ladder, enough 3/4 CPVC pipe and couplings to 
attach the bucket suspended on the ladder to pipe lengths on the tabletop, a 
stop watch and a 3-liter pop bottle (graduated). 

Strategy:

     Working on the principle that water and electricity flow with similar 
characteristics, a discussion of electric current flow is conducted making 
analogies to the flow of water.  In the U-tube, using water dyed blue with food 
coloring, the fact is explained that water does not move unless additional water 
is poured into one side causing a difference in potential.  This causes movement 
in the water until potential equilibrium is reached. 
     Explaining that an excess of electrons at one end of a conductor causes an 
electrical potential difference, electrons will similarly flow until electrical 
equilibrium is reached.  How much flows (introduce the term, current) depends on 
the potential difference or pressure (introduce the term, voltage) and the 
opposition to flow (introduce the term, resistance).
     Using the bucket of water at tabletop height with the three different sizes 
of pipe connected to 3/4 screw adapters, measure the volume of water which flows 
through each pipe in ten seconds using the graduated 3-liter pop bottle.  One 
student uses the stop watch, another measures the volume and a third keeps a 
chart of the results on the chalk board. 
     It can be seen now that if voltage is constant, a smaller pipe (conductor),
carries a smaller current.  Now the formula, E = I x R, is placed on the chalk 
board.  The students discuss the relationships of the values and are asked to 
derive the equation, I = E / R.  We have seen that if resistance is high (the 
smaller conductor), current is low and that, conversely, small resistance (the 
larger conductor), transports a larger current.  At this time the quantities are 
introduced for measuring volts, ohms and amperes. 
     The students are asked, "Looking at the formula how else can we increase 
the current besides decreasing the resistance?"  A student will answer, "By 
increasing the voltage."  At this time the bucket can be placed on the shelf of 
the 10-foot ladder.  The extra 3/4 CPVC pipe and connectors are used to extend 
the water supply to the tabletop where the pipes were before.  Again, three 
students, a timer, a measurer and a recorder, chart the volume of water from 
each pipe in ten seconds.  More water will come out across the chart because the 
pressure of the water is increased.  Its potential difference is increased.  The 
students, therefore, find that increasing the voltage also increases the 
current. 
      If time permits, actual values for voltage and resistance can be supplied
and the value for current can be calculated mathematically or this can be begun 
on the following class meeting.