Part II.  Properties of an Electrical Circuit


This next demonstration shows how flow in electrical systems
is similar to groundwater systems.  We will be using the analogy
of land surface elevations and contours, which most of us have
seen on topographic maps, to help understand the same concepts of
energy potential and energy flow in groundwater and electrical
systems.  Surface water, and ground or subsurface water, flows
from areas of high potential energy to low potential energy.  For
surface water, this is a formal way of saying that water flows
downhill.  The Earth's surface can be described by topographic
maps, which have lines that connect areas of equal elevation (or
potential energy levels).  Electrical flow maps follow the same
principles, as surface maps with the areas of equal voltage
connected by lines are called "equipotential" lines.

For this part you can use an empty plastic tub from the lab
kit, or something smaller such as a glass baking dish (just make
sure it has a level bottom so the water depth will be the same
throughout, and it is not made of metal).  Fill the tub with
water to a depth of about 2 cm (little less than an inch). 
Because we are using small voltages, we will enhance the
sensitivity of our model by using a salt water solution to
improve the conductivity of water.  Add about 1/4 cup of salt to
a 2 liter bottle of water.  The exact amounts are not important.

Figure 1a below will help guide your model set up.  Place
masking tape along the edges of the plastic tub, and use your
meter stick to mark off 5-cm lengths on the tape (start in the
lower left corner as the 0,0 mark).  Take your bare metal pieces,
and bend them so they can sit in the tub bottom with their base
in the water and the top sticking up in the air.  Place them at
opposite ends of the tub, and connect the "+" post of the battery
to one and the "-" post to the other.  This creates an electrical
circuit.

Connect the common (black) lead of the voltage meter to the
"-" post of the battery.  Turn on the meter to the "20" setting. 
With the red lead probe, check the battery voltage by touching to
the "+" post.  If everything is working you should get the same
voltage reading as in Part I (you might get a slightly smaller
voltage after the circuit is connected).

Measure the voltage where the "+" lead of the battery connects
to the metal wire in the tub.  Still the same voltage?  Measure
the voltage at the "-" lead, and it should read zero.  So what
you have is a change in voltage potential, from high to low,
across the water between your two wires.  In elevation analogy,
this would be from the top of a mountain to its base.


1.  Now, put your probe in the water about an inch towards the
other wire (point "x" in Figure 1a).  What do you measure?



Move the probe in a line between the two wires until you read 5
volts.  Mark this position on Graph 1.  As shown in Figure 1b,
continue to the 4 volt mark, 3, 2, and 1, each time marking the
locations on your Graph 1.

	What you see is the change in electrical potential as you move
across the conducting material (water).  We will use the
convention that electrical flow is from positive to negative. 
Draw a "flow line" from the "+" wire to the "-", to represent
the flow of electricity.

2.  Is this the only pathway for electricity to flow?



	Let's find out.  Put your probe back to the 5 volt mark
between the two wires.  Now move the probe around the "+" lead
(keeping the tip in the water), and try to keep the meter reading
steady at 5 volts. 

Mark these positions and sketch the 5 volt equipotential line
on Graph 1.  The lines end at the container boundaries. 

	Using your meter measurements as a guide, sketch in the
approximate equipotential lines for the 4, 3, 2, and 1 volt
levels (these should look somewhat like the example in Figure
1c). 

3.  Since you measure voltage throughout the basin system, what
does this suggest to you about electrical flow - is there more
than one pathway?






	Now we're going to draw some more flow lines to represent some
of these alternate pathways.  Think about standing on a steep
hillslope, and pouring a bucket of water on the ground and
watching it travel to a hole at the bottom. 

4.  Will all the water follow a single line or path?  Will every
pathway reach the hole at the same time?






That's an easy one, but thinking of this analogy is very
useful when trying to describe electrical and groundwater flow. 
Electricity, like water, will move from high potential (high
elevations) to low:  The flow will travel perpendicular to the
equipotential lines.  Where the equipotential lines are straight
this is simple, but when the lines curve you have to make the
flow lines curve also (see Figure 1c).

5.  Draw two other flow paths between the two metal wires on your
graph.

	There are an (almost) infinite number of flow paths in this
system, and the same concept holds for groundwater flow - there
are nearly infinite pathways for water to travel between two
points - some ways are more direct than others, and groundwater
will travel fastest along the most efficient pathway between the
two points.