Return to Physics IndexElectron Current Flow

Greg Zipprich Bloom Trail High School

Cottage Grove & Sauk Trail

Chicago Heights, IL 60411

(708) 758-7000Objectives:

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 90^{o}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.