Instructor: B. Clark – J. Bihari
Phys 670
Student:
Maximiliano Montenegro
Lesson Plan
Characteristics
Title:
Ohm’s Law
Length of lesson:
Two to three class periods
Grade Level:
10th – 12th grade
Subject area:
Physical Science
Instruction Method:
Inquiry-based hands-on activity
Rationale
The Ohm’s Law is the simplest and most recurrent expression in electricity. Discovered
by G. S Ohm at the beginning of XIX century, it wasn’t until the discovered of the electron and
the introduction of the Drude’s model for conductivity in 1900 that it was theoretical explained.
Most of the physical processes involved are hidden on its simplicity and traditionally, teachers
and students regard the Ohm’s law as a formula, useful for mathematical practice. Moreover,
any lab’s activity based on circuits to illustrate this law relay on instruments and circuits
construction challenging the normal students’ abilities and obscuring them.
This lesson plan proposes the construction of a physical model of a conductor, based on
the classical Drude’s model. On it, the electrons moving along the conductor will be
represented by marbles, and the conductor itself will be a cardboard rail. Voltage and current
can be easily identified with the rail’s inclination and the marbles’ speed respectively. This
concrete representation of these concepts will help the student grabbing the fundamental and
abstract concepts present in introductory electricity.
Objectives
1. The students will build a physical model of a conductor
2. The students will able to identify each element of the model with those present
in a conductor.
3. The students will list the variables that affect the falling time of the marbles in
the conductor’s model.
4. The students will obtain an approximate quantitative relation between the
current, voltage and resistance in a conductor.
5. The students will be able to explain the effect of resistance in a conductor.
Materials
Each group needs have access to:
10 marbles
wood’s sticks
cardboard
ruler
Chronometer
Calculator
Lesson Plan: Ohm’s law
measuring tape
2
chronometer
Procedure
1. The instructor will introduce to the class the Ohm’s law, focused on the concepts related
with voltage, current and resistance.
2. The instructor will show to the class a working model of the conductor, making emphasis
between the different components and their equivalents in a real conductor.
3. Each group will construct their own model. The length of each ramp can be different as
well the width. Maybe it is a good idea that each group have their own dimensions and
then to try to make an inference about the shape dependence on conductivity.
4. Only the trial where all the marbles reach the end of the ramp must be count. Any
unsuccessful trial must be discarded.
5. The distance between stick must be greater than the diameter of each marbles. Use
same kind and size marbles.
6. Greater the height of the ramp, less accurate will be the model: keep the inclination less
than 5 degree.
7. Any cardboard box can be use to make the ramp.
8. A discussion about how model have been useful to describe real life phenomena in
physics could be interesting
Discussed Questions
1. Does the falling time depend on the number of sticks?
2. Is the falling time the same for different number of marbles?
3. Does the falling time increases when the high is increased?
4. Why is the current defined as the ratio between the number of marbles and the falling
time?
5. Does it works for greater highs?
6. Can you say that the model is accurate? Why?
7. Do you imagine another way to obtain the relation?
8. How can this model be extended to include different cross sections or lengths?
Extensions
·
Using a CBL set for motion and forces can help in the acquisition of data.
·
Another variation, where you do not take in account the force, is that instead to be
pulled by a bucket, the girl can be pulled by another boy on rollers. The distance
that each one travels is inverse proportional to their respective masses.
·
The procedure can be modified to obtain a Voltage v/s Current dependence.
Lesson Plan: Ohm’s law
·
3
Several rails with different lengths and widths can introduce to dependence on
shape of the conductor’s resistance.
Evaluation
Working in the same previous groups, the evaluation will be based in a portfolio where it will
be included a detailed explanation of the designed device, diagrams or pictures of the
activity, the variables measures and kept constants, the data collected, a graph Current v/s
Resistance. It will be take in account accuracy, concinnity and creativity
Suggested readings
A Drude resistance simulation.
Dresser, Miles J.
The Physics Teacher (Stony Brook, N.Y.). v. 23 (Mar. 1985) p.151-4
A working model of Ohm's law.
Rhodes, Scott F.
The Physics Teacher (Stony Brook, N.Y.). v. 32 (May 1994) p. 319.
Abstract: High school and physical science students often find it difficult to grasp the
relationships between current, voltage, and resistance that are defined by Ohm's law. The
fact that the students cannot visualize these abstract concepts appears to contribute to the
confusion. A model that is based on water flowing through a pipe under different conditions
can be used to simulate different currents, resistances, and voltages. The construction of
such a model is described.
When is Ohm's law valid?
Waltner, Steven A., and Lehman, Thomas A.
The Physics Teacher (Stony Brook, N.Y.). v. 31 (Feb. 1993) p. 102-3.
Web links
Internet guide to electronics
Introductory explanation with examples of Ohm’s law.
http://webhome.idirect.com/~jadams/electronics/ohm.htm
Molecular Expression
Website devote to the introduction of Electricity and Magnetism
http://micro.magnet.fsu.edu/electromag/java/ohmslaw/
Lesson Plan: Ohm’s law
4
Welcome to the Voltage Circuit Simulator
Java applet illustrating the Ohm’s law.
http://jersey.uoregon.edu/vlab/Voltage/
OHM'S LAW
http://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.intro.html
OHM'S LAW
Website with a nice chart containing all the relation between voltage, current, resistance and
power.
http://www.the12volt.com/ohm/ohmslaw.asp
Instructor: B. Clark – J. Bihari
Phys 670
Student:
Maximiliano Montenegro
Worksheet
Rock and roll!
To understand the dependence of the current on the resistance of a conductor, in this activity,
each group will build a physical model of a conductor. The materials that you need are:
5-10 marbles
wood’s sticks
cardboard
ruler
chronometer
calculator
Procedure
1. Cut in the cardboard the following figures:
B
Holes
Holes
C
A
2. Make the holes on the part A and then
fold (or glue) the sides to obtain the
following shape:
3. Fold and glue parts B and C to obtain
the following:
4. Cut the wooden sticks to 3cm of length
and verify that they fit the holes that
Lesson Plan: Ohm’s law
you made on part A:
6
8. Add successively a row of sticks and
repeat the measurement again. When
you don’t have any row left, complete
the following table:
# marbles
Height
Voltage
5. In this model, the cardboard trail
I
the conductor;
the marbles
V represents
represent electrons and, the sticks
represent the conductor resistance. A
greater number of ticks imply a more
R for the marbles to find their way
difficult
along the ramp and therefore resulting
in a greater conductor’s resistance.
The current will be the number of
marbles divide by the falling time
# marbles
I=
Falling time
and the voltage will be the height
divide by the number of marbles:
heigth
V=
# marbles
# sticks
Falling Time
[s]
Current
[marbles/s]
9. Make a graph of Current v/s # sticks.
10. Discuss and try to obtain a qualitative
statement that explains the behavior
that you see. How can it be translated
to a current, voltage and resistance
statement?
6. Tilt the ramp 2 degree, raising one
end. Leave only 1 row of sticks and
then measure the total falling time for
all the marbles. Repeat 5 times and
make and average (if one marble in
any trial gets stuck, discard that trial).
7. Add another row of sticks and repeat
the previous measurement. Always
only count the trials where all the
marbles arrived to the end point.
11. Can you design a procedure to show
the dependence of the Voltage on the
Current for a fixed resistance, using
the above model?