The Play Doh™ Resistor

By Thomas O'Neill
o'neill@csvrgs.k12.va.us
C. Shenandoah Valley R. Governor's School

Objectives:

1. The student will demonstrate knowledge of the relationship between length of a wire and resistance of a wire by experimentally determining that relationship using an Volt-Ohm meter.
2. The student will demonstrate knowledge of the relationship between cross-sectional area of a wire and resistance of a wire by experimentally determining that relationship using an Volt-Ohm meter.

Virginia Standards of Learning: (revised 1988)

Third grade: 3.3, 3.4, 3.5
Fourth grade: 4.4, 4.7
Sixth grade: 6.2
Eighth grade: 8.1, 8.4, 8.9, 8.11,
Twelfth grade: 12.1, 12.3, 12.11, 12.16

Scope, Sequence and Coordination of Secondary School Science
from The Content Core , Vol. 1 (1993). The National Science Teachers Association. Washington, DC.

Grades 6-8
Electricity and magnetism -Electric circuits (pg. 102 and pg. 108-9) Grades 9-10
Electricity and magnetism -Moving charge
(pg. 101 and pg. 115)
Grades 11-12
Electricity and magnets - Electric circuits (pg. 102 and pg. 123)

Project 2061
from Benchmarks for Science Literacy (1993) American Association for the Advancement of Science. Washington, DC.
The Nature of Science: The Scientific World View (1A pp. 5-7)

Kindergarten through Grade 2
Grades 3 through 5

The Nature of Science: Scientific Inquiry (1B pp. 10-13)
Kindergarten through Grade 2

Grades 3 through 5
Grades 6 through 8
Grades 9 through 12

The Physical Setting: Forces of Nature (4G pp. 94-97)
Kindergarten through Grade 2

Grades 3 through 5
Grades 6 through 8
Grades 9 through 12

Equipment and materials:

• Play Doh™ modeling compound from Playskool, Inc., Pawtucket, RI 02682 (available at most toy and department stores)
• Volt Ohm Meter (VOM) alternatively use a known resistance, a galvanometer, a wire with meter stick and a Wheatstone bridge set-up alternatively use voltmeter/ammeter method of determining resistance.

References:

1. Carpenter, Rae and Minnix, Richard. The Dick and Rae Physics Demo Notebook. Dick and Rae, Inc. 1993 pg. E-380
2. Jones, Brian. ‘Resistance Measurements in Play-Doh™’. The Physics Teacher 31:1 (Jan. 1993) pp. 48-49

Construction of equipment:

The Play Doh™ should be fresh and not dried out. Shape the material into a cylinder. Set up the voltmeter and ammeter as diagramed below: The power should be supplied by a 0-15 V laboratory DC source. The voltmeter should be connected across the ends of the Play-Doh™ and not the leads of the power supply as chemical reactions at the interface of electrode and Play-Doh™ will rapidly change the character of those ends.

Alternatively, with a good VOM it is possible to measure the resistance directly although the numbers will vary considerably and will not be consistent with the Volt/Ammeter method. Small fissures in the Play-Doh™ will have large effects on the internal resistance of the cylinder. It is usually best to plot only the results from one run (constantly increasing the length) as the resistance of the cylinder seems to depend on the presence or absence of cracks in the Play Doh™ as it is rolled out.

Suggested activities:

1) Measure the resistance of the Play Doh® for various lengths using the same amount of compound each time.

The general equation for the resistance of a wire is

where R is the resistance, r is the resistivity and is characteristic of a given material (usually in Ohm -cm), length is the length of the wire in centimeters, and area is the cross sectional area of the wire in square centimeters. Since the volume of the cylinder in question is constant and

Volume = Area *(length) then substituting in the equation above we get

where both r and V are constants. This means that a plot of resistance versus the length2 should be a straight line. The following is a typical data plot:

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This page was posted on 2/26/99