Demonstrations

Internal Resistance, Maximum Power and Impedance Matching

Code Number :	5E40.70
Disclaimer:	Reprinted by permission of Dick Berg, University of Maryland, for use on this website. The demonstrations contained and referenced herein are listed for the purposes of cataloging and describing physics demonstrations which should be conducted only under the direction of a trained instructional support professional or physicist. These demonstrations are not presented for the purpose of being conducted by persons unconnected to this Facility and/or persons not consulting with or being supervised by the recognized instructional support professional or physicist and his/her staff. The University is responsible only for those demonstrations carried out using its own equipment using established safety and scheduling policies, and bears no responsibility for those choosing to use this source material for their own purposes. All demonstrations described and contained herein are public domain, and can also be found in reference materials in libraries, bookstores, and electronic sources. Further information regarding legal liability in use of demonstrations and labs will be found on the web site Injuries in School/College Laboratories in USA. The University of Iowa Disclaimers: University of Iowa Disclaimer All Rights Reserved..
Condition :	Excellent
Principle :	Internal resistance of a battery, peaked power of battery, impedance matching of a circuit and a battery
Area of Study :	Electricity & Magnetism
Equipment :	Three BK meters (Volts, Amps, and Ohms), One six volt battery, one 500 Ohm pot and one 10,000 Ohm pot.
Procedure :	Place the battery, 500 Ohm pot, amp meter and 10000 Ohm pot all in series with the 500 Ohm pot set at 500 Ohms exactly. Place the volt meter across the junctions of the 10000 Ohm pot. Adjust the resistance of the 10000 Ohm pot until the battery output reads 6 volts. Plot the current and the voltage. Adjust the 10000 Ohm pot, so that the voltage reads 5.5 Volts. Again plot the current and voltage. Continue to do this in half volt steps down to 0.5 Volts. Computing the total power from the values that have been plotted should give a peak operating voltage curve. Hint: The output power should be at it's peak when the resistance of the 10000 Ohm pot is set at the same resistance as the sum of the 500 Ohm pot plus the internal resistance of the battery. A volt and amp meter can be used in place of the voltage and current sensor with interface.

References

Adam Niculescu, Peter Martin, "Making and Evaluating an Electrical Battery", TPT, Vol. 44, # 6, p. 343, Sept. 2006.

James N. Boyd, "Power and Efficiency", TPT, Vol. 29, # 7, July 1991, p. 457.

Jack A. Kaeck, "Power Transfer in Physical Systems", TPT, Vol. 28, # 3, March, 1990, p. 214.

Mark A. Heald, "Maximum Power Transfer vs. Efficiency", TPT, Vol. 26, # 1, Jan. 1988, p. 10.

J. Gerard Anderson, "Load Matching Mistake", TPT, Vol. 25, # 9, Dec. 1987, p. 546.

L. V. Hmurcik and J. P. Micinilio, "Contrasts Between Maximum Power Transfer and Maximum Efficiency", TPT, Vol. 24, # 8, Nov. 1986, p. 492.

Iain MacInnes, "Notes on the Condition for Maximum Power Transfer to the Load", TPT, Vol. 23, # 4, Apr. 1985, p. 224.

Ellis D. Noll, "Determining the Internal Resistance of an Energy Source", TPT, Vol. 16, # 7, Oct. 1978, p. 478.

Mail Questions and Comments to: Dale Stille