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Instructor Outline:
UM Physics Demo Lab 07/2013
Electrical Resistance and Ohm’s Law
Lab length: 70 minutes
Lab objective: Instruct the students about equivalent resistance for series and
parallel combinations of resistors, Ohmic, and non-Ohmic resistance.
Materials
1
1
1
1
green multimeter (with leads)
battery board
alligator lead card
long resistor 25 Ω, 50W
1 carbon pencil
1 calculator
1 clear plastic ruler
Exploration stage: 30 minutes - Group Lab Work
The students measure the resistance of the long resistors with the multimeter. They
then measure voltage and current to find that the resistance is the slope of the V
versus I graph. Next, they determine the resistance of a light bulb using the slope in
the V-I diagram and compare it to the resistance of a cold light bulb measured with
the multimeter.
Analysis stage: 5 minutes – Lecture
The instructor analyzes with the class the findings from the exploration, and answers
questions formed during that stage. Ohmic and non-ohmic resistors are discussed.
Application stage: 25 minutes – Group Lab Work
The students build resistors by shading in regions with a carbon pencil (1 worksheet
per group is sufficient). They see how, length, width, series and parallel paths
contribute to total equivalent resistance.
Summary: 10 minutes – Discussion
The formulas for calculating series and parallel equivalent resistance are introduced
and superconductivity is demonstrated via the Meissner effect.
Concepts Developed:
1. A resistor for which the potential required to drive a current is proportional
to the current obeys Ohm’s Law: V = I R.
2. The resistance of an “Ohmic” resistor (one that obeys Ohm’s Law) is the
constant of proportionality between current and potential: R = V/I.
3. The units of resistance are volts per ampere (V/A) which are denoted as
Ohms (Ω) in honor of the physicist Ohm who made early studies of electrical
resistance.
4. The graph of potential versus current for a resistor which obeys Ohm’s Law
is a straight line and the slope of the line is the resistance of the resistor
(change in potential divided by change in current).
5. Ohm’s “Law” is not a law at all, it’s actually a definition. Many devices do not
exhibit a simple proportionality between current and voltage. Semiconductor
devices such as diodes and transistors are useful precisely because they are
nonlinear and do not obey Ohm’s “Law”. If Ohm’s Law were truly a law for
solid matter, we would still be using vacuum tubes to build electronics—
transistors would not be possible!
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
6. The equivalent resistance increases as resistors are added in series and is
obtained by adding the individual resistances: Req = R1 + R2 + R3 + …
7. The equivalent resistance decreases as resistors are added in parallel and
is calculated as: 1/ Req = 1/R1 +1/R2 + 1/R3 + …
8. At low temperatures some materials lose all electrical resistance and become
perfect conductors called superconductors. Superconductors can be used to
build very powerful electromagnets and to levitate objects magnetically by
exploiting the Meissner effect whereby a superconductor expels all
magnetic fields from its interior so that a magnet will sit suspended above the
surface of the superconductor supported by magnetic forces.
If
superconductivity can be achieved at room temperature, magnetic levitation of
trains will become truly practical as well as loss-free transmission of electrical
power over wires. To date the highest temperature superconductors operate
near the temperature of liquid nitrogen (77 degrees Kelvin, equivalent to 77
Celsius degrees above absolute zero).
Suggested Demos:
5G50.50-1 – Meissner Effect
5D10.10 - Assorted Resistors
5D10.u1 - Resistance Board
4A50.20 – Temperature Dependence of Resistance
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109