Ohm's Law Lab

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

Property of LS&A Physics Department Demonstration Lab

1 greenmultimeter (with leads)

1 battery board

1 alligator lead card

1 long resistor 25 Ω, 50W

1 carbon pencil

1 calculator

1 clear plastic ruler

Property of LS&A Physics Department Demonstration Lab

Exploration stage: 30minutes - 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-ohmicresistors 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:

A resistor for which the potential required to drive a current is proportional to the current obeys Ohm’s Law:V = I R.
The resistance of an “Ohmic” resistor (one that obeys Ohm’s Law) is the constant of proportionality between current and potential: R = V/I.
The units of resistance arevolts per ampere (V/A) which are denoted as Ohms (Ω) in honor of the physicist Ohm who made early studies of electrical resistance.
The graph of potentialversus 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).
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!
The equivalent resistance increases as resistors are added in series and is obtained by addingthe individual resistances: Req = R1 + R2 + R3 + …
Theequivalent resistancedecreases as resistors are added in parallel and is calculated as: 1/Req = 1/R1 +1/R2 + 1/R3 + …
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

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