MnSTEP Teaching Activity Collection > MnSTEP Activities > Do all types of lights follow Ohm's Law?

# Do all types of lights follow Ohm's Law?

#### Summary

In this physics lab students will investigate whether Ohm's Law applies to common electric devices (incandescent light bulbs and LEDs). Students may submit their findings in a formal written report or an informal class discussion.

## Learning Goals

This activity encourages students to design a procedure for testing a hypothesis. They will have to collect and analyze data. Students may be asked to present their findings in a written or oral report. Students will "discover" that Ohm's Law does not apply to all electric devices under classroom conditions.

## Context for Use

This activity is designed for a senior high physics class. At least one class period will be needed for the activity with additional time (either in class or outside of class) for students to design their procedure and analyze data. Students should have a working knowledge of basic electricity concepts, Ohm's Law and know (or be able to figure out) what a current vs. voltage graph of a resistor that follows Ohm's Law looks like. Students also need to be familiar with construction of a circuit and a method to measure current and voltage in the circuit.

Materials:
-Small incandescent bulb

-Clear light emitting diode (LED)

-Alligator clip wires

-Switch

-Variable power supply (or batteries)

-Digital multimeter (or calculator or computer based meters)

-Graph paper (or graphing programcomputer based meters)

-Graph paper or graphing program

Subject: Physics:Electricity & Magnetism
Resource Type: Activities:Lab Activity

## Description and Teaching Materials

Introduce the activity with the question in number 1 below. Have the materials (listed above) out on the lab tables and turn the students loose.

1. Do small incandescent lights and/or LEDs follow Ohm's Law? Design a controlled experiment to determine if the lights you were given follow Ohm's Law. Write your procedure in such a way that others could easily follow it.
2. Collect enough data to support your conclusion and display it in a data table.

Finish the activity off with the students' written or oral reports. You could extend the activity by using different colors of LEDs and noting the voltage necessary to produce light.

## Teaching Notes and Tips

If students have little experience constructing circuits and using a multimeter, you will want to set up a sample circuit for demonstration purposes. If data is collected using a 10-100 ohm resister and a 9 volt battery a wonderful linear graph of current vs voltage will be produced. Neither the incandescent bulb nor the LED will follow Ohm's Law, neither produces a linear graph. Teachers are strongly encouraged to set up and try this activity ahead of time. Equipment from school to school varies. If it's nonexistent, Radio Shack is a good source of equipment. If time is a problem, you may want to have each lab group investigate a different light. In this case you may want to include holiday lights of various types.

## Assessment

Assessment of this activity can be in either written or oral format. Students can present their findings formally or informally. You may wish to ask questions similar to the following:
1. Based on your data, does the incandescent bulb obey Ohm's law? Based on your data, does the LED obey Ohm's Law?

2. In each case, support your answer. Include qualitative or quantitative data as requested by your teacher.

3. How can a graph of current vs voltage be used to determine if a resister follows Ohm's Law?

4. Is your graph for the incandescent light similar to the graph for the LED? Explain.

5. Site one possible source of error in your procedure and describe its possible influence on your results.

## Standards

This activity addresses the grades 9-12 History and Nature of Science, Part B Scientific Inquiry standard. (The student will design and conduct a scientific investigation.)