The ComPADRE Collections

Think-Pair-Share Analysis of the Operation of a Metal Detector

John Stewart
University of Arkansas
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


Illustration of EM field interacting with a metal object

In a physics lecture, the class is presented with a simplified model of a metal detector. The class is asked to discuss the operation in groups of two or three students. Once the groups come to a conclusion, the class is presented with a list of possible features of the explanation and asked to indicate the options their explanation supports. Two additional questions to clear up misconceptions are presented. The class is asked to correct their explanation after these questions. Once a consensus is reached a simulation demonstrates the correct explanation of the function of a metal detector.

This activity allows the students to use their knowledge of magnetism and Faraday's law to understand an interesting real-world device. The students work cooperatively and the device's operation is ultimately demonstrated by a colorful simulation.

Learning Goals

A group of students who successfully complete the activity have taken their theoretical understanding of magnetism and applied it to a real device. They have demonstrated a mastery of magnetism and Faraday's law and applied that mastery to a real device. Multiple principles are involved in the operation of the device, so the synthesis of multiple principles is required. The activity reinforces two key electromagnetic principles: (1) a changing flux is required for an emf and (2) eddy currents are a different phenomena from a ferromagnetic response.

Context for Use

Educational level: Introductory College
Setting: Lecture
Time required: 20-50 minutes
Special equipment: Computer and Projector
Pre-requisite knowledge: Magnetism, Faraday's Law, and magnetic materials.

Description and Teaching Materials

The file contains a complete description of the activity and a link to the simulation site.
Metal Detector Principles (Microsoft Word 236kB Jul29 07)

Teaching Notes and Tips

The metal detector system is related to another system in which many students are interested, the electric guitar pickup. An electric guitar would make a very good follow-up demonstration or a possible homework question.


Most of the class should have reached the correct answer by the end of the activity.

References and Resources

Simulation Source: Metal Detector Simulation. Accessed from Florida State University through ComPADRE on July 11, 2007.