Magnetic Fields from Electric Currents
One of the classes of problems dealing with magnetic fields concerns the production of a magnetic field by a current-carrying conductor or by moving charges. It was Oersted who discovered back in the early 1800's that currents produce magnetic effects. The quantitative relationship between the magnetic field strength and the current was later embodied in Ampere's Law, an extension of which made by Maxwell is one of the four basic equations of electromagnetism.
PocketLab and Phyphox software, used in conjunction with a long, straight current-carrying wire, offers a great opportunity for students to quantitatively study the relationship between magnetic field strength B and the distance r from the wire's center. Students will be able to confirm that B is inversely proportional to the distance r. An alternative version of the experiment using only PocketLab, and not Phyphox, can be found by clicking here.
Figure 1 shows the experimental setup for this experiment. A wire is connected to a power supply and is carrying a constant current i of about 8 amps. The right-hand rule applied to the wire tells us that the magnetic field B at locations to the right of the wire is perpendicular to the table top and pointing upward. PocketLab Voyager is placed so that its positive y-axis is in the direction of the magnetic field. A small piece of cardboard is placed alongside the wire so that Voyager's rangefinder can measure distance from the wire. Voyager is initially touching the cardboard and moved away from it to a distance of between 10 and 15 cm. Phyphox displays a real-time graph of magnetic field strength versus distance from the wire. A balsa wood stick perpendicular to the white cardboard has been taped to the tabletop. This serves as a guide when moving Voyager away from the wire. The Samsung tablet shows what the graph may look like after data has been collected.
Phyphox (physical phone experiments) is a free app developed at the 2nd Institute of Physics of the RWTH Aachen University in Germany. The author of this lesson has been working with a pre-release Android version of this app that supports BLE (Bluetooth Low Energy) technology to transfer data from multiple Voyagers to the Phyphox app. Since then, a public Android version of the Phyphox app has been released.
The experiment of this lesson is in a file named CurrentWire.phyphox and accompanies this lesson. This file can then be opened in Phyphox and will appear in the PocketLab Voyager category of the main screen, similar to that in Figure 2.
An accompanying pdf file, Lesson Details.pdf, explains how to work with the Phyphox software and suggests a method for analysis of the data.