Tutorial details

Students begin by examining how the orientation of a bar magnet near a current-carrying wire affects a current carrying wire. After making predictions for several different orientations, students perform the designated experiments and resolve any inconsistencies between their predictions and observations. They are led through the reasoning necessary to recognize how thevector equation F = il × B describes the magnetic force on the current-carrying wire. Studentswho have studied magnetic fields of current-carrying wires previously may have difficulty in recognizing that the field of the wire does not contribute to a force on the wire.

In section II, students use magnetic compasses to examine the magnetic field of a current- carrying wire. Some will have difficulty in inferring the direction of the field at a point from the orientation of the compass. They may think they need to locate the north and south poles of the wire in order to determine the field direction. Check their answers to the first question in this section, which tries to help them associate the compass direction with the direction of the field lines at that location.

In section III students extend their results to the case of current loops and solenoids. Exercises guide students to make an analogy between a current loop and a magnet and to predict the behavior of a current-carrying loop in the presence of a bar magnet. Finally students are led to recognize the similarities between the magnetic field of a long solenoid and that of a bar magnet.

As students work through the tutorial, it is important for instructors to check that all the students at a table (not just one or two) are able to apply the relevant right-hand rule. Variations of the questions in the tutorials can serve as a check.

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Prerequisite tutorials

The Magnets and magnetic fields tutorial is a prerequisite to Magnetic interactions.

Other prerequisites

The tutorial assumes that students have seen F = il × B in lecture or elsewhere. Students are expected to be familiar with the magnetic field of a bar magnet. The tutorial Magnets and magnetic fields covers this topic. No knowledge of the field of a current-carrying wire is assumed.

Equipment

List

• bar magnet
• handouts
• magnetic compass
• white board or large sheet of paper with markers
• wire loop

Discussion