Day 6: Map Field of Current
Student Pages

Activity 1

Blow up a small balloon. Rub it on your hair or otherwise give it a static electric charge.

Question: Is the static charge distributed evenly over the entire surface of the balloon?

Question: Can you preferentially charge one region of the balloon while leaving other surface regions relatively charge free?

Hold the balloon still close to the magnetometer. Note any deflection or other indication of the presence of a magnetic field. Repeat with different areas of the balloon closest to the magnet in the magnetometer.

Question: Is an electrically charged object also a magnetic object?

Activity 2

Charge the knotted end of the balloon and bring it near the magnetometer.

• Rapidly move the balloon away from and back toward the magnetometer several times. Note any deflections of the magnetometer.
• Rapidly move the balloon left and right while keeping it level with the magnet in the magnetometer. Note any deflections of the magnetometer.
• Rapidly move the balloon up and down along the line of the string the magnet in the magnetometer is hanging from. Note any deflections of the magnetometer.

Question: For those motions above that produced a deflection, draw a magnetic field map that accounts for the deflection. Use your knowledge of magnetic fields and deflections of the magnetometer to infer the direction of the magnetic field that caused the deflection.

Relate the motion of the charges stuck to the surface of the balloon to the deflection direction of the magnetometer. You need to do this for two different assumptions: the net charge on the balloon is positive and the net charge is negative.

What causes a magnetic field? Test your conclusion using an uncharged balloon. Note results here.

Activity 3

Your teacher has set up a simple direct current (DC) electric circuit. The wire is vertical.

Question: Discuss how a current in a wire is similar and different from the stationary charges on the moving balloon in Activity 1.

Turn on the circuit. Use the magnetometer to measure the direction of the magnetic field in the vicinity of the current carrying wire, I suggest you do this for several different radius circles about the wire. I also note that you can appeal to a symmetry of the set-up to simplify the observing task.

Question: In general, what is the shape of the magnetic field created by a current carrying wire?

Question: How is the field of a current carrying wire similar and different to the field due to a bar magnet? What simple geometric term describes the field direction? What is the direction of the field relative to the direction of the current in the wire, using the convention that the current always flows in the direction positive charges would move. Consider rotating your coordinates to help you give a more complete answer.

Activity 4

Make a circular loop of wire with the set-up. Orient it so that it hangs vertically. Using circles oriented perpendicular to the wire loop and of size greater than one radius of the loop, measure the magnetic field direction outside the loop. (see drawing) Again, position you magnetometer along points of the circle and measure the direction of the magnetic field at each point, producing a map.

Question: In general, what is the shape of the magnetic field created by a current carrying wire formed in a loop?

Question: How is the field of a current carrying wire loop similar and different to the field due to a bar magnet? What is the direction of the field relative to the direction of the current in the wire, using the convention that the current always flows in the direction positive charges would move. Consider rotating your coordinates to help you give a more complete answer.

Activity 5

Use the set-up from Activity 4 and from Activity 1, but connect the circuit without any power source. In other words, your circuit will consist of a wire loop and a voltmeter. Move a strong permanent magnet in the vicinity of the wire and observe any changes in the reading of the voltmeter. Attempt to discover any dependence on speed and direction of magnet motion

Question: Draw a map that shows how the magnetic field was changing over time in the vicinity of the wire.

Homework

Create a consistent explanation of what it takes to create a magnetic field, how a permanent magnet produces a magnetic field, and what it takes to detect a magnetic field, given your experience and observations to date.

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Lesson Development/Writing: Ed Eckel
Web Design: Theresa Valentine
Last Updated: 8/25/2000