Exercise 4: Electric and magnetic fields

Transcription

1 Astronomy 102 Name: Exercise 4: Electric and magnetic fields Learning outcome: Ultimately, to understand how a changing electric field induces a magnetic field, and how a changing magnetic field induces an electric field, and how both are aspects of electromagnetic radiation. Electromagnetic radiation, as we ve seen in previous exercises, is pervasive. Yet until the middle of the 19 th century, physicists widely believed that EM waves could exist in a vacuum. Further, they did not connect EM waves to light, even though some of them suspected that there would be a connection. The magnetic field of a coil of current-carrying wire Equipment needed: a coil of wire, a power supply, the Labquest data loggers with magnetic field strength detectors, a set of small compasses In 1819, Hans Oersted did a demonstration for his graduate students, trying to show that a current in a wire generated a magnetic field; to detect the magnetic field, he held a compass near the wire. At first, his demonstration failed, but then he had the insight to change the orientation of his compass, and the needle turned in response to the current; when he reversed the current flow direction, the needle of the compass turned the other way. Ampère s Law (Maxwell Equation 4) states that an electric current generates a magnetic field, and this exercise will illustrate that point. First, set up the coil and power supply in the middle of the room; if possible, keep the coil upright, but you can lay the coil flat. Don t connect the power supply yet. Array the compasses such that they are scattered inside and outside the coil at differing distances from the coil.. 1. On the next page, sketch the floor plan of the room, indicating where the coil is, and how it is oriented. The instructor will show you which way north is in the room, though the compasses should make that pretty obvious! Turn on the power supply. 2. Show, on your sketch, the orientation of the needles of each compass. Try to make the orientations as accurate as possible, so that you can tell if the needle orientation is only a little affected, or if it is affected a lot. Turn off the power supply. 3. Calibrate all of the Labquest dataloggers and the magnetic field strength detectors by making sure they get the same reading for the terrestrial magnetic

2 field strength. Place a detector near every spot where there is a compass. Turn on the power supply, and note, on the sketch, the value of the magnetic field strength. Do the magnetic field strengths correspond well with the amount of deflection on the compass needles? N 4. As best as you can, draw the magnetic field lines on your sketch. What pattern does this resemble (think back to Exercise 1)?

3 The ring launcher Equipment needed: ring launcher, a set of different mass and material rings, a field extender, and an electronic balance In 1865, James Clerk Maxwell published an article titled A Dynamical Theory of the Electromagnetic Field in the Philosophical Transactions of the Royal Society of London. In the article, he described a set of equations that unified the until-then separate forces of electricity and magnetism as one force called electromagnetism. Eventually, his equations were distilled into the four Maxwell s Equations of Electromagnetism. Because the phenomena were discovered long before Maxwell s time, the individual equations are known by other scientists names. In particular, Faraday s Law (Maxwell Equation 3) suggests that a changing magnetic field induces an electric field. If there is a conductive material, like a wire, in the field, an electric current will be set up in the material. A good illustration of these principles is found in the ring launcher. The launcher is simply a coil of wire attached to an electrical cord when plugged into the wall socket, the alternating current (AC) of a standard building power supply will generate an alternating direction electric current in the coil, which in turn will generate an alternating direction magnetic field within the coil and an alternating direction electric current and magnetic field in any ring of material placed around the coil. The two magnetic fields (inside the coil and inside the ring of material) will repel and move the ring up. Actually, the generation of force is still not that well explained; check out a recent attempt at explanation by a physics professor: This doesn t mean that we can t at least discover some empirical relationships about the force that propels. 5. Place the coil attached to the bulb circuit on the ring launcher and briefly turn on the launcher. What happened to the bulb? Explain this phenomenon, using the word induction or induced. 6. Now carefully launch (or attempt to launch) various rings of different materials and sizes. Record the information in the table below; with the height the ring goes up, you can be approximate.

4 Ring material Ring height (cm) Ring mass (g) Maximum movement height (cm) The rings are all machined to be the same thickness and diameter, so when a ring is double the mass of another ring of the same material, it is twice the height of the other ring. 7. The conductivity of a ring depends on the area though which the current will move for the rings, the area is the inside of the ring. So if one ring is twice the mass of another ring of the same material, what can be said about the ring s area (compared to the other ring s area)? 8. So what is the mathematical relationship between ring height and ring conductivity, roughly?

5 9. Test this theory: copper has about twice the conductivity of aluminum. Look at the data for the aluminum and copper rings of the same height. Which would you predict would have a greater maximum movement height, and why? Was your prediction true? How confident are you about the ring height/conductivity connection in part b? 10. How did the split ring do? Explain the result in terms of Maxwell s Equations. 11. Place the iron core snugly in the middle of the coil, then launch a ring. Record the maximum movement height in centimeters and compare this number to the movement height of the same ring without the iron core. Why does the iron core improve the maximum movement height?