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# Forces and Movement Produced by Magnetic Fields (page 2)

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Author: Janice VanCleave

### Why?

Materials with a force called magnetic force—force produced by the motion of electric charges—are called magnets, and the area around a magnet where its magnetic force can be detected is called a magnetic field. All phenomena associated with magnets are called magnetism. Magnet A moves along the line due to a force between it and the magnets taped to the craft stick. Every magnet has two separated magnetic poles (region of a magnet where the magnetic forces appear strongest), called south and north poles. Near Earth's North Pole (the northernmost point on Earth) is a place called Earth's magnetic north pole. This is the place where the north pole of a free-swinging magnet such as the compass needle points. The south pole of the compass needle points to Earth's magnetic south pole, near Earth's South Pole (the southernmost point on Earth).

Unlike poles of two magnets attract, and like poles repel each other. All the magnets in the experiment are positioned so that like poles face each other. The magnets attached to the craft stick act as driving forces, causing magnet A to retreat. The movement of magnet A is along the center line, which bisects the angle formed by the lines from the magnets attached to the craft stick. The driving force is the result of the magnetic repulsive forces f1 and f2, from the two magnets acting at angles on magnet A The stop-and-go movement of magnet A is due to the friction between the magnet and the paper. The driving force has to be strong enough to overcome the friction so that the magnet can move. The driving force weakens as the distances d1 and d2 increase until the magnet stops. Then, as the distances decrease, the driving force increases, and there is again enough force on magnet A to move it.

### Try New Approaches

1. What effect, if any, does the angle of repulsive forces f1 and f2 have on the direction of the retreating magnet? Repeat the experiment twice. First make the angles smaller, by making the distance between the center lines and the two lines on either side less than 2 inches (5 cm). Then make the angles greater, by making the distance greater than 2 inches (5 cm).
2. What effect would a difference in strength of the driving force have on the results? Repeat the original experiment, increasing the strength of one of the magnets by placing a third magnet on the craft stick. Place the magnet on top of the stick above one of the lower magnets, with unlike faces of the magnet toward each other. In this position the stacked magnets will attract each other, thus holding the magnet on this stick.

### Design Your Own Experiment

1. Every magnet has an area of force around it that can affect other magnets. This area is called its magnetic field. Design an experiment to compare the strengths of magnetic fields. One way is to place a compass on a wooden table. When the compass needle comes to rest in line with Earth's magnetic field, slowly rotate the compass so that the north end of the needle points to N printed on the compass. The needle aligns itself so that it points to the north magnetic pole of Earth. Place a ruler next to the compass and perpendicular to the compass needle. Use one of the marked magnets from the original experiment. Lay the magnet with side N up at the end of the ruler so that it is 12 inches (30 cm) from the compass. Slowly move-the magnet toward the compass, stopping when the compass needle is deflected (turned aside) 90° from N. Note the distance the magnet is from the compass. Repeat, using a magnet with a greater magnetic field strength, such as a stack of three disk magnets from the original experiment, with sides N up.
2. Design an experiment to determine how the distance from a magnet affects the strength of its magnetic field. One method is to draw two perpendicular lines across the center of a sheet of copy paper. Label the lines N, Sand E, W, as shown in Figure 19.2. Use a ruler to mark 12 centimeters along line W, starting with zero at the compass. Place a compass where the lines cross. Rotate the compass so that the N on the compass lines up with line N. Place one of the labeled disk magnets (A) at the far end of line N with side S up. Move the magnet toward the compass until the north end of the compass needle points toward the magnet and is in line with line N. Place a second labeled disk magnet (B), of equal strength, with side S up at the far end of line W. Move magnet B 1 cm at a time toward the compass. Record any deflection of the compass needle from north in a Magnetic Field Data table like Table 19.1. Use the relationship between distance and deflection angle in this experiment to determine the relationship between magnetic field strength and distance.
3. For more information about the relationship of distance and the strength of a magnetic field, see Robert Ehrlich, Why Toast Lands Jelly-Side Down (Princeton, N.J.: Princeton University Press, 1997), pp. 150–151.

4. Design a way to show that magnetic force fields of different magnets overlap. One way is to place four equal-size disk magnets facedown next to a metric ruler, with their north poles facing up. Use tape to secure the magnet nearest the zero end of the ruler. (See Figure 19.3.) Push the magnets as close together as possible. Compare the distance between the magnets, and explain how overlapping magnetic fields cause the differences between distances d1, d2 and d3, and why f1 is greater than any of the other forces, f2 and f3, between magnets.

### Get the Facts

1. A line of flux is a line drawn so that a tangent to it at any point indicates the direction of a magnetic field. What is magnetic flux density? For information, see Corinne Stockley, The Usborne Illustrated Dictionary of Physics (London: Usborne, 2000), p. 72.
2. Ferromagnetism is the property of a substance by which it is strongly attracted by a magnet. How does electron motion produce ferromagnetism? What is a domain? For information, see a physics text.
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