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Name_____________________________________________________ Date_____________________________ Investigation 16 Magnets and Magnetic Fields I. Magnetic materials A. Two small magnets are included in the plastic bag. Choose one of the magnets to be your reference magnet and draw a small mark on it. You are now going to investigate how various objects that you have around your home, including the other magnet, interact with your reference magnet. Class 1 will be objects that are attracted to and repelled from your reference magnet. Class 2 will be objects that are only attracted to the reference magnet. Class 3 will be objects that are only repelled from the reference magnet. Class 4 will be objects that do not interact at all with the reference magnet. Use as many objects made from different materials as you can (e.g., the other magnet, metals: aluminum, copper, and steel, cork, plastic, wood, rubber, paper, etc.). 1. When performing this exercise, first: write the type of object, second: make a guess as to what will happen, and third: write what happened. object your guess what happened? 2. List the objects in each of your classes. Class 1 attracted and repelled 2. Class 2 attracted Class 3 repelled Class 4 no interaction Are all metals in the same class? Do they all interact with magnets? B. Suppose you have one permanent bar magnet and another bar-shaped object that is attracted to the magnet but not repelled. Imagine that you do not know which object is the magnet. Using only these two objects, find a way to determine which object is the permanent magnet. (Hint: Are there parts on either object that do not interact as strongly as other parts? C. The parts of a permanent magnet that interact most strongly with other materials are called the poles of the magnet. How many different types of poles do you have evidence for so far? What are their names? II. Magnetic fields We have observed that magnets interact even when they are not in direct contact. In electrostatics we used the idea of an electric field to account for the interaction between charges that were separated from one another. With magnetic interactions, we similarly define a magnetic field. 2 A. Compass 1. A compass is a small bar magnet that can rotate freely around a vertical axis. Being a bar magnet, it can interact with other magnets, and being small, it does not adversely affect the magnet with which it is interacting. 2. When you hold a compass in the palm of your hand, away from all other objects, and shake the compass, the compass needle wiggles back and forth and then finally comes to rest pointing in one particular direction. Is this the behavior it should have if it is in a magnetic field? What is the needle lining up with? 3. We define the north (seeking) pole of a magnet as the end that points toward the arctic region of the earth when the magnet is free to rotate and is not interacting with other nearby objects. On the basis of this definition and assuming that the earth is a magnet, is the geographic north pole of the earth the north-seeking pole or the south-seeking pole of a magnet? Explain. B. Magnetic field around a bar magnet. Draw the orientation of the compass when placed at points A, B, C, D, and E. The compass can be represented as an arrow with north being the point of the arrow. C A B D S N 3 E C. The magnetic field of the earth. The magnetic field surrounding the earth is produced by convection currents in the outer core of the earth in combination with the rotation of the earth. The shape of the field, however, is very much like that of a bar magnet, and so one can imagine a bar magnet in the earth producing the field. Below is a circle representing the earth with the geographic north and south poles as shown. A bar magnet placed in the outline drawn at the center of the earth produces a magnetic field that looks very similar to the field surrounding the earth. Draw the lines of force surrounding the bar magnet until you have a good feeling for the shape of the magnetic field. Indicate the position of the magnetic north and south poles of the earth and the magnetic north and south poles of the bar magnet in the earth. Also draw arrows to show the directions of the lines of force of the magnetic field. Geographic North Pole Geographic South Pole 4