Electromagnetism: Magnetism from Electricity (page 2)

Try New Approaches

1. What effect would moving the battery to the opposite (west) side of the compass have? With the negative terminal of the battery pointing south as before, repeat step 8 of the original experiment.
2. What effect would changing the direction of the current through the wire have on the deflection of the compass needle? Rotate the battery 180° so that the terminals of the battery have been reversed.

Design Your Own Experiment

1. A current-carrying straight wire is said to have a magnetic field encircling it. Design a way to show that the direction of the magnetic field is in a circle around a current-carrying straight wire. One way is to compare the direction of the magnetic field above and below the wire in the original experiment. Design a way to raise the compass and place the wire below it, such as by forming a stand for the compass by bending the ends of an index card.
2. A magnetic field is made up of imaginary lines called magnetic field lines that indicate the direction and magnitude of the field. Design a way to show the pattern of the magnetic field lines around a magnet. One way is to place a piece of insulated wire through a piece of cardboard, such as the top of a small cardboard box. Sprinkle a thin layer of iron filings on the cardboard around the wire in a circle with about a 4-in (10-cm) diameter. Connect the ends of the wire to the terminals of a l.5-volt battery. Observe the pattern of concentric circles (circles with a common center) around the wire. Repeat the procedure after rotating the battery 180° so the direction of the current is reversed.
3. An electromagnet is a device that uses electric current to produce a concentrated magnetic field. An electromagnet is made of a solenoid (coil of wire through which a current can pass) with a core of magnetic material such as iron. The current-carrying wire in a solenoid produces a magnetic field, which magnetizes (causes a substance to become a magnet) the iron core. Design an experiment to determine the polarity (the direction of the magnetic poles) of an electromagnet. One way is to wrap a 3-foot (90-cm) piece of 22-gauge insulated wire around a 16d finishing nail (also called a 16-penny nail) (see Figure 20.2). Leave about 4 inches (10 cm) of free wire at each end. Use a wire cutter to strip about 1 inch (2.5 cm) of insulation from the ends of the wire. Allow the compass to align with Earth's magnetic north. With a 1.5-volt battery in a battery holder, twist together the bare end of one solenoid wire and the bare end of one battery-holder wire. Hold the electromagnet so that the pointed end of the nail is near but not touching the west side of the compass. While in this position, touch the free solenoid wire and the free battery wire together for about 1 second. Note the direction in which the north end of the compass needle moves. If the end of the nail pointing toward the compass attracts the north end of the compass needle, the end is the south pole of the electromagnet. If the north end of the needle is repelled, the nail's end is the north pole of the electromagnet. Reverse the direction of the battery and repeat the procedure.



4. How does the number of wire coils in an electromagnet affect the strength of the magnetic field? Design an experiment to test the magnetic strength of an electromagnet. One way is to use the electromagnet from the previous experiment made of 3 feet (90 cm) of insulated wire. Assemble a circuit using the electromagnet, a l.5-volt D battery in a battery holder, and a switch. Tape the electromagnet to the edge of a wooden table as shown in Figure 20.3. Use metal paper clips to test the strength of the electromagnet. Bend one paper clip to form a hook that other paper clips can be hung on. Close the switch and touch the paper clip hook to the pointed end of the nail. Add paper clips to the hook one at a time until the weight of the clips causes the hook to pull away from the nail. Then repeat the experiment using twice as much wire—6 feet (180 cm)—to make the electromagnet. If all the coils will not fit on the nail, wind them as tightly as possible, then wind the next layer over the top, still turning in the same direction. CAUTION: If you feel any warmth through the insulated area of the nail, open the switch. Do not touch the bare nail or bare ends of the wire, because electric current flowing through the wire can cause these areas to get hot enough to burn your skin.

Get the Facts

1. Television images are the result of thousands of electrons hitting the television screen. What effect do electromagnets play in the direction in which the electrons move? For information, see P. Erik Gundersen, The Handy Physics Answer Book (Detroit: Visible Ink, 1999), pp. 329–330.
2. MAGLEV stands for "magnetically levitated." How are MAGLEV trains different from conventional trains? For information, see P. Erik Gundersen, The Handy Physics Answer Book (Detroit: Visible Ink, 1999),pp.330–331.
3. Left-hand rules are used to find force on current or moving particles in a magnetic field. They are also used to find direction of a magnetic field caused by current in straight wires as well as in solenoids. What are the left-hand rules? How do left-hand rules and right-hand rules compare? For information, see physics texts.

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