Electromagnetism (page 2)
An electric current is the flow of electric charges through a conductor such as a metal wire. The motion of electric charges also produces a force called a magnetic force. The property of a space in which a magnetic force can be detected is called a magnetic field. A magnetic field is made up of imaginary lines called magnetic field lines that indicate the direction and magnitude of the field. The straight current carrying wire in this experiment produces a magnetic field as indicated by the movement of the compass needle. The direction of the magnetic field produced by the the current-carrying wire is the direction to which the north pole of the compass points when placed in the magnetic field. The compass needle is a magnet (a material with magnetic forces that attracts other magnetic material and is attracted to another magnet). Every magnet has two magnetic poles (the regions around a magnet where the magnetic forces appear strongest) called south and north poles. For a freely swinging straight magnet, such as the compass needle, its south pole (south-seeking pole) is attracted in the direction of geographical South. Its north pole (north-seeking pole) is attracted in the direction of geographical North. The direction of the current flow in the wire is from the battery's negative terminal to its positive terminal. With the current flowing in a south-to-north direction, the magnetic field lines below the wire are directed toward the east as indicated by the deflection of the north end of the compass needle toward the east. The relationship between a magnetic field and an electric current is called electromagnetism.
Try New Approaches
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
- A straight current-carrying wire is said to have magnetic field lines encircling it. Design a way to show that the direction of the magnetic field lines are 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 forming a stand for the compass by bending the ends of an index card to form a table shape.
- 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-inch (10-cm) diameter. Connect the ends of the wire to the terminals of a 1.5-volt battery. Observe the pattern of concentric circles (circles with a common center) around the wire.
- 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 50.2). Leave about 4inches (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 1.5V 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 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 electromagnetic. Reverse the direction of the battery and repeat the procedure.
- How does the number of wire coils in an electromagnet affect the strength of its magnetic field? Design an experiment to test the magnetic strength of an electromagnet. One way is to use the electromagnet from experiment 3 made of 3 feet (90 cm) of insulated wire. Assemble a circuit using the electromagnet, a 1.5-volt D battery in a battery holder, and a switch. Tape the electromagnet to the edge of a wooden table as shown in Figure 50.3. 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—6 feet (180 cm) of wire 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
- 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's The Handy Physics Answer Book (Detroit: Visible Ink, 1999), pp. 329–330.
- MAGLEV stands for "magnetically levitated." How are MAGLEV trains different from conventional trains? For information see Gundersen's The Handy Physics Answer Book, pp. 330–331.
- Left-hand rules are used to find the force on current or moving particles in a magnetic field. They are also used to find the 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 compare? For information see physics texts.
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