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17.4 Electricity, Magnetism, and Motion Energy and Motion – You know that magnetic force can produce motion, and you know that electric current in a wire produces a magnetic field. So, a magnet can move a wire with a current, just as it would move another magnet. Energy Transformation – Energy can be transformed from one form to another. When a wire with a current is placed in a magnetic field, electrical energy is transformed into mechanical energy. The transformation happens when the magnetic field produced by the current causes the wire to move. How does a Galvanometer work? You learned that a straight wire with a current moves when it is placed in a magnetic field. But what happens when you place a loop of wire with a current in a magnetic field? The current in one side of the loop flows in the opposite direction than the current in the other side of the loop. So, the sides of the loop move in opposite directions. Once each side has moved as far up or down as it can go, it will stop moving. As a result, the loop can rotate only a halfturn. Inside a Galvanometer – The rotation of a wire loop in a magnetic field is the basis of a galvanometer. A galvanometer is a device that measures small currents. An electric current turns the pointer of a galvanometer. In a galvanometer, an electromagnet is suspended between opposite poles of two permanent magnets. The electromagnet’s coil is attached to a pointer. When a current is in the electromagnet’s coil, it produces a magnetic field. This field interacts with the permanent magnet’s field, causing the coil and the pointer to rotate. You can use a galvanometer to measure an unknown current, so electricians use them. Some cars use them as fuel gauges. They are also used in lie detectors to measure how much current a person’s skin conducts. People who are stressed sweat more. Water conducts electricity. Therefore, moist skin conducts more electric current. What does an Electric Motor Do? An electric motor is a device that uses an electric current to turn an axle. An electric motor transforms electrical energy into mechanical energy. A simple electric motor has four parts: The commutator consists of two semicircular pieces of metal. It conducts the current from the brushes to the commutator. Brushes conduct current to the rest of the commutator. They do not move. The armature is a loop of wire that current flows through (coil) Permanent magnets produce a magnetic field which causes the armature to spin If current only flowed in one direction, the armature could only rotate half a turn. However, the brushes and commutator enable the current in the armature to change direction. Current always flows from the positive to the negative terminal of a battery. The current in the armature is reversed each time the commutator moves to a different brush. This causes the armature to rotate continuously. 17.5 Electricity from Magnetism Electric motors use electrical energy to produce motion. Can motion produce electrical energy? In 1831, scientists discovered that moving a wire in a magnetic field can cause an electric current. This current allows electrical energy to be supplied to homes, schools, and businesses all over the world. A magnet can make, or induce, current in a conductor, such as a wire, as long as there is motion. An electric current is induced in a conductor when the conductor moves through a magnetic field. Generating electric current from the motion of a conductor through a magnetic field is called electromagnetic induction. Current that is generated in this way is called induced current. Induction of Electric Current – Michael Faraday and Joseph Henry each found that motion in a magnetic field will induce a current. Either the wire can move through the magnetic field, or the magnet can move. The direction of an induced current depends on the direction that the coil or magnet moves. When the motion is reversed, the direction of the current also reverses. Alternating and Direct Current (AC/DC) – A current with charges that flow in one direction is called direct current (DC). A battery produces direct current when it is placed in a circuit and charges flow in one direction. If a wire in a magnetic field changes direction repeatedly, the induced current also keeps changing direction. A constantly reversing current is called alternating current (AC). Alternating current has a major advantage over direct current. An AC voltage can be easily raised or lowered. This means that a high voltage can be used to send electrical energy over great distances. Then the voltage can be reduced to a safer level for everyday use. The electric current in homes, school, and other buildings is alternating current. How does a Generator work? An electric generator is a device that transforms mechanical energy into electrical energy. A generator uses motion in a magnetic field to produce current. The electric company uses giant generators to produce most of the electrical energy you use each day. Huge turbines turn the armatures of the generators. Turbines are circular devices with many blades. They spin when water, steam, or hot air flows through them. What does a Transformer do? The electrical energy generated by electric companies is transmitted over long distances at very high voltages. Transformers change the voltage in your home so you can use the electricity. A transformer is a device that increases or decreases voltage. A transformer consists of two separate coils of insulated wire wrapped around an iron core. The primary coil is connected to a circuit with a voltage source and alternating current. The secondary coil is connected to a separate circuit that does not contain a voltage source. The changing current in the primary coil produces a changing magnetic field. This changing magnetic field induces a current in the secondary coil. The change in voltage from the primary coil to the secondary coil depends on the number of loops in each coil.