DynamoA dynamo converts mechanical energy into electrical energy by taking advantage of a phenomenon known as electromagnetic induction. It is an electric generator that is similar to those that provide electricity to our homes. Electromagnetic Induction In the early 1800s, physicists discovered an important link between electricity and magnetism [1]. It was found that when a conducting wire is placed inside of a changing magnetic field, an electric voltage is induced in the wire. This phenomenon, known as electromagnetic induction, is demonstrated in Figure 4.
The long wire in Figure 4 is wrapped around a cardboard cylinder to form a coil. A voltmeter, a device that measures electric voltage, is connected to the ends of the coil. When the magnet is moved into and out of the coil, the magnetic field around the coil is changed. The voltmeter's needle fluctuates in response, indicating that a voltage is induced within the coil (click on Figure 4's Play button to see this in action!). The faster the magnet moves, the greater the induced voltage. Interestingly, when the magnet stops moving, the voltmeter's needle instantly returns to zero (the middle position), showing that a changing magnetic field is required to generate electricity. A dynamo uses this principle to generate electricity. A simple dynamo
Figure 5 shows the structure of a simple dynamo. A coil made of conducting wire is positioned between the North and South poles (shown in red and blue, respectively) of two permanent magnets. When the coil is stationary, no voltage is induced. When the coil is rotated, however, it cuts through the magnetic field, and the magnetic field around the coil is changed. The changing magnetic field induces a voltage within the coil. The amount of voltage that is induced depends on the strength of the magnetic field, the number of loops in the coil, and the speed at which the coil rotates [2]. The induced voltage takes a sinusoidal shape, as Figure 6a shows below. It also changes polarity (that is, switches from being positive to negative or vice-versa) for every half-rotation of the coil. This type of voltage is called an alternating voltage [2]. You may have noticed a practical problem with the spinning coil. If the two ends of the coil were connected to Bigshot's electronic circuit directly, the entire circuit board would have to spin with the coil to keep the ends from twisting around each other and breaking. Fortunately, the problem can be solved by placing a commutator [3] between the coil and the circuit. The commutator is made of two half-cylinders of smooth conducting material, which are separated by an insulating material, as shown Figure 5. Each half-cylinder of the commutator is permanently attached to one end of the rotating coil, and the commutator rotates with the coil. The two brushes, usually made of carbon, are stationary and press against the commutator as it rotates. The brushes are connected to the external device (for example, Bigshot's circuit board) and act as the terminals of the dynamo.
The commutator also keeps the dynamo's generated voltage from alternating between positive and negative. Each brush slides along the two halves of the commutator, switching halves the instant the voltage in the coil reverses polarity. This ensures that voltage produced by the dynamo is no longer alternating (like Figure 6a) but rather has the form shown in Figure 6b. In other words, the commutator converts alternating voltage into direct voltage, a process called rectification [4].
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