Motors, Generators, and Transformers Quiz

PDF page 679: As we learned previously, a current-carrying wire in a magnetic field experiences a force—recall . Electric motors, which convert electrical energy into mechanical energy, are the most common application of magnetic force on currentcarrying wires. Motors consist of loops of wire in a magnetic field. When current is passed through the loops, the magnetic field exerts a torque on the loops, which rotates a shaft. Electrical energy is converted to mechanical work in the process. Figure 20.23 shows a schematic drawing of an electric motor.

PDF page 686: a. An electric motor transforms electrical energy into mechanical energy.

PDF page 679: As we learned previously, a current-carrying wire in a magnetic field experiences a force—recall . Electric motors, which convert electrical energy into mechanical energy, are the most common application of magnetic force on currentcarrying wires. Motors consist of loops of wire in a magnetic field. When current is passed through the loops, the magnetic field exerts a torque on the loops, which rotates a shaft. Electrical energy is converted to mechanical work in the process. Figure 20.23 shows a schematic drawing of an electric motor.

PDF page 679: As we learned previously, a current-carrying wire in a magnetic field experiences a force—recall . Electric motors, which convert electrical energy into mechanical energy, are the most common application of magnetic force on currentcarrying wires. Motors consist of loops of wire in a magnetic field. When current is passed through the loops, the magnetic field exerts a torque on the loops, which rotates a shaft. Electrical energy is converted to mechanical work in the process. Figure 20.23 shows a schematic drawing of an electric motor.

PDF page 680: the top and bottom segments feel a force perpendicular to the wire, which does not cause a current. However, charges in the vertical wires experience forces parallel to the wire, causing a current to flow through the wire and through an external circuit if one is connected. A device such as this that converts mechanical energy into electrical energy is called a generator.

PDF page 680: From the equation we see that the torque is zero when As the wire loop rotates, the torque increases to a maximum positive torque of when The torque then decreases back to zero as the wire loop rotates to From to the torque is negative. Thus, the torque changes sign every half turn, so the wire loop will oscillate back and forth. For the coil to continue rotating in the same direction, the current is reversed as the coil passes through using automatic switches called brushes, as shown in Figure 20.25.

PDF page 680: the brushes reverse the current and the torque remains clockwise. (b) The coil rotates continuously in the clockwise direction, with the current reversing each half revolution to maintain the clockwise torque.

PDF page 680: the top and bottom segments feel a force perpendicular to the wire, which does not cause a current. However, charges in the vertical wires experience forces parallel to the wire, causing a current to flow through the wire and through an external circuit if one is connected. A device such as this that converts mechanical energy into electrical energy is called a generator.

PDF page 682: positive maximum of to a negative maximum of In between, the emf goes through zero, which means that zero current flows through the light bulb at these times. Thus, the light bulb actually flickers on and off at a frequency of 2f, because there are two zero crossings per period. Since alternating current such as this is used in homes around the world, why do we not notice the lights flickering on and off? In the United States, the frequency of alternating current is 60 Hz, so the lights flicker on and off at a frequency of 120 Hz. This is faster than the refresh rate of the human eye, so you don’t notice the flicker of the lights. Also, other factors prevent various different types of light bulbs from switching on and off so fast, so the light output is smoothed out a bit.

PDF page 683: construction to the apparatus Faraday used to demonstrate that magnetic fields can generate electric currents. The two wire coils are called the primary and secondary coils. In normal use, the input voltage is applied across the primary coil, and the secondary produces the transformed output voltage. Not only does the iron core trap the magnetic field created by the primary coil, but also its magnetization increases the field strength, which is analogous to how a dielectric increases the electric field strength in a capacitor. Since the input voltage is AC, a time-varying magnetic flux is sent through the secondary coil, inducing an AC output voltage.

PDF page 683: Figure 20.31 shows a laminated-coil transformer, which is based on Faraday’s law of induction and is very similar in

PDF page 685: This is known as the transformer equation. It simply states that the ratio of the secondary voltage to the primary voltage in a transformer equals the ratio of the number of loops in secondary coil to the number of loops in the primary coil.

PDF page 685: Transformers are widely used in the electric power industry to increase voltages—called step-up transformers—before longdistance transmission via high-voltage wires. They are also used to decrease voltages—called step-down transformers—to deliver power to homes and businesses. The overwhelming majority of electric power is generated by using magnetic induction, whereby a wire coil or copper disk is rotated in a magnetic field. The primary energy required to rotate the coils or disk can be provided by a variety of means. Hydroelectric power plants use the kinetic energy of water to drive electric generators. Coal or nuclear power plants create steam to drive steam turbines that turn the coils. Other sources of primary energy include wind, tides, or waves on water. Once power is generated, it must be transmitted to the consumer, which often means transmitting power over hundreds of kilometers. To do this, the voltage of the power plant is increased by a step-up transformer, that is stepped up, and the current decreases proportionally because 20.22 in the transmission wires reduces the Joule losses, which is heating of the wire due to a current The lower current flow. This heating is caused by the small, but nonzero, resistance of the transmission wires. The power lost to the environment through this heat is 20.23 must be as which is proportional to the current squared in the transmission wire. This is why the transmitted current small as possible and, consequently, the voltage must be large to transmit the power Voltages ranging from 120 to 700 kV are used for transmitting power over long distances. The voltage is stepped up at the exit of the power station by a step-up transformer, as shown in Figure 20.32.

PDF page 686: c. A step-up transformer increases voltage to transmit power over long distance with minimum loss.