Consult the TEXTS FOR SUPPLEMENTARY READING and complete the information about the generation of electricity (Text 38, 39, 40, 41). Be ready to discuss the information you have read.
35. Fill in the gaps with the prepositions/conjunctions from the box:
| in of (8) to (2) for or (2) over in by (2) if |
Measuring Electricity
Electricity is measured ___ units ___ power called watts. It was named ___ honor James Watt, the inventor ___ the steam engine. One watt is a very small amount ___ power. It would require nearly 750 watts to equal one horsepower. A kilowatt represents 1,000 watts. A kilowatthour (kWh) is equal ___ the energy ___ 1,000 watts working ___ one hour.
The amount ___ electricity a power plant generates ___ a customer uses ___ a period ___ time is measured ___ kilowatthours (kWh). Kilowatthours are determined ___ multiplying the number ___ kW's required ___ the number of hours of use. For example, ___ you use a 40-watt light bulb 5 hours a day, you have used 200 watts ___ power, ___ 0.2 kilowatthours of electrical energy.
36. Read the text below to find answers to the given questions:
Text 10 D
Electric Motors
1. How does an electric motor work?
An electric motor uses the attractive and repulsive forces between magnetic poles to twist a rotating object (the rotor) around in a circle. Both the rotor and the stationary structure (the stator) are magnetic and their magnetic poles are initially arranged so that the rotor must turn in a particular direction in order to bring its north poles closer to the stator’s south poles and vice versa.
The rotor thus experiences a twist (what physicists call a torque) and it undergoes an angular acceleration — it begins to rotate. But the magnets of the rotor and stator are not all permanent magnets. At least some of the magnets are electromagnets. In a typical motor, these electromagnets are designed so that their poles change just as the rotor’s north poles have reached the stator’s south poles. After the poles change, the rotor finds itself having to continue turning in order to bring its north poles closer to the stator’s south poles and it continues to experience a twist in the same direction.
2. How does electric current create magnetic poles in metal? When the current goes through the metal, what makes it positive and negative?
An electric current is itself magnetic — it creates a structure in the space around it that exerts forces on any magnetic poles in that space. The magnetic field around a single straight wire forms loops around the wire — the current’s magnetic field would push a magnetic pole near it around in a circle about the wire. But if you wrap the wire up into a coil, the magnetic field takes on a more familiar shape.
The current-carrying coil effectively develops a north pole at one end of the coil and a south pole at the other. Which end is north depends on the direction of current flow around the loop. If current flows around the loop in the direction of the fingers of your right hand, then your thumb points to the north pole that develops at one end of the coil.
3. In a three-phase induction motor, there is a rotating magnetic field in the stator, which induces a rotating magnetic field in the rotor. Those two magnetic fields will interact together to make the rotor turn. Is the interaction attractive or repulsive?
The magnetic interaction between the stator and the rotor is repulsive — the rotor is pushed around in a circle by the stator’s magnetic field; it is not pulled. To see why this is so, imagine unwrapping the curved motor so that instead of having a magnetic field that circles around a circular metal rotor you have a magnet (or magnetic field) that moves along a flat metal plate. As you move this magnet across the plate, it will induce electric currents in that plate and the plate will develop magnetic poles that are reversed from those of the moving magnet — the two will repel one another. That choice of pole orientation is the only one consistent with energy conservation and is recognized formally in «Lenz’s Law».
For reasons having to do with resistive energy loss and heating, the repulsive forces in front of and behind the moving magnet don’t cancel perfectly, leading to a magnetic drag force between the moving magnet and the stationary plate. This drag force tends to push the plate along with the moving magnet. In the induction motor, that same magnetic drag force tends to push the rotor around with the rotating magnetic field of the stator. In all of these cases, the forces involved are repulsive — pushes not pulls.
4. How does a fan motor work?
A fan motor is an induction motor, with an aluminum rotor that spins inside a framework of stationary electromagnets. Aluminum is not a magnetic metal and it only becomes magnetic when an electric current flows through it. In the fan, currents are induced in the aluminum rotor by the action of the electromagnets. Each of these electromagnets carries an alternating current that it receives from the power line and its magnetic poles fluctuate back and forth as the direction of current through it fluctuates back and forth.
These electromagnets are arranged and operated so that their magnetic poles seem to rotate around the aluminum rotor. These moving/changing magnetic poles induce currents in the aluminum rotor, making that rotor magnetic, and the rotor is dragged along with the rotating magnetic poles around it. After a few moments of starting, the spinning rotor almost keeps up with the rotating magnetic poles. The different speed settings of the fan correspond to different arrangements of the electromagnets, making the poles rotate around the aluminum rotor at different rates.
5. How does an electromagnetic doorbell work?
When you press the button of an electromagnetic doorbell, you complete a circuit that includes a source of electric power (typically a low voltage transformer) and a hollow coil of wire. Once the circuit is complete, current begins to flow through it and the coil of wire becomes magnetic. Extending outward from one end of the coil of wire is an iron rod. When this coil of wire — also called a solenoid — becomes magnetic, so does the iron rod. The iron rod becomes magnetic in such a way that it’s attracted toward and into the solenoid, and it accelerates toward the solenoid. The attractive force diminishes once the rod is all the way inside the solenoid, but the rod then has momentum and it keeps on going out the other side of the solenoid. It travels so far out of the solenoid that it strikes a bell on the far side — the doorbell!
The rod rebounds from the bell and reverses is motion. It has traveled so far out the other side of the solenoid that it's attracted back in the opposite direction. The rod overshoots the solenoid again and, in some doorbells, strikes a second bell having a somewhat different pitch from the first bell. After this back and forth motion, the rod usually settles down in the middle of the solenoid and doesn’t move again until you stop pushing the button. Once you release the button, the current in the circuit vanishes and the solenoid and the rod stop being magnetic. A weak spring then pulls the rod back to its original position at one end of the solenoid.
6. How do electric/magnetic linear drives work?
Linear electric motors are very much like rotary electric motors — they use the forces between magnetic poles to push one object relative to another. But while a rotary motor uses these forces to twist a rotor around in a circle, a linear motor uses these forces to push a carriage along a track.
Both the carriage and the track must contain magnets and at least some of these magnets must be electromagnets that can be turned on and off, or reversed. By timing the operations of the electromagnets properly, the linear motor pushes or pulls the carriage along the track smoothly and continuously.
7. What is the difference between a single-phase electric motor and a three phase motor?
To keep the center component or «rotor» of an electric motor spinning, the magnetic poles of the electromagnets surrounding the rotor must rotate around it. That way, the rotor will be perpetually chasing the rotating magnetic poles. With single-phase electric power, producing that rotating magnetic environment isn’t easy. Many single-phase motors use capacitors to provide time-delayed electric power to some of their electromagnets. These electromagnets then produce magnetic poles that turn on and off at times that are delayed relative to the poles of the other electromagnets.
The result is magnetic poles that seem to rotate around the rotor and that start it turning. While the capacitor is often unnecessary once the rotor has reached its normal operating speed, the starting process is clearly rather complicated in a single phase motor. In a three phase motor, the complicated time structure of the currents flowing through the three power wires makes it easy to produce the required rotating magnetic environment. With the electromagnets surrounding the rotor powered by three-phase electricity, the motor turns easily and without any starting capacitor. In general, three phase motors start more easily and are somewhat more energy efficient during operation than single phase motors.
8. Does the monorail at Disneyland and the metro in D.C. run on the idea of direct current motors?
Those trains probably run on AC motors, because then they can use transformers to transfer power between circuits. Most likely, these trains use induction motors. To reverse the direction of the train, the engineer reverses the direction in which magnetic poles in the motors’ stators circle the motors’ rotors. When the poles reverse directions, the rotor has to reverse its direction, too, so that it chases those poles around in a circle.
TEXT AND VOCABULARY EXERCISES
37. Find in the text words or phrases which mean the same as:
| § постоянная структура | § вращать |
| § направление | § вращающий момент |
| § постоянный магнит | § энергия сопротивления |
| § металлическая пластина | § взаимодействие |
| § магнитный полюс | § пружина |
| § вентилятор | § колебаться |