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Exercise 1. Calculate the current that flows in a simple unbranched circuit consisted of two batteries of 1.5V and light bulb with a resistance of 1W. Internal resistance of each battery is 1W. Explain your actions during calculations. Write down and then read math formula.
Exercise 2. Work with a partner or in a small group. Take the notes you made earlier (page 78, exercise 2) and instruct your classmate how to treat safely some electric instrument or appliance.
Exercise 3. Skim during 5-7 minutes the text below and answer the questions about it content.
An alternating current is an electric current that changes direction at regular intervals. When a conductor is moved back and forth in a magnetic field, the flow of current in the conductor will reverse direction as often as the physical motion of the conductor reverses direction. Most electric power stations supply electricity in the form of alternating currents. The current flows first in one direction, builds up to a maximum in that direction, and dies down to zero. It then immediately starts flowing in the opposite direction, builds up to a maximum in that direction, and again dies down to zero. Then it immediately starts in the first direction again. This surging back and forth can occur at a very rapid rate.
Two consecutive surges, one in each direction, are called a cycle. The number of cycles completed by an electric current in one second is called the frequency of the current. In the United States and Canada, most currents have a frequency of 60 cycles per second.
The strength, or amperage, of an alternating current varies continuously between zero and a maximum. Since it is inconvenient to take into account a whole range of amperage values, scientists simply deal with the effective amperage. Like a direct current, an alternating current produces heat as it passes through a conductor. The effective amperage of an alternating current is equal to the amperage of a direct current that produces heat at the same rate. In other words, one effective amp of alternating current through a conductor produces heat at the same rate as one amp of direct current flowing through the same conductor. Similarly, the voltage of an alternating current is considered in terms of the effective voltage.
Like direct current, alternating current is hindered by the resistance of the conductor through which it passes. In addition, however, various effects produced by the alternating current itself hinder the alternating current. These effects depend on the frequency of the current and on the design of the circuit, and together they are called reactance. The total hindering effect on an alternating current is called impedance. It is equal to the resistance plus the reactance.
1) What are the properties of an electric current?
2) How big is the frequency of industrial alternating current?
3) What is a shape of a diagram of intensity of common alternating current as a function of time?
4) How different is a resistance of the same section of the circuit to alternating and direct currents.
5) What is the common equivalent to the term “cycle per second” in this country?
Date: 2015-12-11; view: 233