Материал: Андрианова English Thesaurus of Student-Physicist 2013

a result of collisions between the charge carriers and fixed atoms in the conductor, causing an increase in the temperature of the conductor. In this way, the chemical energy stored in the battery is continuously transformed into thermal energy.

Regardless of the ways in which you use electrical energy in your home, you ultimately must pay for it or risk having your power turned off. The unit of energy used by electric companies to calculate consumption, the kilowatt-hour, is denned in terms of the unit of power and the amount of time it's supplied.

1.What did the Italian physicist Alessandro Volta invent?

2.Why do we need electric currents ?

3.How do electrons move in conductor?

4.What do we need to measure the current in the bulb?

5.What materials do we call ohmic and nonohmic?

6.For what purpose do we use kilowatt-hour?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

96

3.Fill in the gaps with the missing words from the list: resistor, drift velocity, resistance, superconductors, battery

1.Despite the collisions the electrons move slowly along the conductor with the _____.

2.The _____ does not provide electrons to the circuit; it provides energy to the existing electrons.

3.After Georg Simon Ohm (1789-1854) was the first to conduct a systematic study of electrical _____.

4.A _____ is a conductor that provides a specified resistance in an electric circuit.

5.Today thousands of _____ are known, including such common metals as aluminum, tin, lead, zinc, and indium.

UNIT 18

DIRECT-CURRENT CIRCUITS

Read the following texts. Study the thesaurus on fig. 18, answer the questions after the texts

Batteries, resistors, and capacitors can be used in various combinations to construct electric circuits, which direct and control the flow of electricity and the energy it conveys. Such circuits make possible all the modern conveniences in a home - electric lights, electric stove tops and ovens, washing machines, and a host of other appliances and tools. Electric circuits are also found in our cars, in tractors that increase farming productivity, and in all types of medical equipment that saves so many lives every day.

The analysis of the number of simple direct-current circuits is simplified by the use of two rules known as Kirchhoff's rules, which follow from the principle of conservation of energy and the law of conservation of charge. Most of the circuits are assumed to be in steady state, which means that the currents are constant in magnitude and direction.

Sources of emf

A current is maintained in a closed circuit by a source of emf. Among such sources are any devices (for example, batteries and generators) that increase the potential energy of the circulating charges. A source of emf can be thought of as a "charge pump" that forces electrons to move in a direction opposite the electrostatic field inside the source. The emf в of a

97

source is the work done per unit charge; hence the SI unit of emf is the volt.

The terminal voltage when the current is zero, called the open-circuit voltage. Terminal voltage also equal the potential difference across the external resistance, often called the load resistance. The current in simple circuit depends on both the resistance external to the battery and the internal resistance of the battery. We will assume in our examples and problems that the internal resistance of a battery in a circuit is negligible.

Resistors in series

When two or more resistors are connected end to end, they are said to be in series. The resistors could be simple devices, such as light-bulbs or heating elements. Regardless of how many resistors we have in series, the sum of the potential differences across the resistors is equal to the total potential difference across the combination. The equivalent resistance of a series combination of resistors is the algebraic sum of the individual resistances and is always greater than any individual resistance.

Resistors in parallel

Now consider two resistors connected in parallel. In this case, the potential differences across the resistors are the same because each is connected directly across the battery terminals. The potential drop must be the same for the two resistors and must also equal the potential drop across the battery.

Kirchhoff's rules

We can analyze simple circuits using Ohm's law and the rules for series and parallel combinations of resistors. However, there are many ways in which resistors can be connected so that the circuits formed can't be reduced to a single equivalent resistor. The procedure for analyzing more complex circuits can be facilitated by the use of two simple rules called Kirchhoff's rules.

RC-circuits

So far, we have been concerned with circuits with constant currents. We now consider direct-current circuits containing capacitors, in which the currents vary with time. We assume that the capacitor is initially uncharged with the switch opened. After the switch is closed, the battery begins to charge the plates of the capacitor and the charge passes through the resistor. As the capacitor is being charged, the circuit carries a changing current. The charging process continues until the capacitor is charged to its

98

maximum equilibrium value. Once the capacitor is fully charged, the current in the circuit is zero. If we assume that the capacitor is uncharged before the switch is closed, and if the switch is closed at t = 0, we find that the charge on the capacitor varies with time.

Household circuits are a practical application of some of the ideas presented in this Unit. In a typical installation, the utility company distributes electric power to individual houses with a pair of wires, or power lines. Electrical devices in a house are then connected in parallel to these lines. The potential difference between the two wires is about 120V. (These currents and voltages are actually alternating currents and voltages, but for the present discussion we will assume that they are direct currents and voltages.) One of the wires is connected to ground, and the other wire, sometimes called the "hot" wire, is at a potential of 120V. A meter and a circuit breaker (or a fuse) are connected in series with the wire entering the house. In modern homes, circuit breakers are used in place of fuses.

Many heavy-duty appliances, such as electric ranges and clothes dryers, require 240V to operate. The power company supplies this voltage by providing, in addition to a live wire that is 120 V above ground potential, another wire, also considered live, that is 120V below ground potential . Therefore, the potential drop across the two live wires is 240V. An appliance operating from a 240V line requires half the current of one operating from a 120-V line; consequently, smaller wires can be used in the highervoltage circuit without becoming overheated.

Electrical safety

A person can be electrocuted by touching a live wire (which commonly is live because of a frayed cord and exposed conductors) while in contact with ground. The ground contact might be made by touching a water pipe (which is normally at ground potential) or by standing on the ground with wet feet, because impure water is a good conductor. Obviously, such situations should be avoided at all costs. Electric shock can result in fatal burns, or it can cause the muscles of vital organs, such as the heart, to malfunction. As an additional safety feature for consumers, electrical equipment manufacturers now use electrical cords that have a third wire, called a case ground.

Special power outlets called ground-fault interrupters (GFIs) are now being used in kitchens, bathrooms, basements, and other hazardous areas of new homes. They are designed to protect people from electrical shock by sensing small currents - approximately 5 mA and greater - leaking to ground.

99

Fig. 18. Thesaurus for Unit 18

100