Voltage
Voltage is a measure of the energy carried by the charge. Voltage is sometimes called the potential difference. Voltage is supplied by the battery or power supply. Voltage is used up in components, but not in wires, and we say voltage across a component. Voltage is measured in volts with a voltmeter, connected in parallel. The symbol V is used for voltage in equations.
In electronics voltage at a point is the voltage difference between that point and a reference point of 0V (zero volts). Zero volts could be any point in the circuit, but it is normally the negative terminal of the battery or power supply. Circuit diagrams are often labeled with 0V as a reminder.
Voltage and Current for components in Series
Fig.6
Voltages add up for components connected in series. Currents are the same through all components connected in series.
In this circuit (Fig. 6) the 4V across the resister and the 2V across the LED add up to the battery voltage: 2v + 4V = 6v. The current through all parts (battery, resistor and LED) is 20mA.
V. Запишите перевод текста «Voltage».
Text 2
Current
Current is a flow of charge through a circuit. Current is not used up, what flows into a component must flow out. Current is measured in amperes (amps), A. Current is measured with an ammeter, connected in series. To connect in series you must break the circuit and connect the ammeter in the gap.
Let us consider two main types of current: direct and alternating. A direct current (d. c.) flows through a conducting circuit in one direction only. It flows provided that a direct voltage source is applied to the circuit.
An alternating current (a. c.) is a current that changes its direction of flow through a circuit. It flows provided that an alternating voltage source is applied to the circuit. Alternating current flows in cycles. The number of cycles per second is called the frequency of the current. In a 60-cycle alternating current flows in one direction 60 times and in the other direction 60 times per second.
It is easy to transform a. c. power from one voltage to another by a transformer. Transformers are also used to step down the voltage at the receiving point of the line to the low values that are necessary for use.
When necessary a. c. can be changed into d. c.
VI. Выберите подходящее по смыслу слово:
Current is a flow of … through a circuit.
(circuit, charge, voltage)
2. It flows provided that a direct … is applied to the circuit.
(voltage source, relay, meter)
3. Alternating current flows in … .
(stages, directions, cycles)
4. The number of cycles per second is called … of the current.
(resistance, frequency, flow)
5. It is easy to transform a. c. power from one voltage to another by … .
(a car, a voltmeter, a transformer)
VII. Выберите правильный вариант ответа:
1. D. C. is a current that ____ .
a) changes its direction of flow
b) flows in one direction
2. A. C. flows provided that ____.
a) direct voltage source is applied
An alternating voltage source is applied
3. In an alternating current circuit _____ .
a) current flows in one direction 60 times per second
b) current flows in one direction 60 times and in the other direction 60 times per second
4. A.C. _____ .
a) can be changed into d. c.
b) cannot be changed into d. c.
VIII. Ответьте на вопросы:
1. What is current?
2. What types of current do you know?
3. When does a direct current flow?
4. What type of current is called an alternating current?
5. What type of current is called a direct current?
6. What is called the frequency of current?
7. What device is used to transform a. c. power from one voltage to another?
8. Is it often necessary to change a. c. into d. c.?
IX. Прочитайте текст «Current» еще раз, выпишите ключевые фразы и сделайте реферирование текста на английском языке.
X. Прочитайте текст «Frequency» и задайте 2 вопроса по содержанию текста:
Frequency
The number of cycles per second is the frequency of an alternating current. There are two frequencies: the standard for Europe is 50 cycles per second while the standard for the USA is 60 cycles per second. A standard frequency has a great advantage since different systems can be interconnected.