Pair work. Put these questions to your groupmate. Let him/her answer them.

1. What is the ammeter used for?

2. What is the voltmeter used for?

3. What is the ohmmeter used for?

4. What terminals does a meter have?

5. Should the measured circuit be opened when the voltmeter is used?

6. Should the measured circuit be opened when the ammeter is used?

7. In what way should the voltmeter be connected to the circuit?

8. In what way should the ammeter be connected to the circuit?

9. What is the difference between a voltmeter and an ammeter?

10. What common meters are used to measure the values in a circuit?

 

6. Solve the following problems:

1. Suppose the ammeter scale reads 1.9 amp, the voltmeter scale reads 2.4 V; how much is the value of resistance in the measured circuit?

2. Suppose the ohmmeter scale reads 75 ohms, the voltmeter scale reads 220 V; how much is the value of current in the measured circuit?

3. Suppose that you have a series circuit consisting of three resistors and a voltage source. R₁ = 0.18 ohm, R₂ = 1.15 ohms, R₃ = 2 ohms, I =10 amp. Find the voltage drop across each resistor; find the value of voltage in the circuit. Suppose R₁ gets open. What does it result in?

4. Two resistors are connected in series. R₁ = 7,000, R₂ = 2,200, I = 110 amp. Find the voltage drop across each resistor. Suppose no current passes through the circuit, what does it result from?

 

Unit Eight [8]

1. a) Cover the right column and read the English words. Translate them into Russian and check your translation.

b) Cover the left column and translate theRussian words back into English.

capacity	[kəˈpæsɪtɪ]	емкость
power	[ˈpauə]	мощность
heat		теплота, нагрев
rate		скорость, степень
to produce	[prəˈdjuːs]	производить
to change	[tʃeɪndʒ]	менять(ся)
to vary	[ˈvɛərɪ]	варьировать(ся)
low	[lou]	низкий
high	[haɪ]	высокий
fixed	[fɪkst]	постоянный
any		(зд.) любой
variable	[ˈvɛərɪəbl]	переменный
the (more) … the (more)		чем (больше)... тем (больше)

 

2. Read the words and put down their Russian equivalents:

[ˈtemprətʃə]	temperature	___________________________
[ˈenədʒi]	energy	___________________________
[wɔt]	watt	___________________________
[ˈkɔnstənt]	constant	___________________________
[pouˈtenʃəl]	potential	___________________________

        

3. Translate into Russian using чем... тем.

1. The more one studies nature, the better one knows its laws.

2. The longer one learns, the more one knows.

3. The higher the atmosphere, the less is its pressure.

4. The heavier the object, the more work one has to do in order to lift it.

5. The greater the number of free electrons in any metal, the higher is its conductivity.

 

2. Translate into Russian. Mind no.

1. There is no energy in this machine.

2. No charges move through an open circuit.

3. No material is a perfect conductor of electricity.

4. No electric machinery is used without protection.

5. No special material is needed in this case.

Resistors

A resistor is one of the most common elements of any circuit. Resistors are used:

1. to reduce the value of current in the circuit;

2. to produce IR voltage drop and in this way to change the value of the voltage.

When current is passing through a resistor its temperature rises high. The higher the value of current the higher is the temperature of a resistor. Each resistor has a maximum temperature to which it may be heated without a trouble. If the temperature rises higher the resistor gets open and opens the circuit.

Resistors are rated in watts. The watt is the rate at which electric energy is supplied when a current of one ampere is passing at a potential difference of one volt. A resistor is rated as a 1-W resistor if its resistance equals 1,000,000 ohms and its current-carrying capacity equals 1/1,000,000 amp,since
P = E× I = IR ×I = I²R where P - power is given in watts, R - resistance is given in ohms and I - current is given in amperes.

If a resistor has a resistance of only 2 ohms but its current-carrying capacity equals 2,000 amp, it is rated as a 8,000,000-W resistor.

Some resistors have a constant value - these are fixed resistors, the value of other resistors may be varied - these are variable resistors.

 

5. Complete the sentences using the correct variant:

1. A resistor is used	a) to measure the resistance. b) to reduce the current. c) to change the resistance. d) to produce IR voltage drop.
2. When current passes through a resistor	a) its temperature drops. b) its temperature rises.
3. Resistors are rated	a) in ohms. b) in volts. c) in watts.
4. Power is given	a) in amperes. b) in watts.
5. Fixed resistors have	a) a constant value. b) a variable value.
6. The value of a variable resistor	a) is fixed. b) is varied.
7. A two-ohm resistor rated as a 8,000,000-W resistor	a) has a current-carrying capacity equal to 2,000 amp. b) has a current-carrying capacity equal to 200 amp.
8. The higher the value of current,	a) the lower is the temperature of a resistor. b) the higher is the temperature of a resistor.

6. Complete the sentences using while. Follow the model on page 13.

1. The value of a fixed resistor is constant ….

2. Current-carrying capacity is given in amperes….     

3. The lower the value of current, the lower is the temperature of a resistor….

4. An electric source produces energy ….