陕西科技大学电气专业英语试卷

Exercise 2

一、阅读理解（20points ）: Passage one The circuit element used to model the current-resisting behavior

of a material is the ① . It is the simplest ② element. George Simon Ohm is credited with formulating the ③

relationship for a resistor based on experiments performed in 1826.

This relationship is known as Ohm's Law. Ohm's Law states that the voltage across a resistor is ④ the current flowing through the resistor. The constant of proportionality is the resistance value of the resistor in ohms. The circuit symbol for the resistor is shown in Fig.1. For the current and voltage shown, Ohm's Law is

u(t) = Ri(t) (1)

where R≥0 is the resistance in ohms.

Since R is constant, Eq.(1) is the equation of a straight line. For this reason, the resistor is called a linear resistor. A graph of u(t) versus i(t) for a linear resistor is a line passing through the origin with a slope of R.

Resistors whose resistances do not remain constant for different terminal current are known as ⑤ resistors. For such a resistor, the resistance is a function of the current flowing in the device. In reality, all practical resistors are nonlinear because the electrical characteristics of all conductors are affected by environmental factors such as temperature. Many materials, however, closely approximate an ideal linear resistor over a desired operating region.

Choose the best answer into the blanks.

1 A. capacitor B. inductor

u (t )

i (t ) + _

Fig.1

C. resistor

D. source

2 A. passive B. active

C. linear

D. nonlinear

3 A. current-current B. current-voltage

C. voltage-power

D. current-power

4 A. equal to B. directly proportional to

C. inversely proportional to

D. different from

5 A. linear B. constant

C. nonlinear

D. time-varying

Passage two

The effective value of voltage and current is the common method of expressing the Value of AC. This is also known as the RMS value. The effective value of AC is defined in terms of an equivalent heating effect when compared to DC. One RMS ampere of current flowing through a resistance will produce heat at the same rate as a DC ampere. For purpose of circuit design, the peak value may also be needed. For example, insulation must be designed to withstand the peak value, not just the effective value. It may be that only the effective value is known. To calculate the peak value, multiply the effective value by 1.41.

6.Which equation is true?( )

A.V peak×1.41=V rms

B. V peak÷0.707=V rms

C. V peak×

0.707=V rms

7.According to the passage, if the effective value is 100V, the peak value is

( ).

A 141

B 70.9

C 70.7

D 14.1 Passage three

However, it is usually adequate to sample a signal at twice its highest frequency. Each binary digit used to show a binary value is known as a bit. The accuracy of conversion is limited by the number of binary bits used by the ADC . If the conversion is made using a four-bit ADC, only sixteen different binary values can be produced. The smallest value is 0000 and the largest is 1111. If an 8-bit ADC is used, then 256 different digital values can be produced. When a measurement of the analogue signal does not coincide with one of these binary values, it must be rounded up or down.