题目内容

4. is a circuit element with resistance approaching infinity.A. An open circuit B. A short circuitC. A resistive circuit D. An inductive circuit

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An electric heater draws 10 A from a 120V line. The resistance of the heater is .A. 1200 Ω B. 120 ΩC. 12 Ω D. 1.2 Ω

Translate the following into ChineseIt must be emphasized that the linear resistor is an idealized circuit element; it is a mathematical model of a physical device. "Resistors" may be easily purchased or manufactured, but it is soon found that the voltage-current ratio of this physical device is reasonably constant only within certain ranges of current, voltage, or power and depends also on temperature and other environmental factors. We shall usually refer to a linear resistor as simply a resistor, using the longer term only when the linear nature of the element needs emphasis. Any resistor that is nonlinear will always be described as such. Nonlinear resistors should not necessarily be considered as undesirable elements.

下面错误的是

A. X->Y, Y->Z，则X->YZ
B. X->Y, X->Z，则X->YZ
C. X->Y, WY->Z，则XW->Z
D. WX->Y, WY->Z，则X->Z

U=(A,B,C,D,E)F={A->D, E->D, D->B, BC->D, DC->A}将其分解为ρ={ABCD, ADE}两个子模式。则：

A. 该分解为无损连接。
B. 该分解为有损连接。
C. 该分解破坏函数依赖。
D. 该分解保持函数依赖。