某同学用伏安法测一节干电池的电动势E和内电阻r，所给的器材有：

A．电压表V：0～3～15V

B．电流表A：0～0.6～3A

C．变阻器R₁（20Ω，1A）

D．变阻器R₂（1000Ω，0.1A）

E．电键S和导线若干

某同学用伏安法测一节干电池的电动势E和内电阻r，所给的器材有：

A．电压表V：0～3～15V

B．电流表A：0～0.6～3A

C．变阻器R₁（20Ω，1A）

D．变阻器R₂（1000Ω，0.1A）

E．电键S和导线若干

在“测定金属的电阻率”的实验中，测定阻值约为3﹣5Ω的金属丝的电阻率，实验中所用的电压表规格：量程0﹣3V、内阻3kΩ；电流表规格：量程0﹣0.6A、内阻0.1Ω；还有其他一些器材：

质量为m，电荷量为q的带负电粒子自静止开始，经M、N板间的电场加速后，从A点垂直于磁场边界射入宽度为d的匀强磁场中，该粒子离开磁场时的位置P偏离入射方向的距离为L，如图所示，已知M、N两板间的电压为U，粒子的重力不计．

根据短文理解，选择正确答案。

    My parents knew their wealth: each other, their six children, and their faith. They tried to live simple lives so that they could have time for what was most important.

    They didn't busy themselves buying a bigger house, because that would mean working harder to pay the monthly mortgage(按揭贷款), working overtime or taking a second job. Who would go running with me then? Who would read stories to me?

    They didn't burden themselves with buying an expensive car because that would mean worrying about installment(分期付款) bills. Besides, walking to the shopping centre every Saturday afternoon with me gave my dad his needed exercise, and made me feel so special.

    One of my heart's delights was seeing Dad and Mom in their bedroom at night, after our nightly family prayer. The lights were turned off, and I'd see the figure of my father seated on his old chair and Mom standing behind him, gently massaging his shoulders. I'd hear them talk about what happened during their day. Even as a child, I sensed their quiet pleasure in being together.

    My question today: Could they have done this rich practice each night and nourished(滋润) their marriage if they had been busy paying for expensive clothes for themselves or their kids, or if they had been worrying about monthly bills for new hi-tech equipment? I don't think so. And I've made a choice: I don't want that of life either.

如图所示，两平行金属导轨间的距离L=0.40m，金属导轨所在的平面与水平面夹角θ=37°，在导轨所在平面内，分布着磁感应强度B=0.50T、方向垂直于导轨所在平面的匀强磁场．金属导轨的一端接有电动势E=4.5V、内阻r=0.50Ω的直流电源．现把一个质量m=0.04kg的导体棒ab放在金属导轨上，导体棒恰好静止．导体棒与金属导轨垂直、且接触良好，导体棒与金属导轨接触的两点间的电阻R₀=2.5Ω，金属导轨的其它电阻不计，g取10m/s²．已知sin37°=0.60，cos37°=0.80，试求：

如图所示，两平行金属导轨间的距离L=0.40m，金属导轨所在的平面与水平面夹角θ=37°，在导轨所在平面内，分布着磁感应强度B=0.50T、方向垂直于导轨所在平面的匀强磁场．金属导轨的一端接有电动势E=4.5V、内阻r=0.50Ω的直流电源．现把一个质量m=0.04kg的导体棒ab放在金属导轨上，导体棒恰好静止．导体棒与金属导轨垂直、且接触良好，导体棒与金属导轨接触的两点间的电阻R₀=2.5Ω，金属导轨的其它电阻不计，g取10m/s²．已知sin37°=0.60，cos37°=0.80，试求：

如图所示，两平行金属导轨间的距离L=0.40m，金属导轨所在的平面与水平面夹角θ=37°，在导轨所在平面内，分布着磁感应强度B=0.50T、方向垂直于导轨所在平面的匀强磁场．金属导轨的一端接有电动势E=4.5V、内阻r=0.50Ω的直流电源．现把一个质量m=0.04kg的导体棒ab放在金属导轨上，导体棒恰好静止．导体棒与金属导轨垂直、且接触良好，导体棒与金属导轨接触的两点间的电阻R₀=2.5Ω，金属导轨的其它电阻不计，g取10m/s²．已知sin37°=0.60，cos37°=0.80，试求：

如图所示，R₄是半导体材料制成的热敏电阻，电阻率随温度的升高而减小，这就是一个火警报警器的电路，电流表是安放在值班室的显示器，电源两极之间接一个报警器，当R₄所在处出现火情时，显示器的电流I和报警器两端的电压U的变化情况是（   ）