【判断题】电源力所做的功与被移动电荷之比,定义为电源电动势,电动势的方向规定从电源正极指向电源负极。
【单选题】It usually (takes) much (less) time (to fly) from one country to another than (traveling) by train.
【单选题】One of the disadvantages of traveling as an air courier is that he ______.
A.
cannot decide when and where to travel
B.
cannot take any luggage with him
C.
has to wear two pairs of jeans
D.
saves little money from the travel
【单选题】On a medium-speed main propulsion diesel engine, the crank-pin or crank journalbearings receive lubricating oil from( ).
D.
drilled passages in the crankshaft
【简答题】电动势的方向规定从电源的负极指向正极即____升高的方向。
【判断题】电动势的方向规定从电源的负极经过电源指向电源的正极,与电源两端的电压方向相反。
【单选题】It usually takes much (less) time to fly from one country to (another) than (traveling) (by) train.