通过电阻R的电流强度为I时,在t时间内产生的热量为Q,若电阻为2R,电流强度为I/2,则在时间t内产生的热量为()
A:4Q
B:2Q
C:Q/2
D:Q/4
A.20
B.24
C.30
D.50
E.60
F.70
如图所示,水平地面上一质地均匀的方形木板在力F1的作用下处于静止状态,若把它看作一个以O点为支点的杠杆,请你画出阻力F2及动力F1的力臂l1.
找出下列正确的一项( )
—Yesterday , my father bought me a new mobile phone as a present , but I don't know how to use it.
—Why not read the ________ first before using it ?