✔ 最佳答案
This is just a simple proportion.
You have already found the potential drop on XY be 1.5 volts, and XY has a length of 20 cm. Thus there is 20 cm/1.5volts = (20/1.5) cm along XY for a drop of 1 volt on it.
To have 1.2 v of drop in ptential, we would have 1.2 v x (20/1.5) cm/v = 16 cm
-----------------------------------------------
In your equation, you have missed out the resistance of PY
since the resistance of the whole of XY is 20 ohms, resistance of PX= x ohms, and resistance of PY = (20-x) ohms
hence, {x/[40+x+(20-x)]}4.5 = 1.2
but 40+x+(20-x) just equals to 60 ohms, which is the total resistance of the circuit. Notice that the total resistance of 60 ohms is not affected by the position of P.
this gives (x/60)4.5 = 1.2
solve for x gives x = 16 ohms, therefore, the length of XP = 16 x(20/20) cm = 16 cm
