Unbalanced Wheatstone Bridge

Hi there I'm stuck on the question (25)a)ii)




I know that the formula I need to use is:


but how do/should I know weather to put the 800 ohm resistor on top or the 1600 ohm resistor on top and the same for the other point?

Any help will be appreciated and thanks in advance.

It doesn't really matter so long as you do the same for both P and Q.
I would use the 800 and 4000 ohm resistors on the top of the formula. This gives the value of the pd between P and zero and Q and zero.

When I did it with the 4000 and 800 on top I got the correct answer but with the other 2 on top I got the incorrect answer.

Can you explain to me why I need to use the bottom 2 resistors on the top of the formula?

Because the voltage is zero at the bottom and you want the voltage across the two bottom resistors. This gives you a value for the two voltages at P and Q above "zero", the value at the bottom.
If you use the top two resistors on the top of the formula you get the voltages between P and Q and the 12V at the top.

A simple example.

Let's say you get the voltage at P is 9V and Q is 5V using the "correct" method. This gives you 9V-5V = 4V between P and Q.
If you do it the other way you will get 3V for P (12V-9V) and 7V for Q (12V-5V)
This still gives you 4V between P and Q, which is correct if you are only asked for the pd or meter reading and not asked for which way round the voltage is.