Physics Electric Circuit

請留意下圖

圖片參考：http://imgcld.yimg.com/8/n/HA00639401/o/701011210001313873413180.jpg

1. 電池已經串聯 = 6V + 12V = 18V

2. 電阻已經串聯 = 2Ω + 4Ω = 6Ω

3. 電流 = V/R = 18 / 6 = 3A

4. 電阻電壓 = V = IR
4a. 3*2=6V
4b. 3*4=12V

5. P & Q = 0V

First, you have to find the voltage across the 2ohm and 4ohm resistors.
Since V=IR,
12-6 = I (2+4)
6 = 6I
I = 1A
Therefore, the voltage across 2ohm and 4ohm resistors are 2A and 4A respectively.

How to calculate the voltage across PQ?
As current flows from the positive pole of the 12V cell, at point Q, no energy is lost and so the potential there is still 12V, and the current keeps on flowing to 6V cell, 6V potential is taken by the 6V cell, so the potential after passing the 6V cell will be 6V. Then, the current keeps on flowing to the 2ohm resistor, 2V is taken by it, so the potential at P is 4V (6V-2V).
In short, at Q the potential is 12V, at P the potential is 6V, so the voltage across PQ is 12V-6V=6V.

Feel free to ask question if you have.