(My english is not good)
If I have a battery that is 4v why isnt U = 4 ?
Becouse the voltage U gets divided between the resistors and so if u have two resistors and 4 volts battery, the voltage gets divided between the two resistors. And so u can have 2volts on one resistor and 2 on the other. Or 3 on one resistor and 1 on the other. It deppends entirely on the resistances values.
There is resistance and current but why isn't U equal to the volt of the battery?
What voltage is: Imagine 2 atoms divided by a distance. Atom A and atom B. Atom A looses some of the electrons therefore it becomes more positive(the positive charge of the protons becomes less compensated). Its electric field becomes stronger. Now you should know that every atom has a positive core and that core uses electric field to attract electrons in. However, since both atoms before were in equilibrium and now suddently A looses electrons it field grows and the equilibrium is broken. In order to restore it, A will attract some of the electrons from B until both atoms have the same positive charge. At that point equilibrium is restored and every single electron who left B will make it more positive than A and B will attract it back. ! Take in mind that i use single atoms in my argumentation. In battery is the same but there are billions of atoms more. So Voltage is the work done by the electric field to move an electron from (in this case) B to A. Also B may be 3volts and A6volts. There still will be potential difference of 3volts between them and current will still flow. What happens is that A becomes less positive as it is accepting electrons and B becomes more positive becouse it is giving away electrons leaving behind more uncompensated positive charge. Current will flow until A and B has the same voltage: A-6,5,4 5volts B-3,4,4.5volts. The voltage between this two points is given by V=VA-VB. Does this helped you, let me know. Also you can see Atom steucture in internetBut what is U? I dont understand what it is. The only thing I see is a letter not what it actakly is