In the answer model, they use Ohm's law to calculate I4, there they use V = Va - Vb, why exactly this order and not Vb - Va?

 

Because the current is going from A to B.

 

Because of the completly arbitrary direction the arrow was drawn.

 

Why is that? Because of convention that current flows from high potential to low potential?

 

Why is that? Because of convention that current flows from high potential to low potential?

Because of the definition of voltage as being the work done per unit charge to move a charge from one point to another.

 

View attachment 302247

In the answer model, they use Ohm's law to calculate I4, there they use V = Va - Vb, why exactly this order and not Vb - Va?

Hi,

The simple way this is usually described is as follows.

The arrow is meant to show the flow of current using the CONVENTIONAL CURRENT FLOW concept in a two terminal device.
When that convention is employed, the terminal where the current ENTERS the device is said to have the most POSITIVE potential.
The terminal where the current EXITS the device is said to have the most NEGATIVE potential.

This can be summarized in a more memorable way:
ENTERS-POSTIVE
EXITS-NEGATIVE

This implies that to get the unsigned voltage across the device you would subtract the more negative potential from the more positive potential.

Example...
Top: 12v
Bottom: 5v
Voltage across: 12-5=7 volts

Example...
Top: 12v
Bottom: -5v
Voltage across: 12-(-5)=17 volts

 

If you don’t like it, just reverse the arrow and use Vb - Va. And the answer will be negative instead of positive.