Why does having a ground in middle of center tapped transform cause it to split voltage?
... connecting a wire through the center ...


A centre-tap connects to the centre.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A centre-tapped transformer is the same as two independent secondaries connected in series (by blue wire on diagram).


  why is it grounded tho?


The ground symbol just tells us what point of the circuit we are using as reference. It's convenient to measure all voltages from there - so think of that as the point to connect the black lead of your multimeter or the earth clip of your oscilloscope.


  and why is emf of one of them opp to the other?


It's not. Both halves of the winding are oriented the same way, as indicated by the dots on the coils.







simulate this circuit

Figure 2. An analogy using DC cells.

Hopefully Figure 2 makes it a bit clearer. Depending which point we assign as reference (GND) the other points relative voltage changes.



You could consider the voltage a bit like floors on a building. In Europe, we agree that the ground floor is 0 or G, that floors above it are numbered positively and numbers below it are negatively. You now have the option of measuring everything relative to ground (the floor number) or measuring the difference in level between any two floors (the potential or voltage difference).



In the left image above our man is standing on Floor 2 relative to ground. The electrical analogy is that some point on the circuit is connected to ground / earth and by convention is zero volts and all voltages (heights) are measured relative to this.

An 'all above ground' building will have no negative floors. A bunker or underground car-park will have no positive floors.

If the building is launched off into space he has no ground reference and is free to number the floors any way he wishes, including have Floor 0 at any arbitrary point. This is analogous to having an electrically isolated circuit with no ground connection in that we can call any point 'ground'.