Q) why we don't use 'dc' supply in our house...but only 'ac'?
not only in our house but for many other purposes.

Q) Is there advantage in using 'ac' rather than 'dc'...or due to less power loss in transmission make 'ac' more advantageous?

Q Is there any difference between EMF and voltage?

Q.. What is the meaning of the words 'Straight polarity'
'reverse polarity'?

 

AC is used in electricity transmission lines and in the home so that transformers can be used to reduce the voltage. Transformers do not work with DC.

 

They use AC because it is stepped up/down easily with transformers. To make the same voltage conversions at the same power levels with DC you need very expensive electronics.

Actually, EMF has the same physical meaning as voltage. The term EMF is used more when referring to sources of voltage and the term voltage is used more when referring to voltage drops across components.

The last question floats.

 

the concept of 'reverse polarity' does not make any sense in case of AC-circuits or AC-voltage, but for DC-circuits, it could for example be an electrolytic capacitor, which was turned upside down - that is with its plus pole to ground and the minus pole to a positive voltage. (In case of a high enough voltage the electrolytic capacitor will be destroyed after a while)...