The higher the voltage the less power is lost to heat therefore a 220 v. device is more efficient than a 110 volt device. Of course your house must be wired for 220.
At 20 amps the 220V will supply twice (2x) the power that the 110V will supply. The answer depends upon your meaning of the term 'better'. If you have a specific size load that you are supplying such as a motor that will accept either 220V or 110V then by using 220V you will have less of an impact from voltage drop and the conductor size will be smaller due to the current being 1/2 of the 110V. If by better you mean which one will give you the most power then the obvious answer is 220V.
Use a transformer to lower from 220 to 110V.
No
Do not do this.
No.
220v and 110v are almost the only voltages used around the world because they are the most efficient.
Depends. Some smaller stuff can usually run on both, in which case it will be printed on them. If it isn't, you can't run a 220v item on 110v.
The fact that it's supposed to. Voltage is stated as the difference between the two wires carrying electricity to the load. When they bring power to the house from the utility, you get two wires carrying 110v but they're 180 degrees out of phase. Imagine one carries positive 110v and the other carries negative 110v. If you hook one of these wires plus a neutral (zero volts) to the load, you get 110v--110v over 0v. If you hook both of them to the load, you get positive 110v over negative 110v, or 220v. So...red to white is 110v, black to white is 110v, red to black is 220v.
yes, but you need a transformer to convert it,
All three, on 110V a split receptacle, on 220V a baseboard heater, on 440V a construction heater or similar resistive load.
Yes. If the computer power input is only rated for 110V input, you can power it from 220V by using a voltage converter from 220V to 110V with appropriate wattage. Some computer power supplies also have a switch for setting 110V/220V in which case computer can be powered from 220V after changing the setting. Most of the portable computers/laptops have power adapters which are rated for universal voltage (e.g. my Dell laptop AC adapter is rated for 100-240V 50-60Hz).
The online retailer Amazon currently has an 'Automatic Transformer Adapter 500W 110v/220v 220v/110v' on sale for $38.89. That's 51% lower than the list price of $79.99 and that's before the free shipping!