Depends on the power of the motor. Weak, so-called assist motors aren't covered by laws. More powerful motors are, and then the bicycle pretty much has to fulfil all the requirements put on a scooter/moped.

One that doesn't operate on public roads or require a driver's license. You could put an electric motor on a bicycle and that should be ok. You need to check the vehicle licensing laws in your state to be sure but in most cases it's still considered a bicycle and doesn't need a license.

Depends on the power of the motor. A bicycle can have a low-power motor and still be considered a bicycle, which don't require a license.

It converts mechanical energy into electric energy.

The eBike Store sells a range of cycling accessories including helmets, lights and bicycle stands. It also sells an electric trailer that can be used to easily convert a regular bicycle into an electric bicycle.