Why do plugs leak electricity?

Question

My charger plug and radio plug remain warm even if my mobile is already fully charged or my radio not in use. So why are they leaking energy?

This reminds me of the directive I once heard that you should switch off at the wall and unplug things before going abroad (and I know of people who do this overnight even). Do you really need to unplug, or is switching off at the wall enough?

Answer 1

All of those chargers have a transformer to drop down the voltage . Even when no current is drawn from the secondary winding there is still current passing through the primary winding.
You should always unplug unused electrical devices because even if they are switched off at the plug a lightning strike can possibly destroy them (computers especially).

Answer 2

Plugs do not leak electricity. Your devices do. The reason that they do is that they are on standby or they have clocks that must be kept going. Also, if you have a DC/AC converter plugged into the wall to recharge your cell-phone or whatsit, that device will draw current even while it's not charging the device.