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An electrical Plug and a socket will have 3 pins-

Earth wire- It is a safety wire and connects the metal case of the appliance to the earth (we will not be talking about this here)

Live wire- The live wire carries current to the appliance

Neutral wire- The neutral wire completes the circuit and carries current away from the appliance

That means, for a device to work the electric current from the socket must enter the live wire or pin in the Electric plug, power the appliance connected and return through the neutral wire or pin completing the circuit. All this while the switch is on. 

This is shown in the circuit diagram below.

Ls, Ns- Live wire, Neutral wire of socket.

Lp, Np- Live wire, Neutral wire of plug.

The switch is “ON” and the plug is inserted into the socket (Ls-Lp and Ns-Np are shorted). The current takes the path as shown(Blue colour lines), the device/appliance is powered and electricity is consumed. 

Now if the switch is ON and we have not connected any device to the socket i.e we are not using it. 

We can see that there is no connection between Ls-Lp and Ns-Np. It is an open circuit even when the switch is ON. There is no current/electricity in the circuit. Therefore electricity is not wasted.

The above case is when the switch is ON and the plug is not inserted into the socket (notice in the circuit, there is no connection between plug and socket). 

Now we shall take the case when the switch is ON and the plug is connected but the device also has a switch, lets say Sd, which is turned OFF. For example, a mixer or a table lamp which is connected to the socket but turned off by the switch on the mixer/lamp. Or a mobile charger plugged into the socket but the mobile is not connected to the charger. This is the circuit diagram- 

Ls-Lp and Ns-Np are shorted but the device switch, Sd=0. The zoomed in part shows the internal power circuitry of the device. There is a step down transformer which steps down the voltage to device’s rating. Notice that the device switch Sd, is after the transformer, that means the transformer is working but the device is OFF(Sd=0). 

this is the case when the transformer is on no load. There will be losses/wastage of power in the form of hysteresis and eddy currents. But the transformers are designed such that these losses are minimized to the maximum extent. So in this case, yes there is wastage but it is very less. 

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