According to the fact that a phone charger consists of a transfomer and a recitifier, there should - still - be a difference in potentials, even though no phone is attached to the charger.

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    $\begingroup$ Given that there are losses in the ac-dc conversion, a little current will still flow. But, with no path for current on the dc side to flow, a back emf is built up that opposes current. $\endgroup$ – Jon Custer Nov 16 '16 at 20:36
  • $\begingroup$ What is the cause of this EMF? Is it - maybe - electromagnetic induction on the primary coil? $\endgroup$ – ILoveChess Nov 16 '16 at 20:37
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    $\begingroup$ The most obvious thing to point to is the filtering capacitors on the dc side. They charge up, the diodes don't conduct anymore, so there is no way for current to flow... $\endgroup$ – Jon Custer Nov 16 '16 at 20:38
  • $\begingroup$ Modern chargers are "switching power supplies", not transfomer and a recitifier. You can tell by the lighter weight. You could try electronics.stackexchange.com $\endgroup$ – Keith McClary Nov 16 '16 at 23:39
  • $\begingroup$ These have a high frequency oscillator followed by a transformer (which can be much smaller due to the high frequency) and rectifier. I guess they have a control circuit that turns them off when there is no load. $\endgroup$ – Keith McClary Nov 17 '16 at 1:59

Because the circuit is not complete. Think of it as a potential difference, but the resistance connecting them is infinite, ie, air. When phone is connected, circuit is complete and current flows. If you touch the charger tip with your tongue, you will feel slight tingling taste of electricty, as the circuit would still be complete.

  • $\begingroup$ The primary coil of the transformer is still a closed circuit. $\endgroup$ – Yashas Nov 17 '16 at 4:40
  • $\begingroup$ To be honest, even the secondary coil is a closed circuit. $\endgroup$ – Yashas Nov 17 '16 at 4:46
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    $\begingroup$ That part has some little losses. It dissipates as heat. $\endgroup$ – Kalpak Gupta Nov 21 '16 at 7:10

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