Transformer impedance?

Hi,

Since the primary side of a transformer is a closed circuit (like the cubical adapter for the iphone charger), it would make sense that even if my iphone wasn't plugged into the secondary, the primary would still draw power.

I know this is the case for at least some adapters, because i see arcing sometimes when i plug my laptop adapter into the wall without my laptop being plugged into the secondary.

But some adapters only start getting hot when i plug in load at the secondary. Why?

also

When the secondary load is connected to make the secondary a closed circuit, does its induced emf change the impedance for the primary side?

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Baluncore
2019 Award
A transformer has a turns ratio, primary to secondary, of Np : Ns.
The same power must flow through the primary and secondary circuits.

Given the low impedance mains supply, the primary voltage is fixed at Vp.
The secondary voltage will be Vs = Vp * Ns / Np
The load, Rs, on the transformer will decide the secondary current, Is = Vs / Rs.
The primary current will then be Ip = Is * Ns / Np.
So the transformer transforms the voltage one way, and the current the other.

Looking into the primary from the mains supply is seen; Rp = Vp / Ip.
That is the apparent input impedance of the loaded transformer.
Substituting everything back into everything else gives; Rp = Rs * (Np / Ns)2
The impedance ratio is therefore the square of the turns ratio.

The adapter you show is an electronic switching power supply. It is not a simple transformer, but it does a good imitation of a transformer. It operates at a much higher frequency than the mains supply, it can then use a much smaller transformer to isolate the DC output from the mains supply.

A transformer has a turns ratio, primary to secondary, of Np : Ns.

It operates at a much higher frequency than the mains supply, it can then use a much smaller transformer to isolate the DC output from the mains supply.
why does higher frequency = able to use smaller transformer?

is it perhaps.. because higher freq. = more inductive impedance = lower current = can use smaller wires. and therefore can make the transformer smaller in size?

Baluncore
2019 Award
Voltage is proportional to rate of change of flux.
Higher frequency is faster change of flux, so it needs both less turns and less flux.
Less flux gives a smaller core before saturation. Less turns gives a smaller winding.

There are too many influences to predict size from frequency accurately. Core material will also change.
A crude rule of thumb is something like; Mass ∝ Frequency-2/3

I know this is the case for at least some adapters, because i see arcing sometimes when i plug my laptop adapter into the wall without my laptop being plugged into the secondary.
In Switching Power Suply the mains voltage is always rectified and smoothed with capacitor of 10-100uF. The arcing is caused by charging that capacitor.