Your android phone needs to be waterproof (wireless) so it recharges its battery

Question

Your android phone needs to be waterproof (wireless) so it recharges its battery via a plastic coated high efficiency transformer built into a stand. You took it with you to Australia, but you forgot to buy an electrical adapter, and they don't sell them out where you are staying which is quite a hike from Alice Springs, so you get out a pen knife and splice some wires from your 12 Volt DC truck battery, directly to the prongs of your phones charger pad, hoping that will work. Here is what the charger & phone (load) look like in the US: Here is what the charger and phone look like in Australia after your modifications: What is the power dissipated the resistor (your android phone) in Australia? (Ignore turn on transients, the question is asking about long term steady state)

Explanation / Answer

for US circuit,

Vs / Vp = Ns / Np

Vs / 120 = 1 / 10 => Vs = 12 Volt

VP Ip = Vs Is

120 x 1 = 12 x Is

Is = 10 A

Rload = V / I = 1.2 ohm

now for Australia charge,

Vs / Vp = Ns / Vp

Vs / 12 = 1 / 10

Vs = 1.2 Volt

P = Vs^2 / R = 1.2^2 / 1.2 = 1.2 Watt .........Ans

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