I frequently use an Immersion Rod Water Heater (shown below) and I have noticed that the after unplugging the heater, the neutral pin of the 3 pin plug is quite hot (about 5 degrees more - but not very hot) compared to the live pin. How is this temperature difference possible and why?
I just had one of those last week. The plug was hot, and the socket was hotter. I'm the electrical guy so I popped it off. #12 stranded wire shoved into a smaller #14 backstab hole. Two thirds of the strands had missed the hole!
Dumb things like that happen all the time, and you have to nip 'em in the bud the moment you see them.
Don't use this outlet until it's fixed. It probably feeds thru to other outlets on the same circuit, so check this outlet while using those. Get it fixed ASAP.
If I had used the next outlet over, I would've never found it. It can start a fire. (We use steel junction boxes, that helps.)
The voltage on each pin, live or neutral, is irrelevant. Also the current flowing through each pin will be the same.
The difference is the resistance of the connection the current is flowing through. This will be caused by the springs in the socket not making good contact with the plug pin.
It's not unusual for heavy current users like kettles to make the pins slightly warm. However a cup heater like this takes very little current, so a warm plug pin is warning that the socket could need replacing. With a heavier current using appliance, it could get hot enough to cause a fire.
To put all that was said in a simpler form, in case a layman stumbles upon this question:
This is an indication that the plug/cord, the socket or both are defective and need to be checked ASAP, since there is a potential fire hazard.
The high resistance of the plug connection is often due to oxidation on the surface of one of the mating pins/receptacles. Regardless of which one it is, the heat will often cause oxidation on the other surface which also causes undue heating. If this is now plugged in elsewhere, this heated surface encourages oxidation of the new mating surface and so on and so on. It is the same epidemiology as zoological venereal diseases. Terminology is similar, mating, male connector, female ....