Electronic – What does the plus (+) mean in a voltage source symbol

I think, the answer is relatively simple. Do you know the working principle of a "Schottky diode", which is based on a semiconductor-metal junction? Now - what happens if you connect a voltmeter (or any other load) across the diode? You create two Schottky junctions which exactly compensate the diffusion voltage inside the pn diode. Thus, no voltage can be measured. With other words: You cannot use the diffusion voltage to drive any current through an external load.

First thing you need to do is to understand amps and volts, then it all gets clearer. Electrical power is not like a substance which is supplied, it is a flow. The electrons move around a circuit and as they pass through things they make stuff happen. It is a bit like a hydraulic system where a pump pushes fluid around a system and as it passes through hydraulic motors etc it makes them work. In order for the electrons to go round and round there must be a loop. The battery is like a pump pushing them around. They flow from the high pressure (at the "pump" output) towards the lower pressure ("punp" return). It does not matter how much pressure there is in the system, what counts is how much pressure difference there is between one end and the other, this is what forces the electrons to move. The term for this "pressure" in the electrical circuit is voltage. For this reason, it only makes sense to talk about voltage difference between one place and another, never the absolute voltage at a point. It is a pain to have to keep referring to difference between one point and another when we want to talk about the voltage at some point in the circuit so we just CHOOSE a convenient point in the circuit (often, but not always the negative supply terminal) CALL IT 0V. Then when we say there is 10V at the output, it means "10V difference between the output and the 0V point".

To answer why current flows the opposite way to electrons, in the early days of electricity, cleaver scientists worked out that the strange effects observed in the laboratory (when coils of wire, connected between metal plates in acid, produced magnetic fields) must be produced by something flowing through the wire. They also figured that there must be a force which made the flow happen. They called the force "voltage" and the flow "current". They did not at that stage know what particles were involved so they made an arbitrary decision and assigned + and - to the voltage and a direction to the flow. It turns out that electrons actually flow the other way, but it does not matter, all the maths works and it too late to change it now.

AC is often produced by a rotating machine that uses magnets and coils of wire. As the machine rotates the magnets cause a constantly varying magnetic field to pass through the coils. During 1 rotation, the direction of the magnetic flux will reverse and return to the initial state. This induces varying voltages across the coils. Voltage changes from 0, builds and then falls, reverses polarity, rises then falls back to zero. This cycle repeats. There are some advantages to using AC in transmission of power as it is easy to use a transformer to change the voltage. With a DC system this is more complex, but modern electronics means that it is now easier to change one DC voltage to another, and DC transmission systems are becoming more popular.