The reason the alternator example doesn't work is because, as i said, energy can't be created out of nothing. In a closed system there will always be the same amount of energy. It might be in different forms such as heat, electricity, movement, etc, but always the same amount. This is not something I can explain in greater detail, its just a scientific fact. If you want a thorough explanation of this I suggest you read up on
thermodynamicsIn your example with the alternator, energy in the form of rotational movement is converted into electricity by moving magnets past coils of wire. If the alternator could do a 100% conversion and you could design a electric motor that could also do a 100% conversion, then you had a perpetual motion machine. It would never be able to create more energy than what you originally put into it by jump starting it, because then it would be creating energy out of nothing.
In reality, energy is lost from the alternator in the form of heat because of friction between the moving parts. When the electricity flows through the wires a small amount of it is converted to heat as well because of resistance in the wires. And so on and so on. The second problem is that you can't create a closed system. Energy, usually heat, will always escape from your system. There is no material on earth that can keep heat from escaping. So the net result = the alternator will at some point grind to a halt.
And the same goes for the example with the Balance Balls. You can't get more energy out of them than you put in.
Seriously I'm not making this stuff up, ask your local physics teacher and he/she will tell you the same thing.