Why must we always use a transformer-rectifier when experimenting with electricity from a power plant?
Power plants produce electricity that is suitable for transport over the long distances between the plant ans our houses. Because of this there are two characteristics that make this electricity different from the electricity that we use in the day by day applications.
The first different characteristic is the high voltage of the electricity transported between plant and our homes. The high voltage requirement used in the transport wires between plant and homes is given by the necessity to keep the currents low enough (to not overheat the transport lines). Since the total electric power `P=U*I` , the power will be the same if we have a high voltage and a low current instead of having both with medium values. This gives the necessity to use down transformers before using the electricity in the day by day applications.
The second different characteristic is that the electricity produced in power plants is AC (alternating current). This happens because usually in plants rotating generators are used to produce the electricity. Also generally, by their construction, electric generators produce 3 different phases of AC current at the same time (only AC currents can have different phases), and because of this the transportation of the same electric power is easier (it requires 3 wires with lower currents, instead of one with a higher current) without much heat loss along the the transporting network. Our day by day electric appliances (like TV's, computers, phones, etc, except light-bulbs and heating) use only DC currents and this is why one needs to use a rectifier to transform AC electricity into DC one.