Experiments you can run yourself.
Ohm's law: how voltage and resistance control the current. An interactive circuit simulation, the formula I = U/R, bulb brightness and the flow of electrons.
Connecting resistors in series and in parallel: how to compute the equivalent resistance and why in a parallel circuit a single break does not put out the rest. Interactive simulation.
The terminal voltage U=ε−I·r drops at high current because of the internal resistance r. In a short circuit (R→0) U drops near zero; with an open circuit U=ε.
The sum of currents at a node and the sum of voltages in a loop.
P=U²/R=I²R; lower resistance at constant voltage means higher power.
A transistor lets a small voltage control a large current — the foundation of electronics (Nobel 1956).