According to the book, current is given by the time derivative of charge, that is, I = Q/t. Does this mean that the amount of charge in a wire or a circuit changes with time? If not, what does it mean?

Estimate the resistance of a typical lightbulb.

In the circuit shown, the battery provides a current I = 0.3 A to the resistor. How much current is returned by the resistor to the negative terminal of the battery? How much current flows inside the battery from the negative to the positive terminal?

Estimate the resistance of a typical electrical cord in your home, say, one that attaches a lamp to the wall socket.