What is the current in a series circuit that has two resistors (4.0 ohms and 7.5 ohms) and a power source of 9.0 volts?

- - First, we have to decide how to handle the two resistors.

The effective resistance of resistors in series is the sum

of their individual resistances. That is, they act like a single

resistor, whose resistance is the sum of all of them.

So in this question, the 4.0 ohms and the 7.5 ohms act like a

single resistor of 11.5 ohms.

- - The current in the circuit is

(the supply voltage) / (the total resistance)

= (9.0 volts) / (11.5 ohms)

= 0.783 ... Ampere (rounded)