Charge, current and time are linked by charge = current × time (Q = It). Charge is measured in coulombs (C), current in amperes (A) and time in seconds (s). Potential difference, current and resistance are linked by potential difference = current × resistance (V = IR), with potential difference in volts (V) and resistance in ohms (Ω).
The power of a device is the energy it transfers each second. Power = potential difference × current (P = IV), and since V = IR this can also be written P = I2R. Power is in watts (W). Energy transferred = power × time (E = Pt), or energy transferred = charge × potential difference (E = QV), with energy in joules (J).
In a series circuit the same current flows through every component, and the total (equivalent) resistance is found by adding the individual resistances: Rtotal = R1 + R2. The supply potential difference is shared between the resistors.
Worked example: a 9 V supply is connected to a 10 Ω resistor and a 5 Ω resistor in series. Total resistance = 10 + 5 = 15 Ω. Current = 9 ÷ 15 = 0.6 A. Power = 9 × 0.6 = 5.4 W. In 10 s the energy transferred is 5.4 × 10 = 54 J.