Power units refresher: Power is defined as the rate at which work or energy is transferred. Which unit expression matches this definition?

Introduction / Context:
Power quantifies how fast energy is converted, delivered, or consumed. In electrical and mechanical systems, correct unit handling ensures accurate sizing of supplies, thermal solutions, and protective devices. In SI, the watt is the coherent unit that ties energy and time together for power calculations.

Given Data / Assumptions:

• Energy (or work) uses the joule (J) as the SI unit.
• Time uses the second (s).
• Power is defined as energy per unit time.

Concept / Approach:
Power P is defined by P = dW / dt. For constant power over an interval, this reduces to P = W / t. Therefore, the unit must be joule per second, written as J/s, which is named the watt (W). In electrical form, P = V * I, which remains consistent because 1 W = 1 V * 1 A = 1 J/s.

Step-by-Step Solution:

Verification / Alternative check:
Check dimensional consistency: 1 V = 1 J/C and 1 A = 1 C/s, so V * A = (J/C) * (C/s) = J/s = W, confirming equivalence across formulations.

Why Other Options Are Wrong:

• Joules x charge or joules x voltage: Multiply energy by another quantity, producing non-power units.
• Joules/work: Dimensionless ratio when work equals energy; not a rate.

Common Pitfalls:
Confusing energy (capacity to do work) with power (rate of doing work). Battery energy is in watt-hours (Wh), but instantaneous load demand is in watts (W).

Final Answer:
joules/time