Question

17. Which of the following is the correct formula for finding power in a DC circuit? A. P = I2R B. P = VR C. P = IR D. P = V2I

Answer 1

Answer:

Choice A. P = I² · R where

• P is the power in the DC circuit,
• I is the current through the circuit, and
• R is the total resistance of the circuit.

Step-by-step explanation:

Electrical power is the rate at which the electrical force does work. So what is electrical work? That's the work $W$ that the electrical force do when it moves charges $Q$ across a potential difference $V$:

W = $V\cdot Q$.

The power is the rate at which the electrical force do the work:

$\displaystyle P = \frac{W}{t} = V \cdot \frac{Q}{t}$.

On the other hand, current $I$ is the charge through a cross-section of the circuit in unit time. By the definition of current:

$\displaystyle\frac{Q}{t} = I$.

$\displaystyle P =V \cdot \frac{Q}{t} = V\cdot I$.

Consider Ohm's Law:

$V = I \cdot R$.

Therefore

$\displaystyle P = V\cdot I = (I \cdot R) \cdot I = I^{2}\cdot R$.

Answer 2

Choice-A is one of several useful, correct formulas for electrical power.  It's true in AC circuits as well as DC ones.