Electrical Energy and Power

Understanding how electrical energy is transferred and how power is calculated is essential for circuit analysis. These concepts link the fundamental quantities of current, voltage, and resistance to the real-world behaviour of devices.

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Electrical Power

Power is the rate at which energy is transferred. In an electrical circuit:

P = IV

where:

• P = power in watts (W)
• I = current in amperes (A)
• V = potential difference in volts (V)

One watt means one joule of energy is transferred per second.

Deriving Alternative Forms

By substituting Ohm's law (V = IR) into P = IV, we get two additional useful expressions:

P = I²R (substitute V = IR into P = IV)

P = V²/R (substitute I = V/R into P = IV)

These three forms are all equivalent and are used in different situations:

• Use P = IV when you know both current and voltage.
• Use P = I²R when you know current and resistance (but not voltage).
• Use P = V²/R when you know voltage and resistance (but not current).

Worked Example 1