AP Physics 2- 11.4 Electric Power- Study Notes- New Syllabus

Energy Transfer and Power in Electric Circuits

Power in Electric Circuits:

Power (\(\text{P}\)) describes the rate at which energy is transferred into, out of, or within an electric circuit.

\(\text{P} = \dfrac{\text{W}}{t} = \text{VI}\)

• \(\text{W}\): Work done (or energy transferred), in joules (J)
• \(\text{t}\): Time, in seconds (s)
• \(\text{V}\): Potential difference across the circuit element (volts, V)
• \(\text{I}\): Current through the circuit element (amperes, A)

Alternate Forms of Power:

Using Ohm’s law (\(\text{V} = \text{IR}\)), power can also be expressed as:

\(\text{P} = \text{I}^2 \text{R} \quad \text{or} \quad \text{P} = \dfrac{\text{V}^2}{\text{R}}\)

• \(\text{I}^2 \text{R}\): Describes the rate of thermal energy dissipation in resistors (Joule heating).
• \(\dfrac{\text{V}^2}{\text{R}}\): Useful when only voltage and resistance are known.

Energy Transferred in Circuits:

The total electrical energy transferred over time is given by:

\(\text{E} = \text{Pt} = \text{VIt}\)

Interpretations of Energy Transfer:

• Into the circuit: A battery or EMF source provides energy to charges.
• Within the circuit: Resistors convert energy into heat, lightbulbs convert energy into heat and light, capacitors store energy in electric fields.
• Out of the circuit: Useful work (like powering a motor or producing light).