Overview
This lecture focused on the calculation and interpretation of electrical power in circuits, including how to determine if a device is absorbing or delivering power, and the importance of the conservation of power principle.
Power Calculation Basics
- Power (P) in a device is calculated as P = V × I, where V is voltage and I is current.
- Always check if current enters the positive terminal of the voltage for correct sign convention.
- Positive power means the device is absorbing power; negative power means the device is delivering power.
Example Cases
- Case 1: V = 5 V, I = 2 mA → P = 10 mW (absorbing power).
- Case 2: V = -3 V, I = 2 A → P = -6 W (delivering power).
- Case 3: V = 20 mV, I is actually -1 mA if current leaves the positive terminal; P = -20 μW (delivering power).
Sign Conventions & Measurement
- The sign of voltage indicates measurement direction, not if the voltage is "positive" or "negative."
- Correct power calculation depends on current direction relative to the voltage’s positive terminal.
- If current leaves the positive terminal, use a negative current value for P = V × I.
Conservation of Power
- In any circuit, total power delivered must equal total power absorbed.
- If a voltage source delivers 10 W and a load absorbs 9 W, 1 W must go somewhere else (it cannot disappear).
Key Terms & Definitions
- Power (P) — The rate of energy transfer, P = V × I, measured in watts (W).
- Absorbing Power — When power calculated is positive; device uses energy.
- Delivering Power — When power calculated is negative; device supplies energy.
- Conservation of Power — Principle stating total power delivered equals total power absorbed in a circuit.
Action Items / Next Steps
- Think about where the missing 1 watt of power goes in the voltage source and LED example.
- Prepare for upcoming lessons on Ohm’s Law.
- Complete the related homework assignment or discussion board question about power conservation.