Overview
This lecture explains the concept of electrical power, also known as Watt's Law, presents its key formulas, and demonstrates how to use them in simple circuit problems.
Mechanical vs. Electrical Power
- Power is the energy consumed or work done over a period of time.
- Mechanical power is defined as work done (force × distance × cosθ) divided by time and is measured in joules per second (watts).
Definition of Electrical Power
- In an electrical circuit, power is the work done by moving electric charges through a conductor.
- Electrically, work is defined as charge (q) times voltage (V).
- Power (P) is the work done divided by time.
Fundamental Equations for Electrical Power
- Electric current (I) is charge (q) passing through a conductor per unit time.
- The fundamental formula: P = V × I (Power = Voltage × Current).
- Using Ohm’s Law (V = I × R), derived formulas are:
- P = I² × R (Power = Current squared × Resistance)
- P = V² / R (Power = Voltage squared divided by Resistance)
- Power is measured in watts (W), denoted by the capital letter "W".
Example Problem: Light Bulb
- Given: A 60 W light bulb operates at 110 V.
- To find current: I = P / V = 60 W / 110 V = 0.54 A.
- To find resistance: R = V² / P = (110 V)² / 60 W = 201.5 Ω.
Key Terms & Definitions
- Power (P) — The rate of energy consumption or work done, measured in watts (W).
- Watt (W) — The SI unit for power; 1 watt = 1 joule/second.
- Current (I) — The flow of electric charge per unit time, measured in amperes (A).
- Voltage (V) — The electric potential difference between two points, measured in volts (V).
- Resistance (R) — The opposition to current flow, measured in ohms (Ω).
- Ohm’s Law — States that voltage = current × resistance (V = I × R).
Action Items / Next Steps
- Practice solving circuit problems using the three power formulas (P = VI, P = I²R, P = V²/R).