Notes on Electric Power Lecture

Introduction

• Discussion on electric power and how to determine its sign (absorbed or delivered).
• Definition of electric power: electrical energy transferred per unit time by an electrical circuit or element.
• Simplified: Electric power is the time rate of change of electrical energy.

Electric Power Definition

• The amount of electrical energy transferred per unit time.
• Analyzing a basic case (resistor) to clarify the concept.

Basic Circuit Element: Resistor

• Voltage across the resistance: V volts.
• Potential difference (V): V = V1 - V2 (where V1 > V2).
• Current direction: positive terminal to negative terminal.

Current Direction and Charge Movement

• Current direction is the same as positive charge movement.
• Current is opposite to negative charge (electrons) movement.
• Focusing on positive charge movement for analysis.

Electric Potential Energy Change

• Electric potential energy before and after moving through the resistor:
  • U1 (before) = Q * V1
  • U2 (after) = Q * V2
• Change in electric potential energy (ΔU): ΔU = U1 - U2 = Q * (V1 - V2) = Q * V.
• Electric potential energy lost means energy is transferred.

Energy Conversion

• Electrical potential energy is converted into thermal energy in the resistor due to collisions with atoms/molecules.
• Resistance causes energy loss, leading to heat production.

Power Calculation

• Power (P) = Energy transferred per unit time.
• P = ΔU / T.
• Substituting ΔU:
  • P = (Q * V) / T.
  • Q/T = current (I), hence P = I * V.

Nature of Power

• Positive Power (P = +IV): Current enters positive terminal, power absorbed (element acts as sink).
• Negative Power (P = -IV): Current leaves positive terminal, power delivered (element acts as source).

Summary of Power Signs

• Positive when current enters positive terminal.
• Negative when current leaves positive terminal.
• Terminal sign indicates power sign (current entering positive = positive power, entering negative = negative power).

Examples

1. Current entering positive terminal, power = +IV (sink).
2. Current leaving positive terminal, power = -IV (source).
3. Tricky case: Current enters positive terminal but inside it acts as source (power = -IV).
4. Current entering negative terminal inside, power = +IV (sink).

Homework Problem

• Given: Voltage = 8 volts, current = 2 amperes entering negative terminal.
• Task: Calculate power with proper sign and determine if source or sink.