Overview
This lecture covers the basics of electric current, Ohm's Law, and electric power, and applies these concepts to solve related circuit problems.
Electric Current and Charge
- Conventional current flows from positive to negative terminal, opposite to real electron flow.
- Electric current (I) is the rate of charge flow: ( I = \Delta Q/\Delta t ).
- Charge (Q) is measured in coulombs (C); time (t) in seconds (s).
- 1 ampere (A) = 1 coulomb per second (C/s).
- The elementary charge (charge of one electron) is ( 1.6 \times 10^{-19} ) C.
Ohm's Law and Resistance
- Ohm's Law: ( V = IR ), where V is voltage (volts), I is current (amps), and R is resistance (ohms).
- Increasing voltage increases current (direct relationship).
- Increasing resistance decreases current (inverse relationship).
- Resistance is analogous to traffic lanes—more resistance, less current flow.
Electric Power
- Electric power (P) formulas: ( P = VI ), ( P = I^2R ), ( P = V^2/R ).
- Power is measured in watts (W).
- 1 W = 1 joule/second (J/s).
Example Problems
- To find total charge: ( Q = I \times t ); convert time to seconds before calculating.
- Number of electrons: ( \text{number} = Q/e ) where e is the elementary charge.
- Current in resistor: ( I = V/R ).
- Power dissipated: ( P = I^2R ) or ( P = VI ).
- Resistance of a bulb: ( R = V/I ).
- Cost of running a device: Calculate energy in kWh and multiply by cost per kWh.
- Voltage from power and current: ( V = P/I ).
- Calculating resistance: ( R = V/I ).
- For kilo-ohms, multiply by 1,000: 5 kΩ = 5,000 Ω.
Key Terms & Definitions
- Conventional Current — Flow of positive charge from high to low voltage.
- Ampere (A) — Unit of electric current, 1 C/s.
- Coulomb (C) — Unit of electric charge.
- Ohm (Ω) — Unit of electrical resistance.
- Watt (W) — Unit of electrical power, 1 J/s.
- Ohm's Law — Relationship: ( V = IR ).
- Power — Rate of energy transfer: ( P = VI ).
Action Items / Next Steps
- Practice converting between units (amps, milliamps, coulombs, kilo-ohms).
- Solve additional Ohm’s Law and power problems from textbook or homework.
- Review and memorize key formulas for exams.