Overview
This lecture covers the basics of electric current, Ohm's law, and electric power, with practice problems demonstrating calculation methods for current, resistance, charge, and energy cost.
Electric Current Concepts
- Conventional current flows from the positive to negative terminal (from high to low voltage).
- Electron flow is the actual movement of electrons, from negative to positive terminal.
- Current (I) is defined as the rate of charge flow: ( I = \frac{\Delta Q}{\Delta T} ).
- Electric charge (Q) is measured in coulombs (C); time (T) in seconds (s).
- The unit for current is the ampere (A): 1 A = 1 C/s.
- Charge of one electron is (-1.6 \times 10^{-19}) C.
Ohm’s Law and Resistance
- Ohm’s law: ( V = I \times R ) (Voltage = Current × Resistance).
- Resistance (R) is measured in ohms (Ω).
- Increasing voltage increases current (direct relationship).
- Increasing resistance decreases current (inverse relationship).
Electric Power Equations
- Electric power (P) is the rate of energy transfer: ( P = V \times I ).
- Alternative power formulas: ( P = I^2 \times R ), ( P = \frac{V^2}{R} ).
- Power is measured in watts (W): 1 W = 1 J/s.
Practice Problems
- To find charge: ( Q = I \times t ) (ensure time is in seconds).
- To find number of electrons: ( \text{Number} = \frac{Q}{1.6 \times 10^{-19}} ).
- To find current: ( I = \frac{V}{R} ) or ( I = \frac{Q}{t} ).
- To find resistance: ( R = \frac{V}{I} ).
- To find power: use ( P = V \times I ), ( P = I^2 \times R ), or ( P = \frac{V^2}{R} ) depending on available values.
- To find energy cost: multiply power in kilowatts by operating hours and by the cost per kWh.
Key Terms & Definitions
- Conventional Current — Flow of positive charge from high to low voltage.
- Electron Flow — Movement of electrons from negative to positive terminal.
- Current (I) — Rate of charge flow (C/s); unit: ampere (A).
- Electric Charge (Q) — Quantity of electricity; unit: coulomb (C).
- Voltage (V) — Electrical potential difference; unit: volt (V).
- Resistance (R) — Opposition to current flow; unit: ohm (Ω).
- Power (P) — Rate of energy transfer; unit: watt (W).
- Coulomb — SI unit of charge.
- Ampere (A) — SI unit of electric current.
Action Items / Next Steps
- Practice similar problems using Ohm’s law and power equations.
- Ensure homework units (amps, volts, ohms, coulombs, seconds) are consistent.
- Review definitions and equations before the next lecture.