Overview
This lecture covered multiple-choice questions (MCQs) from past papers on electricity. Key concepts included charge quantization, electric current, drift velocity, calculations involving circuits, and potential difference.
Charge and Current Calculations
- Electric current (I) is the rate of flow of charge (Q) over time (t): I = Q/t.
- The total charge can be found by Q = I × t.
- The charge on an alpha particle is 2 × 1.6 × 10⁻¹⁹ C.
- Number of particles = total charge / charge per particle.
- Electric charges occur in multiples of 1.6 × 10⁻¹⁹ C (quantized).
- Electron charge is -1.6 × 10⁻¹⁹ C.
Electron Flow and Drift Velocity
- Electrons move from the negative to the positive terminal in a wire.
- Drift velocity (v) formula: v = I / (nAe), where n = number density, A = area, e = electron charge.
- Drift velocity is inversely proportional to the cross-sectional area of a conductor.
Resistance, Power, and Energy
- Resistance in series and parallel circuits can be calculated using R_total formulas.
- Power (P) dissipated in a resistor: P = I²R or P = V²/R.
- Energy dissipated: E = I²Rt.
- Internal resistance causes a drop in terminal voltage as current increases.
- Power output depends on both internal and external (load) resistance.
MCQ-Specific Approaches
- Use charge quantization to determine possible or impossible charge values.
- To find current from number of electrons: I = (n × e) / t.
- Voltage divider rule: V_out = (R_target / R_total) × EMF.
- Ratio and proportionality methods simplify complex circuit and drift velocity problems.
Circuit Behavior and Measurements
- Terminal potential difference decreases as current increases due to internal resistance.
- When resistors are in parallel, the voltage across each is the same; power ratio can be compared by V²/R.
- For filament lamps, current vs. voltage graph is nonlinear (curve flattens at high voltage).
Key Terms & Definitions
- Current (I) — Rate at which charge flows; measured in amperes (A).
- Charge (Q) — Fundamental property of matter; measured in coulombs (C).
- Electron — Subatomic particle with charge -1.6 × 10⁻¹⁹ C.
- Drift Velocity — Average speed of charge carriers due to electric field.
- Resistance (R) — Opposition to current; measured in ohms (Ω).
- Power (P) — Rate of energy transfer; measured in watts (W).
- Potential Difference (V) — Energy transferred per unit charge; measured in volts (V).
- EMF (Electromotive Force) — Energy provided per coulomb by a power source.
Action Items / Next Steps
- Review relevant textbook chapters on current, resistance, and circuits.
- Practice additional MCQs on drift velocity, charge quantization, and circuit calculations.
- Prepare questions for clarification in the next class.