Overview
This lecture covers the fundamentals of electricity, focusing on electric charge, static electricity, electric fields, current, voltage, resistance, circuits (series and parallel), electrical safety, and related calculations.
Electric Charge & Static Electricity
- Charge is a property of matter; protons are positive, electrons negative, and neutrons neutral.
- Objects can be positively charged (lose electrons), negatively charged (gain electrons), or neutral (equal protons/electrons).
- Charge is measured in coulombs (C).
- Like charges repel; opposite charges attract; neutral objects are attracted to charged objects due to charge rearrangement.
Conductors and Insulators
- Conductors (metals) have free electrons allowing current flow.
- Insulators (rubber, plastic) do not conduct electricity but can be charged by friction.
Electric Fields
- An electric field is a region where a charge experiences a force.
- Field lines point away from positive charges and toward negative charges.
- Field strength is shown by the closeness of field lines; closer lines mean stronger fields.
- Uniform fields occur between parallel plates; field lines are evenly spaced.
Electric Circuits: Current, Voltage, Resistance
- Current (I): rate of flow of charge (I = Q/T, measured in amperes).
- Direct current (DC) flows in one direction; alternating current (AC) changes direction.
- Conventional current flows from positive to negative; electron flow is actually negative to positive.
- Voltage (V): energy transferred per unit charge (V = W/Q, measured in volts).
- EMF: energy per charge supplied by a source; potential difference is energy used per charge across a component.
- Resistance (R): opposition to current (R = V/I, measured in ohms).
- Resistance increases with wire length, decreases with greater cross-sectional area.
Power and Energy in Circuits
- Power (P): rate of energy transfer (P = IV).
- Other equations: P = I²R, P = V²/R.
- Energy: E = VIT.
- Kilowatt-hour (kWh) is the practical unit for household energy usage (1 kWh = 3,600,000 J).
Components & Their Symbols
- Key components: cell, battery, switch, power supply, diode, LED, lamp, resistor, variable resistor, thermistor, LDR, fuse, buzzer, bell.
- Thermistor: resistance decreases as temperature increases.
- Light-dependent resistor (LDR): resistance decreases with increased light.
Diodes & Rectification
- Diode allows current in one direction only.
- Half-wave rectifier uses one diode; full-wave rectifier (bridge) uses four diodes to convert AC to DC.
Series and Parallel Circuits
- Series: current is the same; resistances add; voltage divides.
- Parallel: voltage is the same; current divides; total resistance decreases (R_total = (R1*R2)/(R1+R2) for two resistors).
- Cells in series add their voltages; in opposite directions, subtract.*
Potential Dividers
- Potential divider splits voltage across components in series based on resistance ratio.
- Variable potential divider allows adjustable output voltage.
Electrical Safety
- Mains electricity has live, neutral, and earth wires.
- Fuses and circuit breakers protect against overcurrent by breaking the circuit.
- Earth wire protects users by providing a low-resistance path if the metal case becomes live.
Key Terms & Definitions
- Charge (Q) — property of matter, measured in coulombs (C).
- Current (I) — rate of flow of charge, measured in amperes (A).
- Voltage (V) — energy per unit charge, measured in volts (V).
- Resistance (R) — opposition to current, measured in ohms (Ω).
- Power (P) — rate of energy transfer, measured in watts (W).
- EMF (Electromotive Force) — energy supplied per unit charge by a source.
- Potential Difference (PD) — energy used per unit charge across a component.
- Conductor — material allowing free movement of electrons.
- Insulator — material lacking free electrons.
- Diode — component allowing current in only one direction.
- Thermistor — resistor whose resistance decreases with temperature.
- LDR (Light-Dependent Resistor) — resistor whose resistance decreases with light.
- Fuse — safety device melting at a set current to break the circuit.
- Circuit Breaker — switch that opens when current is too high.
- Earth Wire — provides low-resistance path to ground for safety.
- Kilowatt Hour (kWh) — unit of energy used in households.
Action Items / Next Steps
- Use the provided checklist to review your understanding of each topic.
- Practice drawing circuit diagrams and identifying component symbols.
- Memorize key equations (Ohm’s Law, power, energy).
- Complete any assigned homework problems or past paper questions on series/parallel circuits and safety features.