Chapter 7: Electricity, Magnetism, and Electronics Basics

Electric circuits

Current is the rate of flow of charge (amperes). Potential difference (voltage) is the energy transferred per unit charge (volts). Resistance opposes current. Ohm’s law: for ohmic conductors. Resistivity: , so a longer or thinner wire has more resistance. In series, and the current is the same through each; in parallel, and the voltage is the same across each. Examples: in series; and in parallel give .

Power and energy: ; energy . Domestic bills use the kilowatt-hour (kWh): a 2 kW heater used for 3 h consumes 6 kWh. Example: a 2 kW kettle on a 230 V supply (Sri Lanka’s mains is nominally 230 V, 50 Hz) draws A and has .

Kirchhoff’s laws: at any junction the currents entering equal those leaving; around any closed loop, the sum of voltage sources equals the sum of voltage drops.

Cells, batteries, and direct and alternating current

A cell converts chemical energy to electrical energy; real cells have internal resistance, so the terminal voltage falls when current is drawn. Batteries may be primary (single-use) or rechargeable (lead-acid, nickel-metal hydride, lithium-ion). Direct current (DC) flows one way; alternating current (AC) reverses direction periodically (50 times per second in Sri Lanka’s 50 Hz supply). The r.m.s. value of an AC voltage of peak value is .

Magnetism and electromagnetism

Magnets have north and south poles; like poles repel and unlike poles attract. A current in a wire produces a magnetic field; a coil with an iron core is an electromagnet (used in relays, bells, cranes, and motors). A current-carrying wire in a magnetic field experiences a force (the motor effect), which drives electric motors. When a conductor cuts magnetic field lines, an e.m.f. is induced (electromagnetic induction), the basis of generators and transformers. For an ideal transformer, ; transformers allow electricity to be transmitted at high voltage with lower losses and then stepped down for use.

Electrical safety

Electricity can kill. Basic safeguards are insulation, earthing (a path for fault current to ground), fuses and miniature circuit breakers (MCBs), and residual-current devices (RCDs) that disconnect quickly when current leaks to earth. Never touch exposed conductors, never work on live circuits, never overload sockets, keep water away from electrical equipment, and obey the national wiring standards (Sri Lanka Standards Institution, n.d.).

Basic electronics

Resistors limit current; their values are shown by a colour code: for yellow–violet–red–gold, digits 4 and 7, multiplier , tolerance , so .

Capacitors store charge: ; they smooth voltage and block DC.

Diodes conduct in one direction; they rectify AC to DC. LEDs emit light. Example: to run a red LED (2 V, 20 mA) from 5 V, .

Transistors act as switches or amplifiers; a small base current controls a larger collector current.

Sensors: a thermistor changes resistance with temperature; an LDR with light.

Logic gates (AND, OR, NOT) process binary signals and are the basis of digital electronics (Chapter 10).

A voltage divider gives .

Common mistakes

Connecting an ammeter in parallel or a voltmeter in series.

Forgetting the current-limiting resistor with an LED.

Treating AC peak values as r.m.s. values in power calculations.

Practice questions

Find the current drawn by a 60 W lamp on 230 V. [0.26 A]

Three resistors of 12 Ω, 12 Ω, and 6 Ω are connected in parallel. Find the total resistance. [3 Ω]

A 3 kW water heater runs for 2 hours each day. How much energy does it use in a 30-day month? [180 kWh]