Electricity and Magnetism: Circuits and Electromagnetic Effects
Electricity and magnetism: circuits and effects
Electric current is the rate of flow of charge, I = Q/t, measured with an ammeter in series. Potential difference (voltage) is energy transferred per unit charge, measured with a voltmeter in parallel across a component. Resistance is opposition to current, defined by V = IR. For ohmic conductors at constant temperature, current is proportional to p.d.; filament lamps and thermistors are non-ohmic because their resistance changes with temperature.
In series, current is the same everywhere; voltages add; total resistance is the sum. In parallel, voltage is the same across each branch; currents add; total resistance is less than the smallest branch resistance. Electrical power is P = VI = I²R = V²/R, and energy transferred is E = Pt. Domestic energy is often billed in kilowatt-hours: energy in kWh = power in kW × time in hours.
Magnetism: like poles repel, unlike poles attract; field lines run from north to south outside a magnet. A current-carrying wire has a magnetic field; a coil with a current becomes an electromagnet. The motor effect arises when a current-carrying conductor sits in a magnetic field, and Fleming's left-hand rule predicts the force direction. Electromagnetic induction produces a voltage when a conductor cuts magnetic field lines or when flux through a coil changes; transformers work only with alternating current.
Traps: connecting an ammeter in parallel or a voltmeter in series; adding parallel resistances directly; forgetting that resistance in parallel is smaller than any branch; mixing power and energy; and failing to earth a metal case.
Worked example: A 6 Ω and 3 Ω resistor in parallel give R = (6×3)/(6+3) = 2 Ω. In series with 4 Ω, total R = 6 Ω. From a 12 V supply, I = V/R = 2 A.