Revise: Electric Current & Magnetic Effect
Current and its effects, Ohm's law, resistors and their combinations, the magnetic field due to a current, electromagnets, and meters.
V = IR for ohmic conductors at constant temperature.
12 V across 4 Ω gives 3 A.
Rₛ = R₁ + R₂ + ...; series total is always greater than any individual resistor.
4 Ω + 6 Ω + 10 Ω = 20 Ω.
1/Rₚ = 1/R₁ + 1/R₂ + ...; parallel total is always less than the smallest resistor.
6 Ω and 3 Ω in parallel give 2 Ω.
Ammeter: series, low resistance. Voltmeter: parallel, high resistance.
An ammeter wired in parallel would create a dangerous short circuit.
I = Q/t; current has heating, magnetic, and chemical effects.
2.5 A for 40 s transfers 100 C of charge.
R = ρL/A; resistance depends on material, length, and cross-sectional area.
Thicker cables have lower resistance, reducing heating losses.
Any current produces a magnetic field; straight wire gives concentric circles.
Oersted's compass deflected near a current-carrying wire.
A coil (+ soft iron core) creates a controllable magnetic field from current.
Scrapyard cranes switch electromagnets on/off to grip and release metal.