Electricity, Magnetism, and Modern Physics
What to know
Electric charge comes in positive and negative; like charges repel, opposites attract. Static electricity involves charge transfer by friction, conduction, or induction. Current is charge flow per time (amperes); voltage is electric potential difference (volts); resistance opposes current (ohms). Ohm's law: V = IR. In series circuits, current is the same everywhere and resistances add; in parallel circuits, voltage is the same across branches and total resistance is less than any branch. Power: P = IV = I²R = V²/R.
Magnetism arises from moving charges. Magnetic field lines run from north to south outside a magnet. A current-carrying wire in a magnetic field experiences a force; a changing magnetic field induces a current (electromagnetic induction), the basis of generators and transformers. Motors convert electrical to mechanical energy; generators do the reverse.
Modern physics: the photoelectric effect shows light behaves as particles (photons) with energy proportional to frequency. The double-slit experiment shows wave behavior. Bohr's model quantized electron energy levels; the modern model uses probability clouds. Radioactive decay - alpha, beta, gamma - releases energy; half-life is the time for half a sample to decay. E = mc² links mass and energy.
Traps
Questions swap series and parallel rules, treat voltage as "used up" around a circuit, or assume light is only a wave or only a particle. Half-life problems often ask for remaining amount after whole and partial intervals.
Rule of thumb and worked example
Two 6 Ω resistors in series give 12 Ω; in parallel they give 3 Ω. With a 12 V source and 3 Ω total, current = V/R = 4 A, and power = IV = 48 W. For half-life, after three half-lives, 1/8 of the original sample remains.