Physics - Electricity, Current, Magnetism, Waves and Sound
This domain tests circuit analysis, the magnetic effects of current, electromagnetic induction, and wave relationships. Expect numerical series/parallel problems and qualitative questions about electromagnets, motors, generators, echoes, and sound.
Current electricity
Current = charge ÷ time. Ohm's law: V = IR. Resistance of a wire increases with length and temperature and decreases with cross-sectional area. In series, current is the same, resistances add, and voltage divides. In parallel, voltage is the same, 1/R_total = 1/R₁ + 1/R₂ + …, and current divides. Power P = VI = I²R = V²/R; electrical energy in kWh = power in kW × time in hours.
Magnetism
A current creates a magnetic field; reversing current reverses the field. A solenoid behaves like a bar magnet. Electromagnetic induction occurs only when magnetic flux changes; Lenz's law says the induced current opposes the change. A motor uses current in a field; a generator uses motion in a field.
Waves and sound
Wave speed v = fλ; period T = 1/f. Sound is longitudinal, needs a medium, and travels about 340 m/s in air, faster in solids and liquids. Frequency determines pitch; amplitude determines loudness. Reflection, refraction, and resonance apply to sound and light.
Worked example
A 6 Ω and a 3 Ω resistor are in parallel across 6 V. 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current = 6/2 = 3 A. Branch currents are 6/6 = 1 A and 6/3 = 2 A, summing to 3 A. Total power = 6 × 3 = 18 W.
Traps
Series current is not divided; parallel voltage is not divided. Ammeters go in series and voltmeters in parallel. Frequency and period are reciprocals. Sound speed does not depend on frequency. Induced current appears only while flux changes.