Physics - Electricity, Magnetism, and Waves
This domain tests electric circuits, magnetic effects, electromagnetic induction, and wave behavior. Niigata questions often combine a circuit diagram with a calculation, then ask about a motor or generator or interpret a wave graph. Keep the rules for series and parallel circuits separate.
Current, voltage, resistance
- Ohm’s law: V = IR. Resistance depends on material, length, cross-sectional area, and temperature.
- Series: current is the same; voltages add; R_total = R1 + R2.
- Parallel: voltage is the same; currents add; 1/R_total = 1/R1 + 1/R2.
- Power P = VI = I²R; electrical energy = P × t.
- Ammeter is connected in series; voltmeter in parallel.
Magnetism and induction
- Magnetic field lines leave N and enter S outside a magnet.
- A current-carrying wire creates a magnetic field; a current in a magnetic field can experience a force, as in a motor.
- Electromagnetic induction: a changing magnetic field induces a current, as in a generator. Relative motion matters, not the presence of a magnet alone.
Waves
- Waves transfer energy, not matter. v = fλ.
- Transverse waves vibrate perpendicular to travel; sound is longitudinal.
- Reflection: angle of incidence = angle of reflection.
- Refraction: speed changes at a boundary; entering a slower medium bends the ray toward the normal.
- Sound needs a medium; frequency determines pitch, amplitude determines loudness.
Worked example
A 6 Ω resistor and a 3 Ω resistor are connected in parallel across 12 V. Find the total current. In parallel, each resistor has 12 V. I₁ = 12 ÷ 6 = 2 A; I₂ = 12 ÷ 3 = 4 A; total I = 6 A. Equivalently, 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω and I = 12 ÷ 2 = 6 A. The check confirms the current-division rule.
Traps
- Adding parallel resistances directly.
- Applying series voltage rules to parallel branches.
- Saying waves carry matter.
- Confusing motors and generators.
- Ignoring meter polarity.