Physics - Waves, Electricity and Magnetism, and Thermodynamics
The combined waves, electricity and magnetism, and thermodynamics section rewards one skill: choosing the correct conservation or circuit rule before substituting numbers. Questions mix wave properties with ray diagrams, resistor networks with power, and heat or gas calculations with the first law.
Core rules
- Wave speed: v = fλ. Frequency is set by the source and does not change on refraction; speed and wavelength do.
- Refraction: n1 sin θ1 = n2 sin θ2. Light bending toward the normal means it entered a slower, higher-n medium.
- Ohm’s law: V = IR. In series, R_eq = ΣR and current is common; in parallel, 1/R_eq = Σ1/R and voltage is common.
- Electrical power: P = VI = I²R = V²/R.
- First law: ΔU = Q − W with a consistent sign convention. For an ideal gas, PV = nRT, so T must be in kelvin.
Worked example
A 12 V battery is connected to 6 Ω and 3 Ω resistors in parallel. Find the total current and each branch current. Parallel resistance: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current I = V/R = 12/2 = 6 A. Each branch has the full 12 V: I6 = 12/6 = 2 A and I3 = 12/3 = 4 A. The branch currents add to 6 A, which is the check.
Traps
- Adding parallel resistances directly instead of using reciprocals.
- Using Celsius in PV = nRT or in ΔU calculations.
- Confusing wave speed with frequency when a wave enters a new medium.
- Reversing the sign of work in the first law.
- Assuming a magnetic field does work on a moving charge-it changes direction, not speed.