Physics: Light, Electricity, Heat and Magnetism
Physics in AP POLYCET tests Class 10 mechanics, light, heat, electricity and magnetism, usually as short numericals or rule-based MCQs. Formulas are simple; marks are lost in sign conventions and unit conversion.
Core rules
- Light: mirror formula 1/f = 1/v + 1/u and lens formula 1/f = 1/v − 1/u; use the Cartesian sign convention consistently. Snell’s law: n₁ sin i = n₂ sin r.
- Electricity: V = IR; series resistance R = R₁ + R₂; parallel 1/R = 1/R₁ + 1/R₂; power P = VI = I²R.
- Heat: Q = mcΔT for temperature change and Q = mL for phase change.
- Magnetism: use the right-hand grip rule for a current-carrying solenoid and Fleming’s left-hand rule for motor force.
- Units: convert cm to m, mA to A, and minutes to seconds before calculating.
Worked example: a 6 Ω resistor and a 3 Ω resistor are connected in parallel across a 6 V battery. Parallel equivalent: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current I = V/R = 6/2 = 3 A. Branch checks: the 6 Ω branch takes 1 A and the 3 Ω branch takes 2 A; they add to 3 A. If the resistors were in series, the equivalent would be 9 Ω and the current only 0.67 A.
Traps
- Forgetting that in parallel the voltage is common, while in series the current is common.
- Dropping the negative sign in mirror/lens calculations.
- Using P = VI when only resistance and current are given; use P = I²R.
- Treating focal length as 2 × radius of curvature; f = R/2.
- Mixing cm and m in the same formula.