Physics - Science Track
What the paper tests
The physics paper is set in Khmer and mixes recall with calculation. Frequent topics are kinematics, Newton’s laws, momentum and energy, direct-current circuits, Ohm’s law, electrical power, waves, and optics. Marks are usually split between the formula, the substitution with units, and the final statement.
Core rules
- Resolve vectors before applying ΣF=ma; choose one positive direction and keep it.
- Momentum is conserved in collisions when external forces are negligible; kinetic energy is conserved only in elastic collisions.
- In series, current is common and voltages add; in parallel, voltage is common and currents add. For two parallel resistors, Req=R1R2/(R1+R2).
- Electrical power is P=VI=I²R=V²/R.
Worked example
A 12 V battery of negligible internal resistance is connected to 6 Ω and 3 Ω resistors in parallel. The equivalent resistance is (6×3)/(6+3)=2 Ω. The total current is I=V/R=12/2=6 A. The 6 Ω branch carries 12/6=2 A; the 3 Ω branch carries 12/3=4 A. Total power is P=VI=12×6=72 W. Check: 2 A and 4 A add to 6 A, and 72 W equals 12²/2.
Traps
Using the series rule for parallel resistors; forgetting to convert mA or cm; omitting units in the substitution line; and mixing conventional current with electron flow.