Physics
What the Physics section tests
The physics paper is numerical and definition-led, drawing on mechanics, thermal physics, electrostatics and current electricity, optics, and modern physics. Success depends on choosing the right equation and handling units and signs, not on lengthy derivations.
Core rules to keep ready
- Dimensional homogeneity: every term in a valid equation has the same base dimensions; use this to reject answers quickly.
- Work–energy theorem: W_net = ΔK. For a conservative force, mechanical energy is conserved; friction converts mechanical energy to heat.
- Kirchhoff's rules: the junction rule is charge conservation; the loop rule is energy conservation. A resistor traversed with the current gives a drop −IR.
- Lens/mirror equation: 1/v − 1/u = 1/f for a lens with the Cartesian sign convention (distances measured from the optical centre, positive in the direction of incident light). Magnification m = v/u.
- Photoelectric equation: K_max = hν − φ, where φ is the work function; the stopping potential satisfies eV_s = K_max.
Worked example
An object is placed 30 cm from a convex lens of focal length 20 cm. Find the image position and magnification. Use 1/v − 1/u = 1/f with u = −30 cm and f = +20 cm: 1/v = 1/20 + 1/(−30) = (3 − 2)/60 = 1/60. So v = +60 cm: a real image 60 cm on the far side of the lens. m = v/u = 60/(−30) = −2: the image is inverted and twice the object height. The positive v and negative m agree with a real, inverted image.
Traps that cost marks
- Substituting u as positive in the lens formula, then misreading the sign of v.
- Using peak voltage where RMS is required (V_rms = V_peak/√2) in AC power calculations.
- Omitting the square in Coulomb's law or in the centripetal force.
- Treating static friction as always equal to μN; it equals μN only at the threshold of slipping.