Physics: Mechanics and Current Electricity
Physics in CG PET checks the Class 11–12 core: vectors, Newton’s laws, work–energy, rotational motion, SHM, electrostatics, DC circuits, optics and modern physics. Most questions are direct formula applications, but marks are lost through sign conventions, unit conversions and mixing scalar/vector quantities.
Mechanics rules
- Resolve vectors before applying ΣF = ma; equilibrium needs both ΣF = 0 and Στ = 0.
- Projectile motion: horizontal velocity stays constant; vertical acceleration is g. T = 2u sinθ/g, H = u² sin²θ/(2g), R = u² sin 2θ/g.
- Work–energy theorem: W_net = ΔK. Friction does negative work; conservative forces can be handled with potential energy.
- Circular motion: centripetal acceleration v²/r points to the centre; banking angle satisfies tanθ = v²/(rg).
Current electricity
- Ohm’s law V = IR applies to metallic conductors at constant temperature.
- Series: same current, R_eq = R1 + R2 + …; parallel: same voltage, 1/R_eq = 1/R1 + 1/R2 + …
- Kirchhoff’s rules: junction ΣI = 0; loop ΣV = 0, including EMFs and IR drops.
- Power P = I²R = V²/R. An ideal ammeter has low resistance and is connected in series; an ideal voltmeter has high resistance and is connected in parallel.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across a 12 V battery of negligible internal resistance. Find the total current and power. The parallel branches have the same 12 V. Branch currents are 12/6 = 2 A and 12/3 = 4 A, so total current is 6 A. Equivalent resistance is 12/6 = 2 Ω, and power is VI = 12 × 6 = 72 W, equal to 2²×6 + 4²×3 = 24 + 48 W.
Traps
Using g = 9.8 m/s² when options assume 10 m/s² (or the reverse); swapping sin and cos in projectile formulae; treating the vertical component of velocity as constant; adding parallel resistances directly; ignoring internal resistance; converting cm, mm or μ units incorrectly.