Physics
Physics in the Bihar Polytechnic entrance tests mechanics, electricity, optics, heat and waves at Class 10 level. Expect formula-based numericals and conceptual one-liners; units and sign conventions decide many answers.
Motion and force
- SI units: metre, kilogram, second, newton, joule, watt.
- For uniform acceleration: v = u + at, s = ut + ½at², v² = u² + 2as. Take the initial direction as positive; write deceleration as a negative acceleration.
- Newton's laws: F = ma; action and reaction are equal and opposite but act on different bodies.
Work, energy and power
- Work W = Fs cosθ; kinetic energy ½mv²; gravitational potential energy mgh.
- Mechanical energy is conserved when only conservative forces act; power = work/time.
Electricity
- Ohm's law V = IR; series R = R₁ + R₂; parallel 1/R = 1/R₁ + 1/R₂; P = VI = I²R = V²/R.
- Conventional current is opposite to electron flow.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across a 6 V battery. Find the equivalent resistance and total current. 1/Rp = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2, so Rp = 2 Ω. I = V/Rp = 6/2 = 3 A. Check: branch currents are 6/6 = 1 A and 6/3 = 2 A; they add to 3 A.
Traps
- Do not add parallel resistances directly; the equivalent is smaller than the smallest resistor.
- A deceleration must carry a negative sign in kinematics equations.
- In mirror and lens formula, follow the sign convention consistently; a wrong sign changes the image position.