Physics (Scientific Branch) - الفيزياء
The physics paper - الفيزياء - covers electrostatics, capacitors, current electricity, magnetic fields, electromagnetic induction, and modern physics (photoelectric effect, Bohr atom). Marks cluster in the numerical problems, where the formula alone earns nothing without correct substitution and units.
Core rules
- Coulomb: F = kq₁q₂/r², with r in metres and charges in coulombs; force direction depends on sign.
- Parallel-plate capacitor: field E = V/d is uniform between the plates; C = Q/V; stored energy = ½CV² = ½QV.
- Capacitors add inversely in series (1/C = 1/C₁ + 1/C₂) and directly in parallel - the reverse of resistors.
- A real source obeys I = ε/(R + r); terminal voltage is V = ε − Ir, always less than the emf while current flows.
- Magnetic force F = BIL sinθ is maximum when the conductor is perpendicular to B and zero when parallel.
- Induced emf: ε = −N ΔΦ/Δt; the minus sign is Lenz's law, opposing the change in flux.
Worked example
A battery of emf 12 V and internal resistance 0.5 Ω is connected to a 5.5 Ω resistor. Total resistance = 5.5 + 0.5 = 6.0 Ω. Current I = 12/6.0 = 2.0 A. Terminal voltage V = 12 − (2.0)(0.5) = 11 V. Power in the external resistor = I²R = 4 × 5.5 = 22 W; the other 2 W is lost inside the cell.
Traps
- Entering r in centimetres in Coulomb's law - a factor-of-10⁴ error.
- Treating internal resistance as a parallel branch instead of a series one.
- Using sinθ = 1 when the wire is not perpendicular to the field.
- Mixing µF and F in capacitor energy calculations.
- Reversing the direction of the induced current in Lenz's-law questions.