O-Level Physics notes

Key idea. Kinematics describes motion using distance, displacement, speed, velocity and acceleration. Speed is distance ÷ time, velocity is displacement ÷ time, and acceleration is change in velocity ÷ time, a = (v − u)/t. On a speed-time graph, the gradient gives the acceleration and the area under the line gives the distance travelled. Near the Earth, objects in free fall accelerate at about 10 m/s².

Key idea. Dynamics links forces to motion. Find the resultant force by adding forces in the same direction and subtracting opposing ones. If the resultant force is zero, an object stays at rest or moves at constant velocity. If it is not zero, the object accelerates according to F = ma. Forces come in equal and opposite pairs acting on different bodies, and weight is W = mg.

Key idea. The moment of a force is its turning effect about a pivot. It equals the force multiplied by the perpendicular distance from the pivot to the line of action of the force, measured in N m. For a body in equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments about any point, and there is no resultant force.

Key idea. Pressure is force per unit area, p = F/A, measured in pascals, where 1 Pa = 1 N/m². In a liquid, pressure increases with depth and density, p = hρg, and acts equally in all directions. Hydraulic machines transmit pressure through a liquid to multiply force, while barometers and manometers use liquid columns to measure atmospheric and gas pressure.

Key idea. Energy is measured in joules and is never created or destroyed, only transferred between stores. Work done is force × distance moved in the direction of the force, W = Fs. Kinetic energy is ½mv² and the change in gravitational potential energy is mgh. Power is the rate of energy transfer, P = E/t, in watts, and efficiency compares useful output with total input.

Key idea. Thermal physics uses the kinetic particle model to explain temperature, gas pressure and heat transfer. Energy moves by conduction, convection and radiation. The energy needed to change a substance's temperature is Q = mcΔθ, and the energy needed to melt or boil it at constant temperature is Q = ml. Temperature stays constant during a change of state because the energy goes into overcoming forces between particles.

Key idea. A wave transfers energy without transferring matter. In a transverse wave the particles oscillate at right angles to the direction of energy transfer, and in a longitudinal wave they oscillate parallel to it. Waves are described by amplitude, wavelength, frequency and period, and their speed is given by the wave equation v = fλ, where f = 1/T.

Key idea. Light reflects with the angle of incidence equal to the angle of reflection, both measured from the normal. It refracts when it changes speed, and the refractive index is n = sin i / sin r = c/v. Total internal reflection happens when light in a denser medium meets a boundary at more than the critical angle, where sin c = 1/n. A converging lens forms real or virtual images.

Key idea. Current is the rate of flow of charge, I = Q/t. Potential difference is the energy converted per unit charge, V = W/Q, and resistance is R = V/I. In a series circuit the current is the same everywhere and resistances add. In a parallel circuit each branch has the same p.d., the branch currents add, and 1/R = 1/R₁ + 1/R₂ gives a smaller combined resistance.

Key idea. Electromagnetism links current and magnetism. A current produces a magnetic field, whose direction is given by the right-hand grip rule. A current-carrying wire in a magnetic field feels a force, given by Fleming's left-hand rule, which drives a d.c. motor. A changing magnetic field induces an e.m.f., which is used in generators and in transformers, where Vs/Vp = Ns/Np.