Cambridge O Level Physics: Revision Notes

Mechanics

Point 1: Mass vs. Weight

Mass: A measure of the amount of matter in an object. Constant everywhere. SI unit: kilogram (kg).

Weight: The gravitational force acting on an object. Varies with gravitational field strength. Formula: Weight = mass × gravitational field strength (W = mg). SI unit: Newton (N).

Point 2: Density

Definition: Mass per unit volume of a substance. Formula: Density = Mass / Volume (ρ = m/V). SI unit: kilograms per cubic meter (kg/m³).

Point 3: Speed, Velocity, Acceleration

• Speed: Rate of change of distance. Scalar quantity.
• Velocity: Rate of change of displacement. Vector quantity (magnitude and direction).
• Acceleration: Rate of change of velocity. Formula: a = (v-u)/t. SI unit: meters per second squared (m/s²).

Point 4: Forces and Newton's Laws

• Force: A push or pull. SI unit: Newton (N).
• Newton's First Law: An object remains at rest or in uniform motion unless acted upon by a resultant force.
• Newton's Second Law: Force = mass × acceleration (F = ma).
• Newton's Third Law: For every action, there is an equal and opposite reaction.

Point 5: Work, Energy, Power

• Work Done: Energy transferred when a force moves an object through a distance. Work = Force × distance moved in the direction of the force (W = Fd). SI unit: Joule (J).
• Energy: The capacity to do work. Types: Kinetic (KE = ½mv²), Gravitational Potential (GPE = mgh). SI unit: Joule (J).
• Power: Rate at which work is done or energy is transferred. Power = Work Done / time (P = W/t). SI unit: Watt (W).

Point 6: Pressure

Definition: Force acting normally per unit area. Formula: Pressure = Force / Area (P = F/A). For liquids: P = hρg. SI unit: Pascal (Pa).

Point 7: Moments

Moment of a Force: The turning effect of a force about a pivot. Moment = Force × perpendicular distance from pivot to line of action of force. Principle of Moments: For equilibrium, sum of clockwise moments = sum of anticlockwise moments.

Thermal Physics

Point 8: Thermal Expansion

Most substances expand when heated and contract when cooled. Linear expansion (solids), superficial expansion, cubical expansion (liquids/gases). Expansion causes stress, can be utilized (bimetallic strip).

Point 9: Heat Transfer

• Conduction: Transfer of heat through direct contact, mainly in solids via vibrating particles and free electrons.
• Convection: Transfer of heat in fluids (liquids and gases) by movement of heated particles (convection currents).
• Radiation: Transfer of heat through electromagnetic waves (infrared radiation), requires no medium.

Point 10: Specific Heat Capacity (c)

The amount of heat energy required to raise the temperature of 1 kg of a substance by 1 °C (or 1 K). Formula: Q = mcΔT. SI unit: J kg⁻¹ °C⁻¹.

Point 11: Specific Latent Heat (L)

The amount of heat energy required to change the state of 1 kg of a substance without a change in temperature. Latent Heat of Fusion (melting/freezing) and Vaporization (boiling/condensing). Formula: Q = mL. SI unit: J kg⁻¹.

Waves

Point 12: Wave Properties

• Transverse Waves: Oscillations are perpendicular to wave direction (e.g., light, water waves).
• Longitudinal Waves: Oscillations are parallel to wave direction (e.g., sound waves).
• Wavelength (λ): Distance between two consecutive identical points on a wave.
• Frequency (f): Number of complete oscillations per second.
• Amplitude: Maximum displacement from the equilibrium position.
• Wave Speed (v): v = fλ.

Point 13: Light - Reflection and Refraction

• Reflection: Bouncing back of light from a surface. Angle of incidence = Angle of reflection.
• Refraction: Bending of light as it passes from one medium to another due to change in speed. Snell's Law: n = sin i / sin r.
• Total Internal Reflection (TIR): Occurs when light travels from a denser to a less dense medium at an angle of incidence greater than the critical angle. Applications: optical fibres, prisms.

Point 14: Lenses

Converging (Convex) Lens: Thicker in the middle, converges parallel light rays to a focal point. Forms real or virtual images. Uses: magnifying glass, camera, human eye.

Diverging (Concave) Lens: Thinner in the middle, diverges parallel light rays. Always forms virtual, upright, diminished images. Uses: correcting short-sightedness.

Point 15: Sound Waves

Longitudinal waves produced by vibrations. Requires a medium to travel. Speed of sound is fastest in solids, then liquids, slowest in gases. Echoes are reflections of sound waves.

Electricity and Magnetism

Point 16: Electric Current, Voltage, Resistance (Ohm's Law)

• Current (I): Rate of flow of charge. I = Q/t. SI unit: Ampere (A).
• Voltage (V) / Potential Difference (p.d.): Energy per unit charge. V = W/Q. SI unit: Volt (V).
• Resistance (R): Opposition to current flow. Ohm's Law: V = IR. SI unit: Ohm (Ω).

Point 17: Series and Parallel Circuits

• Series: Components connected end-to-end. Current is same through each. Total resistance R_total = R1 + R2 + ... . Total voltage V_total = V1 + V2 + ... .
• Parallel: Components connected across the same two points. Voltage is same across each. 1/R_total = 1/R1 + 1/R2 + ... . Total current I_total = I1 + I2 + ... .

Point 18: Electrical Power and Energy

• Electrical Power (P): Rate at which electrical energy is converted to other forms. P = VI = I²R = V²/R. SI unit: Watt (W).
• Electrical Energy (E): Energy = Power × time (E = Pt). SI unit: Joule (J), commercial unit: kilowatt-hour (kWh).

Point 19: Magnetism and Electromagnetism

• Magnetic Fields: Region around a magnet where magnetic forces are exerted. Field lines go from North to South.
• Electromagnetism: Magnetic field produced by an electric current. Direction given by Right-Hand Grip Rule.
• Motor Effect: A current-carrying conductor in a magnetic field experiences a force. Direction by Fleming's Left-Hand Rule.

Point 20: Electromagnetic Induction

Generator Effect: Relative movement between a conductor and a magnetic field (or changing magnetic flux linkage) induces an electromotive force (e.m.f.) across the conductor. Direction by Fleming's Right-Hand Rule.

Answer Key: Key Takeaways

1. Mass vs. Weight: Mass is amount of matter (kg), Weight is gravitational force (N).
2. Density: Mass per unit volume (ρ = m/V).
3. Kinematics: Speed (scalar), Velocity (vector), Acceleration (rate of change of velocity).
4. Newton's Laws: Inertia, F=ma, Action-Reaction pairs.
5. Work, Energy, Power: Work (Fd), Energy (KE, GPE), Power (W/t).
6. Pressure: Force per unit area (P = F/A).
7. Moments: Turning effect of a force (F × distance). Principle of Moments for equilibrium.
8. Thermal Expansion: Substances expand when heated, contract when cooled.
9. Heat Transfer: Conduction (solids), Convection (fluids), Radiation (EM waves).
10. Specific Heat Capacity: Energy to change temperature (Q = mcΔT).
11. Specific Latent Heat: Energy to change state (Q = mL).
12. Wave Properties: Transverse vs. Longitudinal, v = fλ.
13. Light Optics: Reflection (i=r), Refraction (Snell's Law), TIR (denser to less dense, angle > critical).
14. Lenses: Converging (convex) focuses light, Diverging (concave) spreads light.
15. Sound Waves: Longitudinal, require medium, speed varies with medium.
16. Electricity Basics: Current (I), Voltage (V), Resistance (R). Ohm's Law: V = IR.
17. Circuits: Series (same current, R_total = sum R), Parallel (same voltage, 1/R_total = sum 1/R).
18. Electrical Power/Energy: P = VI, E = Pt.
19. Magnetism: Magnetic fields, Electromagnetism (current creates field), Motor Effect (force on current in field).
20. Electromagnetic Induction: Changing magnetic flux induces e.m.f. (Generator Effect).