Physics — Force, Energy, Light & Electricity
1. Matter — Density
- Density = Mass / Volume. Unit: kg/m³ or g/cm³.
- Denser objects SINK. Less dense objects FLOAT. Ice floats because its density (0.92 g/cm³) < water (1 g/cm³).
2. Force and Pressure
Turning Effect (Moment of Force)
Moment = Force × Perpendicular distance from pivot. Unit: Nm. 'The LONGER the lever arm, the GREATER the turning effect. This is why a longer spanner is easier to turn a stubborn nut.'
Pressure
Pressure = Force / Area. Unit: Pascal (Pa). 1 Pa = 1 N/m².
- A sharp KNIFE cuts better (SMALL area → HIGH pressure)
- Snowshoes prevent sinking (LARGE area → LOW pressure)
Liquid Pressure
Increases with DEPTH. The same at the SAME depth in all directions. 'A dam is THICKER at the BOTTOM — because water pressure is GREATER at greater depth.'
Atmospheric Pressure
The WEIGHT of the column of air above us. Decreases with altitude. Measured by BAROMETER.
3. Energy — Kinetic and Potential
Kinetic Energy (KE)
Energy of MOTION. KE = ½mv². 'Doubling the SPEED QUADRUPLES the kinetic energy.'
Potential Energy (PE)
Energy of POSITION. PE = mgh (gravitational). 'Water stored in a dam has potential energy — converted to kinetic as it falls, then to electrical in turbines.'
Energy Transformation and Conservation
Energy CHANGES FORM but is NEVER created or destroyed. 'The total energy of a closed system is CONSTANT.'
4. Light — Refraction and Dispersion
Refraction
Light BENDS when it passes from one transparent medium to another (speed changes). Air → Water: light bends TOWARD the normal. 'A stick partly in water looks BENT — because of refraction.'
Lenses
| Lens | Shape | Effect on Parallel Rays |
|---|---|---|
| Convex (Converging) | Thicker in middle | CONVERGES to a FOCUS |
| Concave (Diverging) | Thinner in middle | DIVERGES (spreads out) |
Dispersion — The Rainbow
White light is a MIXTURE of SEVEN colours. When it passes through a PRISM, the colours SEPARATE (disperse). 'Violet refracts the MOST. Red refracts the LEAST.' ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet.
5. Electricity
Static Electricity
Charges at REST. Rubbing a balloon → picks up paper bits. 'Like charges REPEL. Unlike charges ATTRACT.'
Current Electricity — Ohm's Law
V = I × R (Voltage = Current × Resistance). V in volts (V). I in amperes (A). R in ohms (Ω). 'Resistance opposes the flow of current. Good CONDUCTORS (copper) have LOW resistance.'
Household Wiring
Series: SINGLE path. One off → all off. Parallel: MULTIPLE paths. Each device gets full voltage. 'Homes are wired in PARALLEL. Each appliance works INDEPENDENTLY.'
Safety
- Fuse: 'Sacrificial' wire. Melts if current is TOO HIGH. Breaks circuit. Prevents fire.
- Earthing: Connects appliance body to the EARTH. Leakage current flows to ground instead of through YOU.
