Physics (KS3)
UK Key Stage 3 physics: forces and motion, energy, waves, light and sound, electricity, magnetism and space.
Ämne: Fysik · Nivå: Högstadium (13–15) · 399 kort
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- The unit of force is the newton (N), named after Sir Isaac Newton.
- Contact forces include friction, normal contact, tension, air resistance and upthrust. They only act when objects are touching.
- Non-contact forces — gravity, magnetic and electrostatic — act at a distance without the objects touching.
- When all forces on an object cancel out, the forces are balanced and the object is in equilibrium.
- Newton's first law: an object stays still or keeps moving at constant velocity unless an unbalanced force acts on it.
- Weight is calculated by W = m × g, where g is about 10 N/kg on Earth. A 5 kg mass weighs roughly 50 N.
- Speed = distance ÷ time, measured in metres per second (m/s).
- On a distance-time graph, the gradient (steepness) tells you the speed. A flat line means the object is stationary.
- Velocity is speed with a direction — for example, 5 m/s due north. Acceleration is the change in velocity per second.
- Energy is measured in joules (J). Common stores include kinetic, gravitational, elastic, chemical, thermal, nuclear, magnetic and electrostatic.
- Conservation of energy: energy cannot be created or destroyed, only transferred between stores.
- Efficiency = useful energy out ÷ total energy in. Multiply by 100 to express it as a percentage.
- Energy can be transferred by heating, by mechanical work, by electricity or by waves.
- Conduction happens in solids: vibrating particles pass kinetic energy to their neighbours, gradually heating the material.
- Convection happens in fluids (liquids and gases): warm fluid expands, becomes less dense and rises, while cooler fluid sinks.
- Thermal radiation is carried by infrared waves. Unlike conduction and convection, it can travel through a vacuum — that's how heat reaches Earth from the Sun.
- Power is the rate of energy transfer: power (W) = energy (J) ÷ time (s). One watt is one joule per second.
- In transverse waves the vibrations are perpendicular to the direction of travel — examples include light and ripples on water.
- Sound is a longitudinal wave: particles vibrate back and forth along the direction the wave is moving.
- Wave speed = frequency × wavelength. Frequency is measured in hertz (Hz), and one hertz is one wave per second.
- Light travels in straight lines. Its speed in a vacuum is about 300 000 000 m/s (3 × 10⸠m/s).
- Electric current is measured in amperes (A) using an ammeter, which is connected in series with the component.
- Magnetic field lines run from north to south outside a magnet. Like poles repel each other; unlike poles attract.
- Our Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
- Earth's axis is tilted about 23.5°. That tilt — not its distance from the Sun — is what gives us seasons.
- Hooke's law: the extension of a spring is proportional to the force applied, so F = k × e, where k is the spring constant in N/m.
- The elastic limit is the point past which a spring no longer returns to its original length; beyond it, Hooke's law no longer holds.
- The moment of a force is its turning effect: moment = force × perpendicular distance from the pivot, measured in newton-metres (N·m).
- Principle of moments: a balanced object has its total clockwise moments equal to its total anticlockwise moments around the pivot.
- Newton's second law (qualitative): a bigger resultant force makes an object accelerate more, while a bigger mass needs more force to give the same acceleration (F = m × a).
- Newton's third law: when one object pushes or pulls another, the second object pushes or pulls back with an equal and opposite force. The pair always act on different objects.
- Stopping distance of a car = thinking distance + braking distance. Tiredness, alcohol or distraction increase thinking distance; worn tyres or wet roads increase braking distance.
- Terminal velocity is reached when air resistance balances weight: a skydiver stops accelerating and falls at a steady speed. Opening a parachute lowers terminal velocity dramatically.
- Pressure = force ÷ area. The unit is the pascal (Pa), equal to one newton per square metre (1 N/m²).
- Pressure in a liquid grows with depth and with the liquid's density. That is why a dam is built thicker at the bottom than at the top.
- Atmospheric pressure decreases with altitude because there is less air pushing down from above. At sea level it is about 100 000 Pa.
- Gravitational potential energy (GPE) = mass × gravitational field strength × height, often written GPE = m × g × h.
- Kinetic energy depends on mass and speed: KE = ½ × m × v². Doubling speed gives four times the kinetic energy.
- A Sankey diagram uses arrows whose widths show how much energy is transferred usefully and how much is wasted, usually as heat or sound.
- Water has a high specific heat capacity, which is why it heats up slowly and cools down slowly — useful for hot water bottles and central heating systems.
- Houses are insulated by loft insulation, cavity wall insulation, double glazing and draught-proofing — each one slows a different type of heat loss.
- Renewable energy resources include wind, solar, hydro, tidal, wave, geothermal and biomass. They will not run out on human timescales.
- Non-renewable resources — coal, oil, natural gas and nuclear fuel — give reliable power on demand but will eventually run out and release pollution or radioactive waste.
- In the National Grid, step-up transformers raise the voltage for long-distance transmission to reduce energy lost as heat in the cables, and step-down transformers lower it again for homes.
- The wave equation: speed = frequency × wavelength (v = f × λ). The period T is 1 ÷ f, the time for one complete wave.
- When light passes from one transparent material into another it usually bends — this is refraction. It happens because light changes speed at the boundary.
- Ultrasound (above 20 000 Hz) is used for medical imaging of unborn babies, in industry to check welds, and in cleaning delicate items.
- A UK three-pin plug uses three colour-coded wires: brown for live, blue for neutral and green-and-yellow stripes for earth.
- A light year is the distance light travels in one year — about 9.46 × 10¹ⵠm. Even our nearest star is over four light years away.
- Evidence for the Big Bang includes the redshift of distant galaxies (showing the universe is expanding) and the cosmic microwave background radiation that fills all of space.