Energy, Space & Forces
Energy Transfer
How thermal energy moves
Heat vs Temperature
These terms are often confused but mean different things:
Thermal Energy (Heat)
The total energy of all particles in an object. Measured in Joules (J). Depends on mass AND temperature.
A bath of warm water has MORE thermal energy than a cup of boiling water!
Temperature
How hot or cold something is. Measured in °C or K. Related to the average kinetic energy of particles.
A cup of boiling water is HOTTER than a bath of warm water!
Three Ways Energy Transfers
Conduction
Particles vibrate and pass energy to neighbours. Mainly in solids. Metals are best conductors.
Spoon in hot tea gets warm
Convection
Hot fluid rises, cold fluid sinks. Creates convection currents. Only in liquids and gases.
Radiator heating a room
Radiation
Infrared waves travel through space. No particles needed! All hot objects emit radiation.
Heat from the Sun reaching Earth
Conservation of Energy
Energy cannot be created or destroyed
The Law of Conservation of Energy
Energy cannot be created or destroyed. It can only be transferred from one store to another, or between objects.
Total energy input = Total energy output
This means the total amount of energy in the Universe is constant! When energy seems to "disappear", it has actually been transferred to another form, often as thermal energy (heat).
Sankey Diagrams
Sankey diagrams show energy transfers. The width of arrows represents the amount of energy:
The thicker the arrow, the more energy. Wasted energy is usually shown going downward or to the right.
Efficiency
Efficiency tells us what fraction of energy is usefully transferred:
Efficiency = Useful Energy Output ÷ Total Energy Input
Efficiency = Useful Power Output ÷ Total Power Input
LED light bulb: ~80-90% efficient. Only 10-20% wasted as heat.
Incandescent bulb: ~5-10% efficient. 90-95% wasted as heat!
States of Matter
Expansion and density changes
Thermal Expansion
When materials are heated, particles vibrate more and take up more space. The material expands.
- • Solids: Expand slightly when heated
- • Liquids: Expand more than solids
- • Gases: Expand the most when heated
Application: Gaps in bridges allow for expansion in summer heat!
Density Changes
When materials expand, the same mass takes up more volume, so density decreases.
Hot water is less dense than cold water, so it rises - this causes convection currents!
The Anomalous Expansion of Water
Water behaves unusually near its freezing point. It's most dense at 4°C, not 0°C!
Ice (0°C)
Least dense - floats!
Water (4°C)
Most dense - sinks
Warm water
Less dense - rises
This is why ice forms on top of ponds, allowing fish to survive underneath!
The Solar System
Our cosmic neighbourhood
Historical Models
Geocentric Model (Ptolemy)
Earth at the centre, everything orbits around Earth.
Incorrect - but believed for 1400+ years
Heliocentric Model (Copernicus)
Sun at the centre, Earth and planets orbit the Sun.
Correct - our current understanding
The Eight Planets
Click on a planet to learn more about it:
Click a planet to see details
Beyond the Solar System
Stars, galaxies, and the Universe
Stars
A star is a massive ball of hot gas that produces its own light and heat through nuclear reactions.
- • Our Sun is a medium-sized star
- • Stars appear small because they are very far away
- • Different colours indicate different temperatures
Galaxies
A galaxy is a huge collection of billions of stars, gas, and dust held together by gravity.
- • Our galaxy is called the Milky Way
- • Contains 100-400 billion stars
- • Different shapes: spiral, elliptical, irregular
Scale of the Universe
A light year is the distance light travels in one year (about 9.5 trillion km!)
Gravity & Orbits
The force that shapes the cosmos
Weight vs Mass
Mass
The amount of matter in an object. Never changes. Measured in kilograms (kg).
Weight
The force of gravity on an object. Changes with location. Measured in Newtons (N).
Weight (N) = Mass (kg) × Gravitational Field Strength (N/kg)
On Earth, g ≈ 10 N/kg
Orbits & Satellites
Gravity keeps objects in orbit. An orbit is a balance between forward motion and gravitational pull.
Moons: Natural satellites that orbit planets. Caused by gravity.
Artificial Satellites: Human-made objects orbiting Earth. Used for communication, GPS, weather monitoring.
Magnetism
Magnetic fields and forces
Magnetic Poles
All magnets have two poles - a north pole and a south pole. You cannot have a single pole on its own.
North pole points North when free to rotate
South pole is attracted to the North
Like poles repel (N-N or S-S) | Unlike poles attract (N-S)
Magnetic Fields
A magnetic field is the region around a magnet where magnetic forces can be detected.
- • Field lines go from North to South outside the magnet
- • The closer the lines, the stronger the field
- • Compass needles point along field lines
- • The Earth has a magnetic field - that's why compasses work!
Light & Reflection
How light travels and bounces
Properties of Light
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1Light travels in straight lines - this is why we get sharp shadows.
-
2Light can travel through a vacuum (empty space) - it doesn't need a medium.
-
3Light travels very fast - about 300,000 km/s (300 million m/s).
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4Light is a transverse wave - the vibrations are perpendicular to the direction of travel.
The Law of Reflection
Key Points
- • Angle of incidence = Angle of reflection
- • The normal is an imaginary line perpendicular to the mirror surface
- • Angles are always measured from the normal, not from the mirror
- • Smooth surfaces give specular reflection (clear images)
- • Rough surfaces give diffuse reflection (scattered light)
Practice Questions
Test your understanding
Energy Transfer [3 marks]
Explain how a metal spoon becomes hot when placed in a bowl of hot soup. Name the process involved.
Answer:
Process: Conduction [1 mark]
Explanation: The particles in the hot soup vibrate more and collide with particles in the metal spoon. Energy is transferred from particle to particle through the spoon. [2 marks]
Metals are good conductors because they have free electrons that can move through the metal and transfer energy quickly.
Efficiency Calculation [3 marks]
A motor takes in 500 J of electrical energy. It outputs 350 J of useful kinetic energy. The rest is wasted as heat.
a) Calculate the efficiency of the motor. [2 marks]
b) How much energy is wasted? [1 mark]
Mark Scheme:
a) Efficiency calculation:
Efficiency = Useful energy out ÷ Total energy in [1 mark for formula]
Efficiency = 350 ÷ 500 = 0.7 or 70% [1 mark for correct answer]
b) Wasted energy:
Wasted = 500 - 350 = 150 J [1 mark]
Solar System [3 marks]
a) Name the four planets closest to the Sun in order. [2 marks]
b) Why is Pluto no longer classified as a planet? [1 mark]
Answer:
a) Inner planets: Mercury, Venus, Earth, Mars [2 marks - 1 mark for correct order, 1 mark for correct planets]
b) Pluto is now classified as a dwarf planet because: It has not cleared its orbit of other objects (it shares its orbit with other bodies in the Kuiper Belt). [1 mark]
Reflection [3 marks]
A ray of light hits a mirror at an angle of 40° to the normal.
a) What is the angle of reflection? [1 mark]
b) What is the angle between the incident ray and the reflected ray? [2 marks]
Answer:
a) Angle of reflection = 40° [1 mark]
By the law of reflection: angle of incidence = angle of reflection
b) Angle between rays = 40° + 40° = 80° [2 marks]
The total angle is the sum of the incident and reflected angles.
Year 8 Science Complete!
You've finished Year 8. Continue your journey with Year 9 or review previous years.