💡 Energy
Before You Begin
- Physics basics — Newton's laws, force and motion
📌 After this unit, you'll be able to
- Identify at least 5 forms of energy and give a real-world example of each
- Explain the Law of Conservation of Energy using a roller coaster or pendulum
- State the First and Second Laws of Thermodynamics in plain language
- Compare renewable and non-renewable energy sources and their trade-offs
🌱 Why study energy?
Think of it intuitively
Energy never disappears, it only changes form. When a ball falls: potential energy becomes kinetic energy, then heat and sound. Saying electricity is consumed is technically wrong; it is converted into light, heat, or motion. The universe total energy stays constant.
Energy is the capacity to cause change. Every process in the universe — from a heartbeat to a nuclear explosion — involves energy transformations. The Law of Conservation of Energy is one of the most fundamental laws in all of science, and the Second Law of Thermodynamics explains why perfect efficiency is physically impossible. In an era of climate change, understanding renewable energy is no longer optional — it's essential literacy.
⚡ 30-second summary
- Forms of energy: kinetic, potential, thermal, chemical, electrical, light, nuclear
- Conservation of energy: Energy changes form but total amount stays constant (1st Law)
- 2nd Law: Energy flows from useful to less useful (heat). 100% efficiency is impossible
- Work = Force × distance (J). Power = Work / time (W)
- Renewables: solar (photoelectric), wind (kinetic→electrical), hydrogen (fuel cells)
1. What Is Energy?
Energy: The ability to do work. Measured in joules (J).
- Kinetic Energy (KE): Energy of motion → KE = ½mv² (m = mass in kg, v = velocity in m/s)
- Potential Energy (PE): Stored energy due to position → PE = mgh (m = mass, g = 9.8 m/s², h = height in m)
- Mechanical Energy: KE + PE = constant (in absence of friction)
- Thermal Energy: Kinetic energy of particles in a substance (related to temperature)
- Chemical Energy: Energy stored in chemical bonds (food, fuel, batteries)
- Electromagnetic Energy: Energy carried by light and other electromagnetic waves
- Nuclear Energy: Energy stored in atomic nuclei; released by fission or fusion
2. Energy Transformations
Energy constantly converts from one form to another. Common conversions:
- Solar panel: Light energy → Electrical energy
- Car engine: Chemical energy (fuel) → Mechanical energy + Thermal energy
- Hydroelectric dam: Potential energy (water) → Kinetic energy → Electrical energy
- Photosynthesis: Light energy → Chemical energy (glucose)
- Cellular respiration: Chemical energy (glucose) → ATP + Thermal energy
3. Conservation of Energy AP Exam
Law of Conservation of Energy: Energy cannot be created or destroyed — only converted from one form to another. The total energy of an isolated system remains constant.
Mechanical energy conservation: When friction is negligible, KE + PE = constant throughout an object's motion.
Example: A ball of mass 2 kg is dropped from a height of 5 m (g = 10 m/s²).
PE at top = mgh = 2 × 10 × 5 = 100 J
At bottom, all PE converts to KE: KE = 100 J → v = √(2 × KE / m) = √(100) ≈ 10 m/s
PE at top = mgh = 2 × 10 × 5 = 100 J
At bottom, all PE converts to KE: KE = 100 J → v = √(2 × KE / m) = √(100) ≈ 10 m/s
4. Laws of Thermodynamics AP Exam
- 1st Law (Conservation): Energy is conserved; ΔU = Q − W (change in internal energy = heat added minus work done by system).
- 2nd Law (Entropy): Heat flows spontaneously from hot → cold. Entropy (disorder) of an isolated system increases over time.
- Key implication: No engine is 100% efficient — some energy is always lost as heat.
5. Renewable vs. Non-Renewable Energy
| Renewable | Non-Renewable |
|---|---|
| Solar, Wind, Hydropower | Coal, Oil, Natural Gas |
| Geothermal, Tidal, Biomass | Uranium (nuclear fission) |
| Replenished naturally; low CO₂ | Finite; emit greenhouse gases |
Practice Questions
Q1: A 3 kg object moves at 4 m/s. What is its kinetic energy?
Answer: KE = ½mv² = ½ × 3 × 4² = ½ × 3 × 16 = 24 J
Answer: KE = ½mv² = ½ × 3 × 4² = ½ × 3 × 16 = 24 J
Q2: A roller coaster car has 8,000 J of potential energy at the top of a hill. Assuming no friction, what is its kinetic energy at the bottom?
Answer: By conservation of energy, KE = 8,000 J (all PE converts to KE).
Answer: By conservation of energy, KE = 8,000 J (all PE converts to KE).
💡 Study Tip: Always include units in energy calculations. KE = ½mv² gives joules only if mass is in kg and velocity is in m/s. For conservation problems, set up a table: PEinitial + KEinitial = PEfinal + KEfinal. This framework solves most energy problems systematically.
Pre-Test Checklist
- Law of conservation of energy: energy changes form, total amount stays constant
- Heat always flows from hot to cold spontaneously (not the reverse)
- Wave equation: v = fλ (speed = frequency × wavelength)
- Renewable vs fossil fuels: CO₂ emissions difference
Spaced Repetition — Ebbinghaus Curve
Review this material at increasing intervals to commit it to long-term memory.
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