Energy Transformations and Conservation
Grade 10 · Science · Study Guide
Explore how energy changes from one form to another and why it can never truly be created or destroyed.
Energy is the capacity to do work or transfer heat. One of the most fundamental laws in all of science is the Law of Conservation of Energy: energy cannot be created or destroyed, only transformed from one form to another. The total energy in a closed system always remains constant.
Common forms of energy include kinetic energy (energy of motion: KE = ½mv²), potential energy (stored energy based on position or state: gravitational PE = mgh), thermal energy (internal energy due to particle motion), chemical energy (stored in molecular bonds), electrical energy, and radiant energy (light and other electromagnetic radiation). Every time energy is transformed, some useful energy is lost as heat (thermal energy) — this is why machines are never 100% efficient.
The efficiency of an energy conversion is calculated as: efficiency = (useful energy output / total energy input) × 100%. In real-world systems, friction and resistance always reduce efficiency. The Law of Conservation of Energy tells us where that 'lost' energy went — it wasn't destroyed, just converted to a less useful form (usually heat). Power describes how quickly energy is transferred or transformed: Power = Work / Time, measured in Watts (W = J/s).
| Device/Process | Energy Input | Useful Energy Output | Wasted Energy |
|---|---|---|---|
| Light bulb (incandescent) | Electrical | Light (radiant) | Heat |
| Car engine | Chemical (fuel) | Kinetic | Heat, sound |
| Solar panel | Radiant (light) | Electrical | Heat |
| Falling object | Gravitational PE | Kinetic | Sound (on impact) |
| Human body | Chemical (food) | Kinetic + thermal | Heat |
Key terms
- Law of Conservation of Energy
- Energy cannot be created or destroyed; it can only be transformed from one form to another.
- Kinetic energy (KE)
- The energy an object has due to its motion: KE = ½mv², where m is mass and v is velocity.
- Gravitational potential energy (GPE)
- Energy stored in an object due to its height above a reference point: GPE = mgh.
- Efficiency
- The ratio of useful energy output to total energy input, expressed as a percentage.
- Power
- The rate at which energy is transferred or work is done: Power = Work / Time (Watts).
- Thermal energy
- The total internal energy of particles in a substance due to their random motion; commonly called heat.
Flashcards
- Joule (J)
- The SI unit of energy and work; 1 J = 1 kg·m²/s².
- Watt (W)
- The SI unit of power; 1 W = 1 J/s (1 Joule of energy transferred per second).
- Mechanical energy
- The sum of kinetic energy and potential energy in a system.
- Chemical energy
- Energy stored in chemical bonds; released during chemical reactions (e.g., food, fuel, batteries).
- Elastic potential energy
- Energy stored in a stretched or compressed elastic material (e.g., a spring or rubber band).
- Energy efficiency
- (Useful output / Total input) × 100%; no real device is 100% efficient due to energy losses as heat.
Practice questions
A 2 kg ball moving at 4 m/s has how much kinetic energy?
- 8 J
- 16 J
- 32 J
- 4 J
Show answer
Answer: 16 J
KE = ½mv² = ½ × 2 × (4²) = ½ × 2 × 16 = 16 J.
According to the Law of Conservation of Energy, energy can be destroyed when it is converted to heat.
Show answer
Answer: False
Energy is never destroyed. The Law of Conservation of Energy states that energy simply changes form — what appears to be 'lost' energy has been converted to thermal energy (heat) or another form.
The formula for gravitational potential energy is GPE = m × g × ___.
Show answer
Answer: h
GPE = mgh, where m = mass (kg), g = gravitational acceleration (~9.8 m/s² on Earth), and h = height above a reference point (m).
A machine uses 400 J of input energy but only does 100 J of useful work. What is its efficiency?
- 25%
- 40%
- 75%
- 100%
Show answer
Answer: 25%
Efficiency = (useful output / total input) × 100% = (100 / 400) × 100% = 25%. The other 75% was wasted, mostly as heat.
Power is measured in Watts, where 1 Watt equals 1 Joule per second.
Show answer
Answer: True
Power = Work / Time. 1 Watt = 1 Joule per second. A 60-Watt light bulb uses 60 Joules of electrical energy every second.
Fun facts
- An incandescent light bulb converts only about 5% of electrical energy into visible light — the rest is wasted as heat!
- The human body is about 25% efficient as a mechanical engine — most of the energy from food is released as heat, which is why you warm up when you exercise.
- Lightning converts electrical energy into light, heat, and sound in milliseconds — a single bolt contains about 1 billion joules of energy.
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