Energy: Forms and Transformations
Discover the different forms of energy, how energy changes from one form to another, the law of conservation of energy, and why no energy transfer is 100% efficient.
What is Energy?
Energy is the capacity to do work or cause change. It is measured in joules (J). Energy cannot be created or destroyed — it can only be transferred from one object to another or transformed from one form to another.
Forms of Energy
Energy exists in many different forms. The two broad categories are kinetic energy (energy of motion) and potential energy (stored energy).
Kinetic Energy
Energy of movement. Any object that is moving has kinetic energy. The faster it moves and the heavier it is, the more kinetic energy it has.
Examples: Running person, moving car, flowing water, wind
Gravitational Potential Energy
Energy stored in an object due to its height above the ground. The higher the object and the greater its mass, the more GPE it has.
Examples: Ball held above ground, water at top of waterfall, skier at top of slope
Chemical Energy
Energy stored in the chemical bonds of substances. Released during chemical reactions (e.g. combustion, digestion).
Examples: Food, fuels (petrol, coal, gas), batteries
Elastic Potential Energy
Energy stored in an object that has been stretched or compressed. When released, it converts to kinetic energy.
Examples: Stretched rubber band, compressed spring, drawn bow
Thermal Energy (Heat)
The internal energy of a substance due to the movement of its particles. Hotter objects have faster-moving particles and more thermal energy.
Examples: Hot cup of tea, the Sun, friction-generated heat
Electrical Energy
Energy carried by moving electric charges (current) through conductors. Easily transformed into other forms.
Examples: Electricity in wires, lightning, static electricity
Light Energy (Radiant)
Energy carried by electromagnetic waves. Can travel through a vacuum (no medium needed). Includes visible light, UV, infrared, etc.
Examples: Sunlight, laser, light bulb
Sound Energy
Energy carried by vibrations of particles in a medium (solid, liquid, or gas). Cannot travel through a vacuum.
Examples: Music, speech, thunder, vibrating guitar string
Energy Transformations
An energy transformation (or conversion) is when energy changes from one form to another. Most devices and processes involve a chain of energy transformations.
Energy Transformation Chains
Torch (Flashlight):
Car Engine:
Solar Panel:
Rollercoaster (top to bottom):
Law of Conservation of Energy
The law of conservation of energy states that energy cannot be created or destroyed — it can only be transferred or transformed from one form to another. The total amount of energy in a closed system remains constant.
Efficiency
No energy transfer is 100% efficient. Some energy is always wasted, usually as thermal energy (heat) that dissipates into the surroundings. Efficiency measures how much of the input energy is converted to useful output energy.
Efficiency Formula
Efficiency = (Useful Energy Output ÷ Total Energy Input) × 100%
Efficiency is always less than 100% in real-world devices.
| Device | Approx. Efficiency | Main Waste Energy |
|---|---|---|
| LED light bulb | 80-90% | Thermal (heat) |
| Incandescent light bulb | 5-10% | Thermal (heat) |
| Car engine (petrol) | 25-30% | Thermal, sound |
| Electric motor | 85-95% | Thermal, sound |
| Solar panel | 15-25% | Thermal, reflected light |
Key Vocabulary
Joule (J)
The SI unit of energy. 1 joule = the energy needed to push with 1 Newton over 1 metre.
Energy Transfer
Energy moving from one object or system to another (e.g. heat transferring from a flame to a pot).
Energy Transformation
Energy changing from one form to another within a system (e.g. chemical to kinetic in a car).
Efficiency
The percentage of input energy that is converted to useful output energy. Higher is better.
Dissipation
When wasted energy (usually heat) spreads out into the surroundings and becomes less useful.
Conservation of Energy
Energy cannot be created or destroyed, only transferred or transformed. Total energy in a closed system is constant.
Worked Examples
Describe the energy transformations when you eat food and then run.
Step 1: Food contains chemical energy stored in chemical bonds.
Step 2: During digestion and cellular respiration, chemical energy is transformed into kinetic energy (movement of muscles) and thermal energy (body heat).
Answer: Chemical energy (food) → Kinetic energy (running) + Thermal energy (body heat) + Sound energy (footsteps)
A light bulb uses 100 J of electrical energy and produces 10 J of light energy. Calculate its efficiency.
Step 1: Write the formula: Efficiency = (Useful Output ÷ Total Input) × 100%
Step 2: Substitute: Efficiency = (10 ÷ 100) × 100%
Step 3: Calculate: Efficiency = 0.1 × 100% = 10%
Answer: The light bulb is 10% efficient. The remaining 90 J is wasted as thermal energy (heat).
A ball is dropped from a height. Describe the energy changes as it falls.
Step 1: At the top, the ball has maximum gravitational potential energy (GPE) and zero kinetic energy.
Step 2: As it falls, GPE is transformed into kinetic energy (KE). Some energy is lost to thermal energy due to air resistance.
Step 3: Just before hitting the ground, the ball has maximum KE and minimum GPE.
Answer: GPE → KE + Thermal (air resistance) + Sound (on impact)
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
A ball sitting at the top of a hill has gravitational potential energy. As it rolls down, what is the main energy transformation?
Question 2
A toaster uses 2000 J of electrical energy. It converts 1600 J to useful thermal energy for toasting. What is its efficiency?
Question 3
The law of conservation of energy states that:
Question 4
What type of energy is stored in a stretched rubber band?
Question 5
In most energy transformations, the "wasted" energy is usually lost as:
Key Concepts Summary
- ●Energy is measured in joules (J) and exists in many forms: kinetic, gravitational PE, chemical, elastic PE, thermal, electrical, light, sound.
- ●Energy transformations change energy from one form to another; energy transfers move energy between objects.
- ●The law of conservation of energy: energy cannot be created or destroyed.
- ●Efficiency = (useful energy out ÷ total energy in) × 100%. No device is 100% efficient.
- ●Most wasted energy is lost as thermal energy (heat) that dissipates into the surroundings.