Energy Transformation
Energy transformation is the change of energy from one form to another, such as chemical to kinetic or electrical to light and heat.
Definition
An energy transformation happens when energy changes from one kind into another: energy in a battery becomes light, energy in food becomes running, energy in a stretched spring becomes motion. Every such conversion, whether potential to kinetic, chemical to thermal, or electrical to light, obeys conservation of energy so that the total stays constant, but most transformations are not perfectly efficient, and some energy ends up as thermal energy that spreads into the surroundings. Transformations are governed by the first law (total energy conserved) and constrained by the second law (entropy of an isolated system cannot decrease). Efficiency is $\eta = E_{\text{useful}}/E_{\text{input}}$, and for a heat engine operating between reservoirs at $T_h$ and $T_c$ the Carnot limit is $\eta_{\max} = 1 - T_c/T_h$. Conversions of ordered energy (mechanical, electrical) to thermal energy can approach $100\%$, but the reverse is fundamentally limited.
Example
A toaster turns electrical energy into heat and a little light, and a flashlight turns chemical energy in the battery into light. A $60\ \text{W}$ incandescent bulb converts about $3\ \text{W}$ into visible light and roughly $57\ \text{W}$ into heat, while an LED giving the same light uses about $9\ \text{W}$ total, so much less is wasted. A hydroelectric plant that drops $500\ \text{kg}$ of water per second through $80\ \text{m}$ has available power $mgh/t = 500 \times 9.8 \times 80 = 392{,}000\ \text{W}$. At $90\%$ turbine-generator efficiency it delivers about $353\ \text{kW}$ of electrical power, and the rest becomes turbulence and heat.
Key Insight
Almost every transformation gives off some heat as a side effect, which is why phones, laptops, and light bulbs get warm. Follow the chain: sunlight to plant (chemical), plant to you (chemical), you to bicycle (kinetic), bicycle brakes to air (thermal); almost all the energy on Earth started as sunlight. Not all joules are equal, though. A joule of electricity can become a joule of heat easily, but a joule of room-temperature heat can never fully become electricity, so entropy, not energy, is what actually gets used up.