Energy Conservation ⚡️
Energy conservation means that the total energy in a closed system stays constant. It’s like a big, invisible “energy bank” that never loses money, only changes form.
What is Energy?
Energy is the ability to do work. It comes in many forms: kinetic, potential, thermal, electrical, chemical, etc.
Why is Conservation Important?
- It lets us predict how systems behave.
- It helps design efficient machines.
- It explains why some energy is “lost” as heat.
Efficiency 🏗️
Definition
Efficiency, denoted by \$η\$, is the ratio of useful output energy to input energy:
\$η = \\frac{E{out}}{E{in}}\$
It is usually expressed as a percentage: \$η\\% = η \\times 100\$.
Analogy
Think of a water pump. If you pour 10 litres of water into the pump but only 7 litres come out, the pump’s efficiency is 70 %.
Solving Problems with Efficiency ⚙️
Step‑by‑Step Guide
- Identify the input energy (\$E_{in}\$).
- Identify the useful output energy (\$E_{out}\$).
- Insert into \$η = \\frac{E{out}}{E{in}}\$.
- Calculate \$η\$ and convert to a percentage if required.
- Check if the result makes sense (e.g., \$0\\leq η\\leq1\$).
Example Problem
⚡️ A light bulb receives 100 J of electrical energy. Only 60 J is converted into visible light; the rest becomes heat.
What is the bulb’s efficiency?
| Energy Type | Energy (J) |
|---|
| Input (electrical) | 100 |
| Output (visible light) | 60 |
| Efficiency | \$\\frac{60}{100}=0.60\\;\\text{or}\\;60\\%\$ |
??
The bulb is 60 % efficient. The remaining 40 % is lost as heat.
Common Mistakes to Avoid ❌
- Mixing units – always use the same units for \$E{in}\$ and \$E{out}\$.
- Forgetting that \$η\$ is a fraction, not a percentage, until you multiply by 100.
- Assuming \$η\$ can be greater than 1 – that would mean you created energy!
Quick Quiz 🧠
- A car engine uses 200 kJ of fuel energy and delivers 120 kJ of mechanical work. What is the engine’s efficiency?
- If a solar panel receives 500 W of sunlight and outputs 350 W of electricity, what is its efficiency?
Answers:
- Efficiency = \$\\frac{120}{200}=0.60\\;\\text{or}\\;60\\%\$.
- Efficiency = \$\\frac{350}{500}=0.70\\;\\text{or}\\;70\\%\$.
Take‑Away Points 📌
- Energy is conserved, but its form can change.
- Efficiency tells us how well a device uses the energy it receives.
- Always check units and remember \$0\\leq η\\leq1\$.