Conservation of Energy
Section: Physics | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What Does "Conserved" Mean?
In everyday language, "conserve energy" usually means use less energy (turn off lights, unplug devices, walk instead of driving). However, in physics, "conserved" has a completely different meaning.
- In physics, saying energy is "conserved" means the total quantity of energy stays the same – it cannot be created or destroyed, only transferred or transformed
- When energy is stored, changed, transferred, or even dissipated, the total amount stays exactly the same
The Law of Conservation of Energy
Energy cannot be created or destroyed, only transferred or transformed from one form to another. This is one of the most fundamental laws in physics, and it applies to all systems and all situations.
- Cannot be created – you can't make energy appear from nothing
- Cannot be destroyed – energy never disappears completely
- Can be transferred – energy can move from one object to another
- Can be transformed – energy can change from one type to another
Examples of Energy Transformation
- A battery doesn't "create" electrical energy – it stores chemical energy and transforms it to electrical energy
- When a ball stops bouncing, the kinetic energy isn't "destroyed" – it transforms into thermal energy and sound
- When you eat food, your body doesn't "destroy" the chemical energy – it transforms it into kinetic energy, thermal energy, etc.
What is a System?
A system is something that has been chosen to be studied, especially in terms of energy changes (inputs and outputs).
- Examples of systems:
- an electric lamp (input: electrical energy, outputs: light + thermal energy)
- a car engine (input: chemical energy in fuel, outputs: kinetic + thermal + sound energy)
- your body (input: chemical energy in food, outputs: kinetic + thermal energy)
- a phone (input: electrical energy, outputs: light + sound + thermal energy)
- The total energy output can never be greater than the total energy input in any system, because energy cannot be created
Energy Input and Output
- Energy input is the energy supplied to a system (energy going in)
- Energy output is the energy coming out of a system (energy going out)
- According to the law of conservation of energy, the total energy input always equals the total energy output. This is true for every system, every time
- Total Energy Input = Total Energy Output
Useful vs Wasted Energy
Not all energy output is useful to us. We can divide energy output into two categories: useful energy (does what we want) and wasted energy (doesn't help us achieve our goal).
| Type | Description | Examples |
|---|---|---|
| Useful Energy | Energy output that does what we want | Light from a lamp, sound from a speaker, kinetic energy from a car engine |
| Wasted Energy | Energy output that doesn't help us achieve our goal | Thermal energy from a lamp, sound from a car engine, thermal energy from a phone |
- "Wasted" energy is not destroyed. It's still energy – it's just not useful for what we want to do. Usually, wasted energy becomes thermal energy that spreads out (dissipates) into the surroundings
Energy Diagrams (Sankey Diagrams)
Energy diagrams (also called Sankey diagrams) show how energy flows through a system, displaying how much energy goes in, how much comes out, what types of energy are involved, and which energy is useful versus wasted.
- The width of the arrows shows how much energy (wider = more energy)
- The input arrow goes in from the left
- The output arrows go out to the right
- All output arrows added together equal the input arrow (conservation)
Worked Example: Electric Lamp Energy Flow
Museli has an electric lamp with an energy input of 100 J of electrical energy, producing 10 J of light energy and 90 J of thermal energy, and wants to verify that energy is conserved.
- Step 1: write down the input. Input = 100 J of electrical energy
- Step 2: write down the outputs. 10 J of light energy (useful) + 90 J of thermal energy (wasted)
- Step 3: check conservation. Input (100 J) = Output (10 J + 90 J = 100 J) ✓ – energy is conserved, but the lamp is only 10% efficient since most energy (90%) is wasted as heat
Worked Example: Car Engine Energy Flow
Muumbe's car engine receives 100% chemical energy from fuel, producing 25% kinetic energy, 35% thermal and sound energy, and 40% chemical energy in exhaust gas, and he wants to verify conservation.
- Step 1: write down the input. Input = 100% chemical energy in fuel
- Step 2: write down the outputs. 25% kinetic energy (useful – makes the car move) + 35% thermal and sound energy (wasted) + 40% chemical energy in exhaust gas (wasted)
- Step 3: check conservation. 100% = 25% + 35% + 40% = 100% ✓ – car engines are only about 25% efficient, which is why cars get hot and make noise
Dissipated Energy
- Dissipated energy is energy that spreads out into the surroundings and becomes less useful, usually in the form of thermal energy that can't easily be used again
- When energy dissipates, it spreads into the surroundings, becomes less concentrated, and becomes harder or impossible to use
Worked Example: Hot Cup of Tea
Muchindu leaves a hot cup of tea on a table, and after some time the tea cools down.
- Step 1: track the thermal energy. It gradually transfers to the cooler surroundings (air, table)
- Step 2: describe what happens. The thermal energy dissipates – spreads out into the room
- Step 3: conclude. The energy hasn't been destroyed – it's just spread out so much that it can't be used anymore. Energy is still conserved, just redistributed
Why Wasted Energy Matters
Even though energy is conserved, we still want to reduce wasted and dissipated energy.
- We have to pay for energy (electricity, fuel)
- Producing energy can damage the environment
- Wasted energy is useless to us even though it still exists
- More efficient devices save money and resources
Energy Efficiency
- Efficiency is a measure of how good a device is at transforming input energy into useful output energy. A more efficient device wastes less energy and produces more useful output
- Formula: Efficiency = (Useful energy output ÷ Total energy input) × 100%, or using power: Efficiency = (Useful power output ÷ Total power input) × 100%
- Understanding efficiency values:
- efficiency is given as a percentage (%)
- 100% = perfectly efficient (impossible in reality)
- 0% = completely useless (all energy wasted)
- higher percentage = more efficient = less waste
- no device can be more than 100% efficient (that would mean creating energy)
Efficiency of Common Devices
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