Conservation of Energy and Dissipation

In this lesson you will learn about the principle of conservation of energy, what energy dissipation is, and how to reduce unwanted energy transfers. This is a key part of AQA GCSE Combined Science Trilogy (8464), Section 6.1.

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The Principle of Conservation of Energy

Energy cannot be created or destroyed — it can only be transferred from one store to another.

This is one of the most important principles in physics. It applies to every process, in every system, without exception.

In a closed system (one where no energy or matter enters or leaves), the total energy remains constant. Energy may transfer between stores, but the total is always the same.

Example: A Swinging Pendulum

graph LR
    A["GPE store\n(top of swing)"] -->|"Mechanically"| B["KE store\n(bottom of swing)"]
    B -->|"Mechanically"| A
    B -->|"By heating (friction/air resistance)"| C["Internal store\n(surroundings)"]
    style C fill:#ffcccc,stroke:#cc0000

At the top of the swing: maximum GPE, zero KE. At the bottom: maximum KE, minimum GPE. Some energy is dissipated with each swing, so the pendulum gradually swings lower.

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Energy Dissipation

Dissipation occurs when energy is transferred to the internal (thermal) energy store of the surroundings in a way that is not useful. This energy is often described as "wasted."

Why Dissipation Happens

Every energy transfer involves some dissipation. Common causes include:

Cause	Mechanism
Friction between moving surfaces	Kinetic energy → internal energy of surfaces
Air resistance on moving objects	Kinetic energy → internal energy of air
Electrical resistance in wires	Electrical energy → internal energy of wires
Sound produced by vibrations	Energy → sound waves → internal energy of surroundings

Why Dissipated Energy Is "Wasted"

Once energy has been dissipated to the surroundings, it is spread across a very large number of particles. The temperature rise of the surroundings is negligible, and the energy cannot practically be collected and used again. The energy still exists — it has not been destroyed — but it is no longer useful.

Exam Tip: Never say energy is "lost" or "destroyed." Instead say it is "dissipated to the internal (thermal) energy store of the surroundings." This is the language AQA expects.

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Reducing Unwanted Energy Transfers

Method	How It Reduces Dissipation	Example
Lubrication	Reduces friction between moving parts	Oil in a car engine
Thermal insulation	Reduces energy transfer by heating	Lagging on hot water pipes
Streamlining	Reduces air resistance	Aerodynamic shape of a car
Using components with low resistance	Reduces heating in circuits	Superconducting wires (theoretical)

Reducing Dissipation in the Home

Insulation Method	Energy Transfer Reduced
Loft insulation	Conduction and convection through the roof
Cavity wall insulation	Convection in the air gap; conduction through walls
Double glazing	Conduction and convection through windows
Draught excluders	Convection through gaps around doors/windows
Reflective foil behind radiators	Radiation into the wall

graph TD
    A["Energy input\n(heating system)"] --> B["Useful: Heating rooms"]
    A --> C["Wasted: Through roof"]
    A --> D["Wasted: Through walls"]
    A --> E["Wasted: Through windows"]
    A --> F["Wasted: Through draughts"]
    style C fill:#ffcccc,stroke:#cc0000
    style D fill:#ffcccc,stroke:#cc0000
    style E fill:#ffcccc,stroke:#cc0000
    style F fill:#ffcccc,stroke:#cc0000

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Worked Example

A light bulb has a total input energy of 100 J. It produces 5 J of useful light energy. Describe what happens to the rest of the energy.

The remaining $100 - 5 = 95$100−5=95 J is dissipated to the internal (thermal) energy store of the surroundings by heating and radiation. This energy heats the air and nearby surfaces but is not useful for lighting.

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Conservation of Energy in Calculations

Because energy is conserved:

$\text{Total input energy} = \text{Useful output energy} + \text{Wasted energy}$Total input energy=Useful output energy+Wasted energy

This relationship is used in efficiency calculations (covered in the next lesson).

Exam Tip: In calculation questions, if you know the input and the useful output, find the wasted energy by subtraction. AQA often uses this in multi-step problems.

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Common Mistakes

Mistake	Correction
Saying energy is "lost"	Energy is dissipated, not lost
Saying energy is "used up"	Energy is transferred and conserved
Forgetting that dissipated energy still exists	It is spread thinly across the surroundings
Thinking efficiency can be 100%	In practice, some dissipation always occurs

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Summary

• Conservation of energy: energy cannot be created or destroyed, only transferred.
• In a closed system, total energy is constant.
• Dissipation is the transfer of energy to the internal (thermal) store of the surroundings — this energy is wasted.
• Dissipation can be reduced by lubrication, insulation, streamlining, and low-resistance components.
• Total input = useful output + wasted energy.
• Always use AQA-approved language: "dissipated," not "lost" or "used up."
• This topic is assessed in AQA GCSE Combined Science Trilogy (8464), Section 6.1.

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Extended Worked Examples

Worked Example 2 — Falling Ball with Air Resistance

A 0.25 kg ball is dropped from 3.2 m and strikes the ground at 7.0 m/s. Show that some energy has been dissipated and calculate the amount. ($g = 10$g=10 N/kg.)

Initial $E_p$Ep​:

$E_p = 0.25 \times 10 \times 3.2 = 8.0 \text{ J}$Ep​=0.25×10×3.2=8.0 J

Kinetic energy at impact:

$E_k = \tfrac{1}{2} \times 0.25 \times 7.0^2 = 6.125 \text{ J}$Ek​=21​×0.25×7.02=6.125 J

Dissipated:

$8.0 - 6.125 = 1.875 \text{ J}$8.0−6.125=1.875 J

This energy has been transferred to the internal (thermal) store of the air by air resistance, and a tiny amount by radiation as sound.

Worked Example 3 — Sliding Block on a Table

A 2 kg block is pushed with 30 J of kinetic energy along a rough table. It comes to rest after 1.5 m. Describe the energy transfer and calculate the average friction force.

All 30 J is dissipated by friction to the internal (thermal) store of the block and table. Using work done = $F \times d$F×d:

$F = \frac{W}{d} = \frac{30}{1.5} = 20 \text{ N}$F=dW​=1.530​=20 N

Worked Example 4 — Electrical Dissipation

A 0.5 Ω resistor carries a current of 2.0 A for 60 s. Calculate the energy dissipated.

Power: $P = I^2 R = 4 \times 0.5 = 2$P=I2R=4×0.5=2 W.

Energy: $E = Pt = 2 \times 60 = 120$E=Pt=2×60=120 J.

This 120 J is transferred by heating to the internal (thermal) store of the resistor and surroundings.

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Comparison Table — Where Does Wasted Energy Go?