Efficiency

Energy is transferred between different stores, but not all of this energy is useful. Some of it is often wasted or dissipated, especially into the surroundings as heat. This is why we talk about efficiency – how much of the input energy is useful versus how much is wasted.

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Efficiency measures how well a device converts input energy into useful energy. A highly efficient device wastes little energy, while an inefficient device wastes most of its input energy.

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Key Principle:

Energy is only useful when it is transferred from one store to a useful store.

Useful devices are designed to transfer energy from one store to another useful store.
However, some of the input energy is often dissipated or wasted, usually to thermal energy stores in the surroundings.

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Calculating Efficiency:

Efficiency can be calculated using the formula:

$\text{Efficiency} = \frac{\text{Useful energy transferred}}{\text{Total energy supplied}} \times 100 \, \%$

Both useful energy and total energy are measured in joules (J).

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Example:

The energy supplied to an LED bulb = 140 J.
Useful energy transferred = 108 J. Efficiency calculation:

Efficiency= \frac{108}{140} ×100=77.1%

Give your answer as a %. The LED bulb is quite efficient since most of the energy is transferred usefully.

Key points:

Efficiency is always less than 100%, as some energy is always wasted (e.g., as heat or sound).
The law of conservation of energy states that energy cannot be created, so no device can be more than 100% efficient.

Increasing Efficiency:

Lubrication reduces friction between moving parts in machines, preventing energy loss as heat.
Insulation reduces the dissipation of thermal energy to the surroundings, especially in systems designed to transfer heat. By reducing wasted energy transfers, devices can be made more efficient, ensuring more of the input energy is usefully transferred.

Wasted Energy:

When energy is wasted, it typically dissipates as heat in the surroundings.

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For example:

Mechanical Work and Friction:

When work is done mechanically (e.g., a car engine running or an object moving), frictional forces must be overcome.
- Friction from moving parts rubbing together or air resistance creates wasted heat.
- This wasted energy goes to the thermal energy stores of the surroundings, reducing overall efficiency.

The Conservation of Energy Principle

The conservation of energy principle states that energy cannot be created or destroyed, only transferred from one store to another.

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This leads to the equation:

\text{Total Energy} = \text{Useful Energy Output} + \text{Wasted Energy}

If more energy is transferred to useful stores and less is wasted, the device is more efficient.

Improving Efficiency:

To increase efficiency and reduce wasted energy:

Lubricate moving parts to reduce friction.
Streamline shapes to reduce air resistance.
Use better materials to minimise energy loss (e.g., insulation to reduce heat loss).

Efficiency Simplified Revision Notes for GCSE Edexcel Physics

Efficiency

Key Principle:

Calculating Efficiency:

Example:

Key points:

Increasing Efficiency:

Wasted Energy:

Mechanical Work and Friction:

The Conservation of Energy Principle

Improving Efficiency:

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