Edexcel IGCSE Physics: Double Science 复习笔记：4.1.3 Efficiency

Edexcel IGCSE Physics: Double Science 复习笔记：4.1.3 Efficiency

Efficiency

The efficiency of a system is a measure of how well energy is transferred in a system
Efficiency is defined as:

The ratio of the useful energy transferred by the device to the total energy supplied to the device

If a system has high efficiency, this means most of the energy transferred is useful
If a system has low efficiency, this means most of the energy transferred is wasted
Determining which type of energy is useful or wasted depends on the system
- When electrical energy is converted to light in a lightbulb, the light energy is useful and the heat energy produced is wasted
- When electrical energy is converted to heat for a heater, the heat energy is useful and the sound energy produced is wasted

Remember that conversation of energy still applies, so the total energy supplied goes to both the useful and wasted energy transferred

Efficiency is represented as a percentage, and can be calculated using the equation:

4.6.2-Energy-efficiency-equation

The energy can be of any form e.g. gravitational potential energy, kinetic energy

Worked Example

The blades of a fan are turned by an electric motor. In one second, the motor gets 300 J of electrical energy from the mains supply. 85 J of the energy is wasted as heat and sound.Calculate the efficiency of the motor.

Efficiency-Worked-Example-3

Exam Tip

Efficiency can be in a ratio (between 0 and 1) or percentage format (between 0 and 100%)If the question asks for efficiency as a ratio, give your answer as a fraction or decimal.If the answer is required as a percentage, remember to multiply the ratio by 100 to convert it: if the ratio = 0.25, percentage = 0.25 × 100 = 25 %Remember that efficiency has no units (only %)

Sankey Diagrams

Diagrams are used to represent energy transfers
- These are sometimes called Sankey diagrams
The arrow in a Sankey diagram represents the transfer of energy:
- The end of the arrow pointing to the right represents the energy that ends up in the desired store (the useful energy output)
- The end that points down represents the wasted energy
The width of each arrow is proportional to the amount of energy going to each store
As a result of the conversation of energy:

Total energy in = Useful energy out + Wasted energy

Total energy in, wasted energy and useful energy out shown on a Sankey diagram

A Sankey diagram for a modern efficient light bulb will look very different from that for an old filament light bulb
A more efficient light bulb has less wasted energy
- This is shown by the smaller arrow downwards representing the heat energy

Sankey diagram for modern vs. old filament light bulb

Worked Example

An electric motor is used to lift a weight. The diagram represents the energy transfers in the motor.

How much energy is wasted?

Step 1: State the conservation of energy

- Energy cannot be created or destroyed, it can only be moved from one store to another
- This means that:

Total energy in = Useful energy out + Wasted energy

Step 2: Rearrange the equation for the wasted energy

Wasted energy = Total energy in – Useful energy out

Step 3: Substitute the values from the diagram

500 – 120 = 380 J

Exam Tip

Drawing good Sankey diagrams takes practice.
Start by planning your diagram:
- How wide are you going to make the input arrow?
- How wide will the useful energy out arrow need to be?
- How wide must the wasted arrow be?
Next, start drawing the diagram one step at a time:
- Draw the left hand side of the arrow, along with the line going across the top
- Next add the useful energy out arrow, making sure it is the correct width
- Now carefully mark the start and end of the wasted arrow – make sure your marks are the correct distance apart
- Finally join the markings together, finishing the wasted energy arrow