1.2.2a Efficiency of an energy transfer

Efficiency of an energy transfer

1. In any device, some input energy is transferred usefully and some in less useful ways, and the efficiency tells you how good it is at the intended transfer.
2. Calculate it with $efficiency=\frac{useful\; output\; energy\; transfer}{total\; input\; energy\; transfer}$.
3. Both energy values must be in the same unit, usually joules, $J$.
4. For example, a useful output of $60J$ from a total input of $100J$ gives $\frac{60}{100}=0.60$, or $60\%$.

Efficiency using power

1. You can also calculate efficiency from power, since power is the rate of energy transfer.
2. Use $efficiency=\frac{useful\; power\; output}{total\; power\; input}$.
3. Both powers must be in the same unit, watts, $W$.
4. This version is handy when a question gives the electrical power supplied to a device and the useful power output from it.

Decimal and percentage efficiencies

1. Efficiency can be written as a decimal or as a percentage.
2. Convert a decimal to a percentage by multiplying by $100$, so $0.72\times 100=72\%$.
3. Convert a percentage to a decimal by dividing by $100$, so $85\%=\frac{85}{100}=0.85$.
4. A decimal efficiency is normally between $0$ and $1$, and a percentage efficiency between $0\%$ and $100\%$.

1.2.2b Increasing the efficiency of an energy transfer

Increasing the efficiency of an intended energy transfer

1. A transfer is more efficient when a greater proportion of the input energy goes to the intended, useful output.
2. No device is perfectly efficient, because some energy is always transferred to unwanted stores, often heating the surroundings.
3. In a car engine, the useful transfer fills the kinetic store of the car, while some energy also goes to the surroundings by heating and sound.

Reducing energy wasted by friction

1. One common way to raise efficiency is to reduce the friction between moving parts.
2. Friction transfers energy by heating to the thermal stores of the parts and surroundings, where it spreads out and becomes less useful.
3. Lubricating with oil or grease forms a slippery layer between the surfaces, so less work is done against friction and less energy is dissipated by heating.
4. Everyday examples include oil in a car engine, grease on a bicycle chain, bearings in wheels, and smooth surfaces in moving mechanisms.
5. A full answer links the steps: lubrication reduces friction, so less energy is transferred by heating to the surroundings, so a greater proportion of the input is transferred usefully, so the efficiency increases.

Reducing unwanted thermal transfers

1. Another way to raise efficiency is to reduce unwanted thermal transfers to the surroundings.
2. This helps when the intended transfer is heating something, such as water in a kettle or a house in winter, because less energy leaks away.
3. Thermal insulation cuts this unwanted heating: insulated kettle sides, loft insulation in a house, and a jacket around a hot water tank all keep more energy where it is wanted.
4. The idea is always the same: reducing unwanted thermal transfer leaves more energy available for the useful transfer.

Improving the design for the intended transfer

1. The design of a device also affects its efficiency, because a better design directs more energy into the useful output.
2. A streamlined vehicle loses less energy to air resistance, so more of the engine's energy maintains the vehicle's kinetic store.
3. In an electrical device, reducing unnecessary resistance cuts the energy wasted heating the wires, so more reaches the useful output.
4. You do not need engineering detail; focus on the physics reason that the design reduces unwanted energy transfers.