Efficiency and powerIB MYP Sciences: Revision notes
Section 1
Wasted energy
Every device transfers some energy to stores we do not want. This wasted energy is usually transferred by heating to the thermal store of the surroundings, where it dissipates (spreads out). It cannot easily be collected and used again.
Examples: a filament lamp wastes most of its energy as thermal energy, a motor warms up, and friction heats the moving parts of a machine. Energy is never destroyed, so useful output + wasted energy = total input.
Section 2
Efficiency
Efficiency tells you how much of the energy put into a device is transferred usefully.
efficiency = useful energy output / total energy input (as a decimal), or x 100% (as a percentage)
Worked example: a motor takes in 500 J and transfers 350 J usefully. Efficiency = 350 / 500 = 0.70, or 70%. The wasted energy is 500 - 350 = 150 J.
No device is 100% efficient, because some energy is always wasted. Efficiency has no unit.
Do not divide by the wasted energy. Always divide by the total input energy.
Efficiency can never be more than 100%. If your answer is, check you have not turned the fraction upside down.
Section 3
Power
Power is the rate of transferring energy: how much energy is transferred each second.
power (W) = energy transferred (J) / time (s)
One watt (W) is one joule per second. Worked example: a kettle transfers 90 000 J in 45 s. Power = 90 000 / 45 = 2000 W (2 kW).
A device with a higher power transfers the same energy in a shorter time.
Efficiency is a ratio of energies. Power is about time. A powerful device is not always an efficient one.
Section 4
Calculating and comparing efficiencies
To compare devices, calculate each efficiency with the same method, then compare the percentages. The higher efficiency wastes a smaller fraction of its input.
Example: an LED takes in 5 J and gives 4 J of light, so efficiency = 4 / 5 × 100% = 80%. A filament lamp takes in 80 J for the same 4 J of light, so efficiency = 5%. The LED wastes 1 J and the filament lamp wastes 76 J for the same light.
Section 5
Reducing wasted energy
We can raise efficiency by:
- Lubricating moving parts to reduce friction
- Insulating hot objects, such as a hot water tank, to reduce heating of the surroundings
- Using a more efficient design, such as LED lamps or streamlined shapes that reduce air resistance
- Using thicker wires with less resistance
Reducing waste saves money and cuts the fuel burned, so it helps the environment.
Must know
- Wasted energy dissipates into the surroundings, mainly as thermal energy
- Efficiency = useful energy output / total energy input (x 100%)
- Power (W) = energy transferred (J) / time (s)
- A more efficient device wastes a smaller fraction of its input
- Waste is reduced by lubrication, insulation and better design
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Efficiency and power
- A student in Lagos compares a filament lamp with an LED lamp. Both are connected to the mains. After a few minutes the filament lamp is too hot to touch, but the LED lamp is only warm.Explain why the filament lamp is less efficient than the LED lamp.2 marks
- A construction company in Mumbai uses a crane powered by an electric motor to lift steel beams to the top of a tall building.In one lift the motor takes in 6000 J of electrical energy and transfers 4200 J as gravitational potential energy of the beam. Calculate the efficiency of the motor.2 marks
- Students in Cape Town compare two electric kettles, P and Q. Each kettle heats 1.0 kg of water through 20 °C. Kettle P has a thick plastic body and kettle Q has a thin plastic body. Kettle P took in 100 kJ of electrical energy and kettle Q took in 120 kJ. In both cases the water gained 84 kJ of energy.State a hypothesis that the students could test about the two kettles, giving a scientific reason.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).