Describe how devices convert energy from one form into another, define efficiency as useful output over total input, and explain why some energy is always transformed into unwanted thermal energy (MA STE Introductory Physics, Energy, HS-PS3-3).

What this topic is asking

This topic is the engineering-design standard of the Energy module of the Massachusetts Introductory Physics MCAS, HS-PS3-3. You must describe how a device converts energy from one form into another, define efficiency as the useful energy output divided by the total energy input, and explain why some energy is always transformed into unwanted thermal energy. The crosscutting ideas are energy and matter together with the engineering practice of designing and refining solutions within constraints, especially the constraint of efficiency.

Devices convert energy between forms

The MCAS expects you to identify the energy transformation a device performs, written as an "energy chain." Common examples:

• Electric motor: electrical energy goes to kinetic energy (plus some thermal energy).
• Generator: kinetic energy goes to electrical energy (the reverse of a motor, see electromagnetic induction).
• Solar cell: light energy goes to electrical energy.
• Battery: chemical energy goes to electrical energy.
• Wind turbine: kinetic energy of the wind goes to electrical energy.
• Light bulb: electrical energy goes to light (and a lot of thermal energy).

Each step is a transformation, and a device can chain several: a flashlight runs chemical to electrical to light.

Efficiency

Efficiency is a ratio with no units (the joules cancel), and a percent is just the ratio times 100. The MCAS uses it to test energy accounting: if you know the input and the useful output, you can find the efficiency, and the difference between them is the wasted energy. A higher efficiency means less of the input is wasted. You can also work with power instead of energy, since efficiency is the same ratio for useful output power over input power.

Why no device is perfect

This is the conceptual heart of the standard, and the most common explanation question. The key is to name the wasted form (unwanted thermal energy, from friction and resistance) rather than saying energy is "lost." A device is improved, or "refined," by reducing these losses, for example by lubricating moving parts to cut friction or by using thicker wires to cut resistance. That is the engineering-design action HS-PS3-3 asks for: build a device that converts energy within constraints, then refine it to waste less.

Worked example

Reference-sheet note

The reference sheet gives the energy formulas you need to find inputs and outputs ($W = Fd$, $KE = \tfrac{1}{2}mv^2$, $PE = mgh$, $P = \dfrac{W}{t}$, and the electrical $P = IV$). It does not print the efficiency ratio, so you recall that efficiency is useful output over total input, that the result is often a percent, and that the wasted energy is unavoidable thermal energy from friction and resistance.

Try this

Q1. A device takes in $300$ J and produces $240$ J of useful output. Calculate its efficiency as a percent. [2]

• Cue. $\dfrac{240}{300} = 0.80$, so 80 percent.

Q2. Name the form most wasted energy takes in a real device, and one cause. [2]

• Cue. Unwanted thermal energy (heat); caused by friction in moving parts or electrical resistance in wires.