A filament lamp lights a desk. The bulb runs hot, and the fixture above it is warm.
The lamp's job is making light. What should we call the energy that leaves it as heat?
Name the device's job first. The lamp is there to light the desk.
The energy that leaves as light does that job. It is the lamp's useful energy output.
The energy that leaves as heat warms the bulb, the fixture, and the room air.
The lamp's job cannot use that heat. It is the lamp's wasted energy output.
You have seen where such energy goes: energy that seems lost has been transferred to the surroundings as thermal energy.
In a word you already have, the wasted heat dissipates.
Useful and wasted are decided by the device's job, never by the kind of energy.
A space heater's job is warming a room. The heat it sends into the room is its useful output.
The faint hum the same heater makes carries energy its job cannot use. The hum is wasted output.
Q: A cordless drill drives screws into a deck board. Sort the drill's energy outputs into useful and wasted.
The drill's job is turning the screw.
The kinetic energy that turns the screw does the job: useful output.
The heat that warms the motor and the casing does nothing the job can use: wasted output.
Useful: the turning of the screw. Wasted: the heat from the motor.
Q: An electric kettle heats water for tea. Sort the kettle's energy outputs into useful and wasted.
The kettle's job is heating the water.
The heat that enters the water does the job: useful output.
The heat that warms the kettle body and the countertop does nothing the job can use: wasted output.
The same kind of energy gets both verdicts: the water's heat is useful, the kettle body's heat is wasted. The job decides.
A leaf blower's job is to blow leaves along a sidewalk with fast-moving air. While it runs, its motor casing gets warm. Is the energy leaving through the warm casing useful output or wasted output?
A hair dryer's job is to dry hair with warm moving air. Lumi says: 'The heat in the air the dryer blows out is useful output.' Is she correct?
A projector's job is to light a screen. An electric griddle's job is to cook food on its hot plate. Each device sends out heat. For which device is the heat it sends out part of its useful output?
An electric fan's job is to move air across a room. Which output is the fan's useful output?
In which device is the heat it sends to its surroundings counted as useful output?
Two students sort a toaster's energy outputs into useful and wasted. What decides the sort?