Two winch motors sit side by side, and each takes in 400 J of electrical energy. The gray motor delivers 140 J of useful work. The black motor delivers 220 J.
Something separates the two motors, and it is not the energy they take in. Which motor turns the greater share of its input into useful output?
The black motor does. Both motors take in 400 J, and more of the black motor's 400 J comes out as useful work: 220 J against 140 J.
Now the same pair with one change: suppose the gray motor also delivers 220 J. Same input, same useful output: neither motor turns the greater share into useful output.
Back to the first pair: 220 J against 140 J from the same 400 J. The black motor again.
The scene can change completely and the test stays the same. Two phone chargers each take in 60 J from the outlet. The slim charger puts 48 J into the battery; the older charger puts 39 J in. The slim charger turns the greater share of its input into useful output.
The deciding feature is never the device's size, age, or brand. With equal inputs, it is the useful output alone.
The rest of each input leaves as wasted output, mostly heat: the gray motor's casing runs warmer than the black motor's.
The warm casing is evidence you can read. Suppose two devices take in equal energy, and you cannot see either useful output. Each input splits into useful output plus wasted output. So with equal inputs, the device that wastes the greater share delivers the smaller useful output. Most wasted output leaves as heat. That heat warms the casing. So with equal inputs, the hotter casing means the greater wasted share. The cooler casing belongs to the device that turns the greater share of its input into useful output.
Q: Two desk fans each take in 150 J of electrical energy. The metal fan's useful output, the moving air's kinetic energy, is 90 J. The plastic fan's useful output is 75 J. Which fan turns the greater share of its input into useful output?
Both fans take in 150 J.
The metal fan's useful output is 90 J; the plastic fan's is 75 J.
Same input, greater useful output: the metal fan.
The metal fan turns the greater share of its input into useful output.
Q: Two LED floodlights each take in 80 J. Each sends out 56 J as light. Which floodlight turns the greater share of its input into useful output?
Both floodlights take in 80 J.
Both useful outputs are 56 J.
Same input, same useful output: nothing separates the two.
Neither. The two floodlights turn equal shares of their inputs into useful output.
Two toy race cars have motors that each take in 120 J from their battery packs. The red car's motor delivers 66 J of useful work; the blue car's motor delivers 54 J. Which motor turns the greater share of its input into useful output?
Two aquarium pumps each take in 90 J. The quiet pump delivers 63 J of useful work; the rattling pump delivers 45 J. Tamsin says: 'The quiet pump turns the greater share of its input into useful output.' Is she correct?
A newer string trimmer and an older string trimmer each take in 200 J. Each delivers 70 J of useful work to its spinning line. Which trimmer turns the greater share of its input into useful output?
Four electric scooters have motors that each take in 360 J per test run. The silver scooter's motor delivers 288 J of useful work, the black scooter's 252 J, the red scooter's 306 J, and the white scooter's 270 J. Which motor turns the greatest share of its input into useful output?
Two conveyor motors take in equal energy. The left motor delivers more useful work than the right motor. Which statement is correct?
Two drills, identical apart from their motors, run side by side on the same job all afternoon with equal energy input. The yellow drill's casing gets much hotter than the green drill's casing. Which statement follows?