To read an energy-flow diagram, convert every label to one unit, check that the outputs add up to the input, then use the useful part to find efficiency. The balance check catches most errors.
This lesson belongs to the electricity and energy cluster. If the power-and-time formula is new, read calculating energy from a device table first.
What do I check first?
Look at the units on every arrow. A diagram may label the input in kJ and an output in J, and adding them without converting gives nonsense. Convert everything to the smaller unit, joules, unless the question asks for kJ.
Then label the input, the useful output and the wasted output with their converted numbers.
Worked example: an invented desk lamp
An invented desk lamp diagram shows these labels.
| Arrow | Label |
|---|---|
| Input: electrical energy | 2.0 kJ |
| Output: light energy | 340 J |
| Output: heat energy | 1.66 kJ |
Step 1: convert to joules. Input = 2.0 × 1 000 = 2 000 J. Heat = 1.66 × 1 000 = 1 660 J. Light stays 340 J.
Step 2: check the balance. 340 + 1 660 = 2 000 J, which equals the input, so the diagram is consistent.
Step 3: find the efficiency. Useful output is light: 340 ÷ 2 000 × 100% = 17%.
Step 4: interpret. 83% of the input is wasted as heat, which is 1 660 ÷ 2 000 × 100%. That is the reason the lamp feels warm.
The mistake that costs marks
The common slip is to add numbers without converting: 2.0 + 340 + 1.66 = 343.66, then write an efficiency of 340 ÷ 2.0 = 170, or 17 000%.
The clue that something is wrong is an efficiency far above 100%. The fix is the conversion line, and the balance check confirms it: if the outputs do not add to the input, the units were mixed.
Check yourself
An invented fan motor diagram shows an input of 5.0 kJ, a useful output of 3 500 J of movement energy and a wasted output labelled 1.5 kJ of heat. Check the balance and find the efficiency.
Answer
Convert: input 5 000 J, wasted 1 500 J, useful 3 500 J.
Balance: 3 500 + 1 500 = 5 000 J, so the diagram balances.
Efficiency = 3 500 ÷ 5 000 × 100% = 70%.
What to study next
Continue to why efficiency cannot exceed the stated energy input, then test the skills in the integrated practice set.
The units and significant-figure checker gives unit drills, and the mistake log and paper-error review tracks slips. For a teacher who checks your diagrams with you, see online one-to-one Science tuition.