Grade 9 · Science
Account for useful and transferred energy practice
Use a stated system boundary to calculate efficiency and account for energy transferred to other forms.
Opening your practice adventure…
Try a sample
A device receives 50 J and a report claims 70 J of output with no other energy source or stored-energy change. What should be questioned?
- Whether joules can measure energy.
- Whether 70 is greater than 50.
- Whether output must always be zero.
- The measurements or completeness of the energy account.
See the answer & why
The measurements or completeness of the energy account.
The stated account supplies less energy than its claimed output.
A little hint: Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.
A teaching tip
Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.