Evaluate Device Energy Efficiency - Physics

1. Identify the more efficient device: A has $\%\text{ efficiency}=85\%$, B has $\%\text{ efficiency}=0.90\times 100\%$.

   Device B. B has 90% efficiency, higher than A's 85%.

2. What is the percentage efficiency if $E_\text{useful}=60\,\text{J}$ and $E_\text{input}=80\,\text{J}$?

   $75\%$. $\frac{60}{80}\times 100\% = 75\%$

3. What is the formula to convert efficiency (decimal) into percentage efficiency?

   $\text{efficiency (\%)}=100\times\text{efficiency}$. Multiply decimal efficiency by 100 to get percentage.

4. Identify the correct unit for efficiency (not percentage efficiency).

   No unit (dimensionless ratio). Efficiency is a ratio of same units, so they cancel.

5. What is the formula for percentage efficiency using useful output power and total input power?

   $\%\text{ efficiency}=\frac{P_\text{useful}}{P_\text{input}}\times 100\%$. Multiply power ratio by 100 for percentage.

6. Identify the correct relation between power, energy, and time used to find input power.

   $P=\frac{E}{t}$. Power equals energy divided by time.

7. What is the percentage efficiency if $P_\text{useful}=18\,\text{W}$ and $P_\text{input}=24\,\text{W}$?

   $75\%$. $\frac{18}{24}\times 100\% = 75\%$

8. What is the total input energy if $E_\text{useful}=150\,\text{J}$ and $\text{efficiency}=0.60$?

   $250\,\text{J}$. $E_\text{input} = \frac{150}{0.60} = 250\,\text{J}$

9. Choose the correct equation for percentage efficiency: $\frac{E_{\text{in}}}{E_{\text{useful}}}\times100$ or $\frac{E_{\text{useful}}}{E_{\text{in}}}\times100$?

   $\frac{E_{\text{useful}}}{E_{\text{in}}}\times100$. Useful output goes in numerator, total input in denominator.

10. Which option is the correct unit for power used in efficiency calculations: $\text{J}$ or $\text{W}$?

    $\text{W}$. Watt (W) is the SI unit for power.

11. What is the efficiency if $P_\text{useful}=45\,\text{W}$ and $P_\text{input}=60\,\text{W}$?

    $0.75$. $\frac{45}{60} = 0.75$

12. What is the formula for wasted energy in terms of input energy and useful output energy?

    $E_\text{wasted}=E_\text{input}-E_\text{useful}$. Energy not converted to useful form is wasted.

13. Find the useful output energy if $\text{efficiency}=0.80$ and $E_{\text{in}}=500\,\text{J}$.

    $400\,\text{J}$. Rearrange: $E_{\text{useful}}=0.80\times500=400\,\text{J}$.

14. Calculate the efficiency when $E_{\text{useful}}=60\,\text{J}$ and $E_{\text{in}}=100\,\text{J}$.

    $0.60$ (or $60\%$). Apply $\text{efficiency}=\frac{60}{100}=0.60$.

15. Identify the correct statement about energy in a device: $E_\text{input}$, $E_\text{useful}$, and $E_\text{wasted}$.

    $E_\text{input}=E_\text{useful}+E_\text{wasted}$. Energy conservation: input equals useful plus wasted.

16. State the formula for efficiency in terms of useful output power and total input power.

    $\text{efficiency}=\frac{P_{\text{useful out}}}{P_{\text{total in}}}$. Same ratio applies whether using energy or power values.

17. Identify the term for energy that is not transferred to the intended output of a device.

    Wasted energy. Energy lost as heat, sound, or other unwanted forms.

18. Calculate input power if a device uses $E=600\,\text{J}$ in $t=20\,\text{s}$.

    $30\,\text{W}$. Use $P=\frac{600}{20}=30\,\text{W}$.

19. What is the formula for percentage efficiency using useful output energy and total input energy?

    $\%\text{ efficiency}=\frac{E_\text{useful}}{E_\text{input}}\times 100\%$. Multiply efficiency ratio by 100 to get percentage.

20. Identify the correct statement: efficiency compares useful output to total input (energy or power).

    True. Efficiency definition: useful output divided by total input.

21. Which option correctly gives the relation between efficiency and wasted fraction of input energy?

    $\text{wasted fraction}=1-\text{efficiency}$. If efficiency is 0.7, then 0.3 (30%) is wasted.

22. Find the efficiency if a device outputs $900\,\text{J}$ useful energy from $1.2\,\text{kJ}$ input energy.

    $0.75$. $\text{efficiency} = \frac{900}{1200} = 0.75$

23. Calculate wasted energy when $E_{\text{in}}=1200\,\text{J}$ and $E_{\text{useful}}=900\,\text{J}$.

    $300\,\text{J}$. Subtract: $1200-900=300\,\text{J}$ wasted.

24. What is the formula for efficiency in terms of useful output power and total input power?

    $\text{efficiency}=\frac{P_\text{useful}}{P_\text{input}}$. Same ratio applies to power as to energy.

25. State the formula for efficiency in terms of useful output energy and total input energy.

    $\text{efficiency}=\frac{E_{\text{useful out}}}{E_{\text{total in}}}$. Efficiency is the ratio of useful output to total input.

26. Calculate useful output power if $P_{\text{in}}=200\,\text{W}$ and efficiency is $0.30$.

    $60\,\text{W}$. Calculate $P_{\text{useful}}=0.30\times200=60\,\text{W}$.

27. What is the wasted energy if $E_\text{input}=500\,\text{J}$ and $E_\text{useful}=350\,\text{J}$?

    $150\,\text{J}$. $E_\text{wasted} = 500 - 350 = 150\,\text{J}$

28. What is the formula for efficiency in terms of useful output energy and total input energy?

    $\text{efficiency}=\frac{E_\text{useful}}{E_\text{input}}$. Ratio of useful output to total input energy.

29. What is the input power if $\%\text{ efficiency}=80\%$ and $P_\text{useful}=120\,\text{W}$?

    $150\,\text{W}$. $P_\text{input} = \frac{120}{0.80} = 150\,\text{W}$

30. What is the useful output power if $P_\text{input}=500\,\text{W}$ and $\%\text{ efficiency}=30\%$?

    $150\,\text{W}$. $P_\text{useful} = 500 \times 0.30 = 150\,\text{W}$

31. Which option is the correct unit for energy used in efficiency calculations: $\text{J}$ or $\text{W}$?

    $\text{J}$. Joule (J) is the SI unit for energy.

32. What is the formula for wasted power in terms of total input and useful output power?

    $P_{\text{wasted}}=P_{\text{total in}}-P_{\text{useful out}}$. Same principle applies to power: input minus output equals waste.

33. What is the wasted power if $P_\text{input}=200\,\text{W}$ and $\text{efficiency}=0.65$?

    $70\,\text{W}$. $P_\text{wasted} = 200 - (200 \times 0.65) = 70\,\text{W}$

34. Calculate the percentage efficiency when $P_{\text{useful}}=75\,\text{W}$ and $P_{\text{in}}=150\,\text{W}$.

    $50\%$. Calculate $\frac{75}{150}\times100=50\%$.

35. What is the formula for wasted energy in terms of total input and useful output energy?

    $E_{\text{wasted}}=E_{\text{total in}}-E_{\text{useful out}}$. The difference between input and output gives waste.

36. Find the total input energy if $\text{efficiency}=0.25$ and $E_{\text{useful}}=50\,\text{J}$.

    $200\,\text{J}$. Rearrange: $E_{\text{in}}=\frac{50}{0.25}=200\,\text{J}$.

37. Which device is more efficient if A is $0.70$ efficient and B is $65\%$ efficient?

    Device A. Compare: $0.70>0.65$, so A is more efficient.

38. Which option correctly states the allowed range of efficiency values (as a decimal) for real devices?

    $0\leq \text{efficiency}\leq 1$. Efficiency cannot exceed 1 (100%) due to energy conservation.

39. What is the useful output energy if $\text{efficiency}=0.30$ and $E_\text{input}=200\,\text{J}$?

    $60\,\text{J}$. $E_\text{useful} = 0.30 \times 200 = 60\,\text{J}$

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