Physics Flashcards: Evaluate Device Energy Efficiency

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This deck focuses on Evaluate Device Energy Efficiency, giving you a quick way to review the definitions, rules, and examples that matter most for Physics.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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