Physics Quiz: Evaluate Device Energy Efficiency

A solar panel converts $18\%$ of incoming solar energy into electrical energy. If $E_{in}=2500\ \text{J}$ of solar energy strikes the panel, how much electrical energy is produced, $E_{out}$?

A heat engine takes in thermal power $P_{in}=900\,\text{W}$ from burning fuel and produces useful mechanical power $P_{out}=270\,\text{W}$. The rest is waste heat to the surroundings. What is the engine's efficiency (as a percentage)?

An electric motor has efficiency $\eta=80\%$ and delivers useful mechanical power $P_{out}=320\ \text{W}$. What electrical input power $P_{in}$ is required? (Use $\eta=(P_{out}/P_{in})\times 100\%$.)

A battery charger takes in $P_{in}=120\ \text{W}$ from the wall and stores useful chemical energy in the battery at a rate of $P_{out}=102\ \text{W}$. What is the efficiency $\eta$ (in %)?

An electric motor has efficiency $\eta=85\%$ and receives input power $P_{in}=200\,\text{W}$. How much useful mechanical output power $P_{out}$ does it deliver?

Two light bulbs produce light (useful) and heat (waste).

• Incandescent bulb: $P_{in}=100\ \text{W}$ electrical, $P_{out}=5\ \text{W}$ light.
• LED bulb: $P_{in}=20\ \text{W}$ electrical, $P_{out}=8\ \text{W}$ light.

Which bulb is more efficient, and by how many percentage points (LED efficiency minus incandescent efficiency)?

A heat engine takes in $E_{in}=5000\ \text{J}$ of thermal energy from fuel and produces $E_{out}=1400\ \text{J}$ of useful mechanical work. What is the efficiency of the engine (as a percentage)?

A solar panel receives $E_{in}=2000\ \text{J}$ of solar radiation in a short time interval and produces $E_{out}=320\ \text{J}$ of electrical energy. The rest becomes thermal energy in the panel. How much energy is lost as waste heat, $E_{lost}$?

Use $E_{in}=E_{out}+E_{lost}$.

A battery charging system stores $E_{out}=760\ \text{J}$ of chemical energy while $E_{in}=800\ \text{J}$ of electrical energy is supplied. What fraction of the input energy is wasted (lost as heat), expressed as a percentage of the input?

An electric motor takes in electrical power $P_{in} = 500 \, \text{W}$ and delivers useful mechanical power $P_{out} = 400 \, \text{W}$. The rest is lost as thermal energy in the motor. What is the motor's efficiency (as a percentage)?

Use $\eta = \dfrac{P_{out}}{P_{in}} \times 100\%$

Two light bulbs convert electrical power into light (useful) and heat (waste). Bulb 1 has $P_{in}=100\ \text{W}$ and produces $P_{light}=5\ \text{W}$. Bulb 2 has $P_{in}=20\ \text{W}$ and produces $P_{light}=8\ \text{W}$. Which bulb is more efficient at producing light, and by how many percentage points?

Use: $\eta=\left(\frac{P_{light}}{P_{in}}\right)\times 100\%$

A heat engine takes in thermal energy $E_{in}=5000\ \text{J}$ from fuel and produces useful mechanical work $E_{out}=1500\ \text{J}$. What is the engine efficiency $\eta$ (as a percentage)?

Use: $\eta = \left(\frac{E_{out}}{E_{in}}\right)\times 100\%$

An incandescent light bulb uses $P_{in}=100\,\text{W}$ of electrical power and produces $P_{out}=5\,\text{W}$ of useful light. The rest is released as heat. How much power is lost as thermal energy, $P_{lost}$?

A battery charger transfers $E_{in}=1500\,\text{J}$ of electrical energy to a rechargeable battery. After charging, the battery has stored $E_{out}=1200\,\text{J}$ of chemical energy. The rest is lost mainly as heat. How much energy is lost, $E_{lost}$?

A device converts energy with $E_{in}=3600\,\text{J}$ and produces useful output $E_{out}=900\,\text{J}$. The remainder is waste (mostly thermal). What percentage of the input energy is wasted?

A battery charging system receives $E_{in}=4000\ \text{J}$ of electrical energy. Measurements show $E_{lost}=600\ \text{J}$ is dissipated as heat. How much useful chemical energy is stored in the battery, $E_{out}$?

Use: $E_{in}=E_{out}+E_{lost}$

A solar panel receives $E_{in}=2000\ \text{J}$ of solar radiation in a short time interval and produces $E_{out}=320\ \text{J}$ of useful electrical energy. How much energy is lost as waste (mostly thermal), $E_{lost}$?

Use: $E_{in}=E_{out}+E_{lost}$

A solar panel produces $P_{out}=90\ \text{W}$ of electrical power from $P_{in}=600\ \text{W}$ of incoming solar power. What percentage of the input power is lost as waste (mostly thermal)?

Use: $P_{lost}=P_{in}-P_{out}$ and $\%\text{ lost}=\left(\frac{P_{lost}}{P_{in}}\right)\times 100\%$

A solar panel operates at an efficiency of $18\%$. If it receives $E_{in}=3000\ \text{J}$ of solar radiation, how much useful electrical energy $E_{out}$ does it produce?

Use: $E_{out}=\eta E_{in}$ with $\eta$ as a decimal.

A heat engine produces $P_{out}=300\ \text{W}$ of useful mechanical power at an efficiency of $30\%$. What input thermal power $P_{in}$ is required?

Use: $\eta = \left(\frac{P_{out}}{P_{in}}\right)\times 100\%$.