Energy stores, work and efficiency
مُقدَّم| English | المعنى |
|---|---|
| power/ˈpaʊə/ | القدرة |
| efficiency/ɪˈfɪʃənsi/ | الكفاءة |
What would explain this observation?
- A motor can transfer some input energy to lifting and the rest to heating. Useful output is part of the total energy transfer.
Build the model
- Work done by a constant force parallel to displacement is force multiplied by distance.
- Kinetic energy depends on speed squared.
- Energy is conserved when all transfers and stores are included.
- power 功率: Energy transferred per unit time.
- efficiency 效率: Useful output divided by total input.
What happens to kinetic energy when speed doubles at fixed mass?
الصعوبة 1/5 · 100 عملة + مكافأة صغيرة عند الإكمال الأول اليوم
Kinetic energy depends on v squared: KE = 1/2 m v^2. Doubling v squares the factor 2, giving four times the energy at the same mass - which is why braking distances grow so quickly with speed.
Match each technical term to its precise meaning.
الصعوبة 2/5 · 140 عملة + مكافأة صغيرة عند الإكمال الأول اليوم
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Define the system and useful output before calculating efficiency.
- Doubling speed quadruples kinetic energy at constant mass.
- Power describes transfer per time, not total energy.
In the lab: Measure a lifting height and load, time the lift, and record electrical input with suitable instruments. Repeat trials and account for heating or friction as transfers, not missing energy.
Which two habits make the investigation or model in this case more defensible?
الصعوبة 2/5 · 140 عملة + مكافأة صغيرة عند الإكمال الأول اليوم
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: a device receives 600 J and transfers 420 J usefully.
- Efficiency = useful output / total input.
- Efficiency = 420/600 = 0.70 = 70%.
- Over 3 s, useful power = useful energy/time = 420/3 = 140 W.
Your turn: A device receives 250 J and gives 150 J useful output. Find efficiency as a percentage.
A device receives 250 J and gives 150 J useful output. Find efficiency as a percentage.
الصعوبة 2/5 · 140 عملة + مكافأة صغيرة عند الإكمال الأول اليوم
The result is 60 %. Known: input 250 J; useful output 150 J. Use efficiency = useful output / total input. Efficiency = 150/250 = 0.60 = 60%. The remaining 100 J is dissipated, typically as heating of the surroundings.
Check the conclusion and its limits
- Efficiency cannot exceed 100% for a properly defined energy balance.
- Energy dissipated by heating is still conserved.
Not true: Dissipated energy is destroyed. Energy is always conserved. "Dissipated" means spread into the surroundings as heating, where it becomes hard to use again - not destroyed.
- Work done by a constant force parallel to displacement is force multiplied by distance.
- Define the system and useful output before calculating efficiency.
- Watch for: efficiency cannot exceed 100% for a properly defined energy balance.
Write from an efficiency of 25 percent
- A device receives 800 J and transfers 200 J usefully in 4 s.
- Find the efficiency and the useful power. Dissipated energy is not destroyed.
Energy is always conserved. "Dissipated" means spread into the surroundings as heating, where it becomes hard to use again - not destroyed.
الصعوبة 1/5 · 100 عملة + مكافأة صغيرة عند الإكمال الأول اليوم
True. Energy is always conserved. "Dissipated" means spread into the surroundings as heating, where it becomes hard to use again - not destroyed.
Energy transferred per unit time: write the technical term.
الصعوبة 1/5 · 100 عملة + مكافأة صغيرة عند الإكمال الأول اليوم
power means Energy transferred per unit time.
A device receives 800 J and transfers 200 J usefully in 4 s. What are the efficiency and the useful power, and what happens to dissipated energy?
Efficiency is useful output divided by total input. Useful power is useful energy divided by time.
What is the efficiency?
الصعوبة 1/5 · 100 عملة + مكافأة صغيرة عند الإكمال الأول اليوم
200 divided by 800 is 25 percent. The 50 W figure is the useful power, not the efficiency. Other options — 50 W (the useful power); 70 percent (the worked efficiency).