Energy stores, work and efficiency
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| power/ˈpaʊə/ | công suất |
| efficiency/ɪˈfɪʃənsi/ | hiệu suất |
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?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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?
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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 45 percent
- A device receives 200 J and gives 90 J of useful output.
- Find the efficiency. The other energy is dissipated, not destroyed.
Energy is always conserved. "Dissipated" means spread into the surroundings as heating, where it becomes hard to use again - not destroyed.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
power means Energy transferred per unit time.
A device receives 200 J and gives 90 J of useful output. What is the efficiency, and what happens to the other energy?
Efficiency is useful output divided by total input. The difference is still energy.
What is the efficiency?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
90 divided by 200 is 0.45, so the efficiency is 45 percent. Other options — 60 percent (the lesson device that gives 150 J from 250 J); 110 J (the dissipated energy).