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All 11 Flashcards — Power & efficiency
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Question
Define power.
Answer
The **rate of energy transfer** — the energy transferred (or work done) each **second**. Unit: the watt (W).
Question
What is a watt?
Answer
**1 watt = 1 joule per second** (1 W = 1 J s⁻¹).
Question
Two given formulas for power?
Answer
$P = \dfrac{\Delta W}{\Delta t} = Fv$ — energy ÷ time, or force × speed.
Question
Which power formula do you use for an object moving at constant speed?
Answer
**P = Fv**, where F is the **resistive (drag) force** — it equals the driving force at constant speed.
Question
Average vs instantaneous power?
Answer
**Average** = total energy ÷ total time (ΔW/Δt). **Instantaneous** = the power at one instant, using Fv with the speed right now.
Question
Define efficiency.
Answer
The fraction of the energy put in that comes out as **useful** energy: η = useful out ÷ total in (× 100 for a %). It has no unit.
Question
Can efficiency be more than 100%?
Answer
**No** — you can never get more useful energy out than you put in; some is always wasted (mostly as heat).
Question
Where does the 'wasted' energy in a machine usually go?
Answer
Mostly to **thermal energy (heat)**, plus some sound — energy spread out and no longer useful.
Question
Find the average power if 600 J is transferred in 5.0 s.
Answer
P = ΔW/Δt = 600 ÷ 5.0 = 120 W.
Question
A car cruises at 30 m s⁻¹ against 400 N of drag. Engine power?
Answer
P = Fv = 400 × 30 = 12 000 W = 12 kW.
Question
Drag force F = cv. How do you get the drag constant c from power and speed?
Answer
At constant speed P = Fv = cv², so c = P ÷ v². Its SI unit is kg s⁻¹.
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Topic 1.3 hub
Work, energy and power
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