Efficiency: Question bank — Hinglish deep dive

1.3.10 · D5 · Physics › Work, Energy & Power › Efficiency

Ek rule jisko har cheez yahan test karti hai: $η = \frac{E _{useful out}}{E _{total in}} = 1 - \frac{E _{wasted}}{E _{in}}, 0 \leq η \leq 1$

jahan $η$ (Greek letter eta, efficiency ke liye hamara symbol) ek pure fraction hai: useful energy out divided by total energy in.

True ya false — justify karo

Ek machine jo exactly utni hi energy output karti hai jitni leti hai, uska

η = 100%

hota hai.

Principle mein True hai, lekin practice mein impossible: iske liye zero waste chahiye (

E_{wasted} = 0

), aur har real process mein kuch heat, sound ya friction leak hoti hai. Toh

η = 100%

ek aisi limit hai jiske paas tum jaate ho, kabhi reach nahi karte.

Ek bahut achhi tarah se oiled machine ke liye efficiency

100%

se zyada ho sakti hai.

False. Iska matlab hoga

E_{useful out} > E_{in}

, yaani energy kuch nahi se bani — yeh Conservation of Energy ki violation hai. Oiling friction ko kam karti hai,

η

100%

ki taraf push karti hai, kabhi usse aage nahi.

100%

ke paas efficiency wala ek lever phir bhi "energy multiply" karta hai kyunki ek choti force badi weight uthati hai.

False. Lever force multiply karta hai, energy nahi: choti force ek lambi distance se move hoti hai, isliye

F \times d

\approx F \times d

out. Energy amplify nahi hoti — sirf force aur distance ke beech trade hoti hai.

Agar koi device bilkul bhi energy waste na kare, toh uski efficiency

0%

hoti hai.

False — yeh toh ulta hai. Zero waste ka matlab hai

E_{wasted} = 0

, toh

η = 1 - 0 = 100%

. Log "no waste" ko "no output" ke saath confuse karte hain; yahan iska matlab hai perfect output.

Efficiency ki units joules hain.

False. Yeh do energies ka ratio hai, toh joules cancel ho jaate hain —

η

dimensionless hai, 0 aur 1 ke beech ek bare number (ya percentage).

Series mein do machines, har ek

70%

efficient, overall

140%

efficiency deti hain.

False, aur yeh impossibly bhi hai. Series efficiencies multiply hoti hain:

0.70 \times 0.70 = 0.49 = 49%

. Inhe add karna

100%

se zyada ho jaata, jisko conservation forbid karta hai.

Ek machine electricity ko heat mein convert karne mein

100%

efficient ho sakti hai.

True — ek electric heater. Agar heat hi woh cheez hai jo tum chahte the, toh sab input useful output ban jaata hai, toh

η \approx 100%

. Efficiency poori tarah iss baat par depend karti hai ki tum kya useful kehte ho.

Ek motor ki

30%

wasted energy destroy ho jaati hai.

False. Yeh poori tarah conserved hai, bas un forms mein convert ho jaati hai jo tum nahi chahte the — zyaadatar windings mein heat, plus sound aur vibration (dekho Heat and Internal Energy). "Wasted" ka matlab hai "galat form," na ki "chali gayi."

Error dhundho

"Ek bulb 100 J light aur 400 J heat emit karta hai, toh uski efficiency

500/500 = 100%

hai."

Error yeh hai ki heat ko useful output count kiya gaya. Ek lamp ke liye useful output sirf light hai:

η = 100/500 = 20%

. Total output hamesha input ke barabar hota hai (conservation) — yeh efficiency ke baare mein kuch nahi bataata.

"Pump 2 kW leta hai aur 1400 W deliver karta hai, toh

η = 2000/1400 = 143%

Ratio ulta hai: efficiency output÷input hai, input÷output nahi. Sahi hai

1400/2000 = 70%

. Jab bhi

η > 1

mile, samjho fraction flip ho gayi hai.

"Motor input = 500 kJ, useful = 300 J, toh

η = 300/500 = 60%

Units mismatch hai: 500 kJ = 500000 J. Pehle convert karne par

η = 300/500000 = 0.06%

milta hai.

η

ek ratio hai, toh divide karne se pehle dono quantities ek hi unit mein honi chahiye.

"Ek box uthane ki efficiency

η = m g h / (electrical input)

hai, aur

m g h

input hai."

Galti yeh hai ki

m g h

ko input kaha gaya. Yahan

m g h

useful output hai (gravitational PE gained, dekho Gravitational Potential Energy); supply ki gayi electrical energy input hai. Output upar jaata hai.

"Yeh engine

60%

efficient hai, toh yeh

40%

heat ke roop mein waste karta hai plus jitna friction leta hai."

Double-counting ho rahi hai.

40%

waste mein pehle se friction, sound aur heat sab shamil hai — har woh cheez jo useful nahi.

E_{wasted} = E_{in} - E_{useful}

sab losses ka total hai, sirf unme se ek nahi.

"Chained machines ke liye mein efficiencies average karta hoon:

(80% + 50%) /2 = 65%

Average karna galat hai kyunki doosri machine sirf pehli ka reduced output hi dekhti hai. Tum multiply karte ho:

0.80 \times 0.50 = 40%

— hamesha sabse weak stage se neeche, kabhi average nahi.

Why questions

Efficiency kabhi

100%

se zyada kyun nahi ho sakti, chahe design kitna bhi clever ho?

Kyunki

E_{useful out} \leq E_{in}

Conservation of Energy se forced hai: output sirf woh hissa hai jo aaya tha.

η > 1

milne ka matlab energy create karna hoga, jo koi machine nahi kar sakti.

Agar energy hamesha conserve hoti hai, toh hum efficiency ki kyon parwah karte hain?

Conservation kehta hai total amount fixed hai, lekin yeh nahi ki woh useful hai. Efficiency measure karti hai ki kitna us form mein raha jo tum chahte the, toh hum machines compare kar sakte hain aur waste cut kar sakte hain — yeh usefulness ke baare mein bookkeeping hai, quantity ke baare mein nahi.

Friction specifically efficiency kyun kum karta hai?

Friction ordered kinetic/mechanical energy ko scattered heat mein convert karta hai, jo (usually) woh output nahi jो tum chahte the. Woh heat

E_{wasted}

mein jaati hai, toh

η = 1 - E_{wasted} / E_{in}

drop ho jaata hai.

Formula mein energies ki jagah powers kyun use kar sakte hain?

Kyunki power

P = E / t

hai, aur input aur output ek hi elapsed time

t

share karte hain. Jab tum ratio banate ho toh

t

cancel ho jaata hai, isliye

η = P_{useful} / P_{in}

same number deta hai (dekho Work and Power).

"Ek achhi machine thandi chalti hai" ek fair rule of thumb kyun hai?

Machine se nikali heat usually wasted energy hai. Kam heat produce hone ka matlab smaller

E_{wasted}

hai, isliye higher

η

. Ek garam machine literally woh energy radiate kar rahi hai jiske liye tumne pay kiya.

Real heat engines ka ek hard ceiling

100%

se neeche kyun hota hai, motor ke unlike?

Ek heat engine ko chalte rehne ke liye kuch heat ek cold reservoir mein dump karni padti hai — yeh thermodynamics set karta hai, sirf sloppy engineering nahi. Carnot limit

η

ko 1 se bahut neeche cap karta hai perfect frictionless engine ke liye bhi (dekho Heat Engines & Carnot Efficiency).

Edge cases

Ek aisi machine ki efficiency kya hai jiska useful output zero hai (sab input woh heat ban jaata hai jo tum nahi chahte the)?

η = 0/ E_{in} = 0%

. Yeh lower bound hai: energy phir bhi conserved hai, lekin kisi ne bhi intended kaam nahi kiya.

Kya efficiency exactly

0%

aur exactly

100%

ho sakti hai?

0%

achievable hai (ek device jo koi useful output produce nahi karta).

100%

sirf ek theoretical limit hai — iske liye zero waste chahiye, jise real friction aur heat loss forbid karte hain, toh real

η

strictly beech mein rehta hai.

Agar tum redefine karo ki "useful" kya count hota hai, toh kya machine ki efficiency change hoti hai?

Haan — dramatically. Ek heater

\sim 100%

efficient hai agar heat useful hai, lekin near

0%

agar tum light chahte the. Efficiency tumhare goal ke relative define hoti hai, toh hamesha pehle batao ki "useful" ka matlab kya hai.

Ek single machine

90%

efficient hai. Tum uske baad ek doosra, perfect,

100%

stage add karte ho. Overall efficiency kya hai?

0.90 \times 1.00 = 90%

. Ek perfect stage na help karta hai na hurt karta hai — 1 se multiply karna total unchanged rakhta hai, series-multiplication rule confirm karta hai.

Das identical stages, har ek

95%

efficient, chain hain. Kya puri cheez abhi bhi highly efficient hai?

Nahi:

0.9 5^{10} \approx 0.60 = 60%

. Chote losses multiplicatively compound hote hain, toh series mein kaafi "achhe" stages ek mediocre system ban sakte hain — chained efficiency ki tyranny.

Jab machine ki wasted energy full input ke paas jaati hai toh

η

ka kya hota hai?

Jaise

E_{wasted} \to E_{in}

, formula

η = 1 - E_{wasted} / E_{in} \to 1 - 1 = 0

. Efficiency smoothly

0%

tak girta hai — ek completely useless machine ka limiting case.

Recall Yahan har trap ki ek-line summary

Teen deadly mistakes hain: (1) ratio flip karna (input÷output), (2) units mix karna, aur (3) "useful" galat count karna — hamesha poocho mujhe actually kya chahiye tha? Aur do conservation facts sab kuch pin karte hain: $η \leq 100%$ hamesha, aur series efficiencies multiply hoti hain.

Connections

Efficiency — parent note; definition aur derivations jinhein ye traps probe karte hain.
Conservation of Energy — woh single law jiske peeche " $η$ 100% nahi beat kar sakta" hai.
Work and Power — kyun power form same number deta hai.
Gravitational Potential Energy — usual "useful output" $m g h$ jise log galat jagah rakhte hain.
Heat and Internal Energy — jahan "wasted" energy actually jaati hai.
Friction — roz ka woh reason jiske liye $η < 1$ hota hai.
Heat Engines & Carnot Efficiency — kyun engines ka ek hard sub-100% ceiling hota hai.