Home › Exam notes › Railway RRB › Work, Power, Energy & Efficiency

Work, Power, Energy & Efficiency

Basic Science & Engineering · Measurement, Drawing & Mechanics

Useful for: Railway RRB

Overview

Work and energy are measured in joules, power in watts; efficiency compares useful output with input.

Key points

Work = force × displacement in the direction of force.
Power = work ÷ time; 1 hp = 746 W.
Efficiency = useful output ÷ input × 100%; always below 100% in real machines.
Machine efficiency = mechanical advantage ÷ velocity ratio × 100%.
Energy used (kWh) = power (kW) × time (h).

Explanation

Losses come from friction, heat and sound.
Higher efficiency means less wasted energy.
Electric motors convert electrical to mechanical energy.
Generators convert mechanical to electrical energy.

Step by step

Identify the concept, tool or rule involved.
Recall the definition, value or procedure.
Check units, safety logic and the direction of the effect.

Examples

A motor takes 2 kW and delivers 1.6 kW → efficiency 80%.
A machine with MA 4 and VR 5 → efficiency 80%.
A 2 kW heater for 3 h uses 6 kWh.

Traps and advanced points

Trap: Efficiency above 100%.
Trap: Mixing kW and kWh.
Trap: Using MA alone as efficiency.

Quick revision

η = output/input.
η = MA/VR.
kWh = kW × h.

Practice MCQs

Malaria is caused by which type of organism?
1. Fungus
2. Virus
3. Bacterium only
4. Plasmodium parasite
Read chapter notes · Similar questions
Show answer
Answer: (d) Plasmodium parasite. Malaria is caused by Plasmodium parasite.
The functional unit of the kidney is what?
1. Villus
2. Nephron
3. Neuron
4. Alveolus
Read chapter notes · Similar questions
Show answer
Answer: (b) Nephron. The functional unit of the kidney is Nephron.
What is the SI unit of electric current?
1. coulomb
2. volt
3. ohm
4. ampere
Read chapter notes · Similar questions
Show answer
Answer: (d) ampere. Ampere is the SI unit of electric current.
Which component of blood is central to clotting?
1. Plasma water only
2. Red blood cells only
3. Neurons
4. Platelets
Read chapter notes · Similar questions
Show answer
Answer: (d) Platelets. Platelets is central to clotting.
One horsepower is equal to about
1. 746 W
2. 1,000 W
3. 500 W
4. 100 W
Read chapter notes · Similar questions
Show answer
Answer: (a) 746 W. This is the British (mechanical) horsepower.
1 kilowatt-hour equals
1. 1,000 J
2. 3.6 × 10³ J
3. 3.6 × 10⁶ J
4. 36 J
Read chapter notes · Similar questions
Show answer
Answer: (c) 3.6 × 10⁶ J. 1,000 W × 3,600 s.
A machine takes 500 J of input and gives 400 J of useful output. Its efficiency is
1. 20%
2. 80%
3. 125%
4. 90%
Read chapter notes · Similar questions
Show answer
Answer: (b) 80%. Efficiency = 400 ÷ 500 × 100.
Work done is zero when the force and displacement are
1. both very large
2. in the same direction
3. in opposite directions
4. perpendicular to each other
Read chapter notes · Similar questions
Show answer
Answer: (d) perpendicular to each other. W = Fs cos 90° = 0.