Physics

Power, Energy, and Efficiency

443 Questions

Power, energy, and efficiency are fundamental physics concepts that quantify work and the rate of doing work. Questions cover electrical energy consumption in kilowatt hours, the efficiency of thermodynamic cycles like the Brayton cycle, and mechanical power lifting loads. This topic is essential for general science and engineering exam preparation.

Electrical energy consumptionBrayton cycle efficiencyPower calculation formulasCommercial electricity unitsHydraulic lift mechanics

Power, Energy, and Efficiency Questions

Multiple choice

In a single pass rolling operation, a 20 mm thick plate with plate width of 100 mm, is reduced to 18 mm. The roller radius is 250 mm and rotational speed is 10 rpm. The average flow stress for the plate material is 300 MPa. The power required for the rolling operation (in kW) is closest to

  1. 15.2

  2. 18.2

  3. 30.4

  4. 45.6
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\text{Given} : t_\gamma=20m m, t_f=18mm, b=100 mm, \\ R=250mm,N=10rpm.\sigma_0=300MPa\\ \text {We know, Roll strip contact length is given by,} $

Multiple choice

A 600 mm × 300 mm flat surface of a plate is to be finish machined on a shaper. The plate has been fixed with the 600 mm side along the tool travel direction. If the tool over-travel at each end of the plate is 20 mm, average cutting speed is 8 m/min, feed rate is 0.3 mm/stroke and the ratio of return time to cutting time of the tool is 1:2, the time required for machining will be

  1. 8 minutes

  2. 12 minutes

  3. 16 minutes

  4. 20 minutes
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Multiple choice

A welding operation is time-studied during which a operator was pace-rated as 120%. The operator took, on an average, 8 minutes for producing the weld-joint. If a total of 10% allowances are allowed for this operation, the expected standard production rate of the weld-joint (in units per 8 hour day) is

  1. 45

  2. 50

  3. 55

  4. 60
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\text{Hence, standard production rate of the weld joint}\\ \hspace{6cm}=\frac{8\times60}{10.67}\\ \hspace{6cm}=45units$

Multiple choice physics electric current and its effects coil and fuse heating effect of electric current- uses applications of heating effect of electric current

A coal based thermal power plant producing electricity operates between the temperatures $27^o C$ and $227^o C$ The plant works at 80% of its maximum theoretical efficiency. Complete burring of 1 kg of coal yields 3600 KJ of heat. A house needs 10 units of electricity each day. Coal used for supplying the amount of energy for the house in one year is 

  1. 1141 kg

  2. 580 kg

  3. 605 kg

  4. 765 kg
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Carnot efficiency: η_max = 1 - T_c/T_h = 1 - 300/500 = 0.4. Actual efficiency at 80%: η = 0.4 × 0.8 = 0.32. Yearly electricity needed: 10 × 365 = 3650 units. Each unit requires energy. With 3600 kJ/kg coal and 32% efficiency, usable energy per kg = 3600 × 0.32 = 1152 kJ/kg. Converting units to kJ and calculating: Coal needed ≈ 3650 × (energy per unit in kJ)/(1152) ≈ 580 kg after unit conversions.