Physics

Power, Energy, and Efficiency

463 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 evs - i substances, objects and energy renewable resources alternative fuels and energy sources alternative sources of energy

If K.E. body is increased by 100 % then % change in its momentum is:

  1. 50%

  2. 41.4%

  3. 10%

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

Momentum p is related to kinetic energy K by p = sqrt(2mK). If kinetic energy increases by 100 percent, the new kinetic energy becomes K2 = K + 1.0K = 2.0K. The new momentum p2 = sqrt(2m(2K)) = sqrt(2) * p = 1.414 * p, representing a 41.4 percent increase.

Multiple choice physics solar equipment production of electricity from solar energy solar power alternate sources of energy solar power plant solar energy and its applications renewable and non-renewable resources renewable and non-renewable sources of energy generation of electricity

The efficiency of a solar cell is 20% and its surface area is $4  {cm}^{2}$. Find the electrical energy generated in one second, by 3000 cells, connected in a solar panel, if $640  J$ of solar energy is incident on one meter square area, in one second.

  1. 183.6 J

  2. 153.6 J

  3. 193.6 J

  4. 123.6 J
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The total surface area of solar cells on which solar energy is falling = area of each cell $\times$ number of cells $= 4 \times 3000 {cm}^{2} = 1.2  {m}^{2}$
The solar energy falling on 3000 cells 
${Q} _{1} = 1.2 \times 640  J = 768  J$
The energy converted into electric energy $= \displaystyle\frac{20}{100} \times 768 = 153.6  J$
The electric energy generated in one second is $153.6  J$

Multiple choice physics option b: engineering physics buoyancy floatation fluid pressure

A pump is required to lift 1000 kg of water per minute from a well 20 m deep and eject it at a rate of 20 $ms^-1$. What (horsepower engine is required for the purpose of lifting water)?

  1. $4.46 HP$
  2. $4.36 HP$
  3. $3.96 HP$
  4. $8.85 HP$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Power required to lift water = m*g*h / t. Given mass per minute m/t = 1000 kg/min = 1000/60 kg/s, height h = 20 m, g = 9.8 m/s^2. Potential energy rate = (1000/60) * 9.8 * 20 = 3266.67 W. Power required to eject water at velocity v = 20 m/s is (1/2) * (dm/dt) * v^2 = 0.5 * (1000/60) * 20^2 = 3333.33 W. Total power = 3266.67 + 3333.33 = 6600 W = 6.6 kW. Converting to horsepower: 6600 / 746 = 8.85 HP.

Multiple choice physics option b: engineering physics buoyancy floatation fluid pressure

Power of a water pump is 2 kW. If $g=m/{ sec }^{ 2 }$, The amount of water it can raise in one minute to a height of 10 m/s 

  1. 100 Litre

  2. 1200 Litre

  3. 1000 Litre

  4. 2000 Litre
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Power P = 2 kW = 2000 W. Work needed to lift mass m to height h in time t: W = mgh. P = mgh/t, so m = Pt/(gh) = (2000×60)/(10×10) = 120000/100 = 1200 kg. This equals 1200 liters of water.

Multiple choice physics option b: engineering physics buoyancy floatation fluid pressure

A pump motor is used to deliver water at a certain rate from the given pipe. To obtain 'n' times water from the same pipe in the same time, the amount of power of the motor should be increased to:

  1. np
  2. ${n^3}$
  3. ${n^2}$

  4. 2np
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Power P = (1/2)mv^2/t = (1/2)(rho*A*v*t)v^2/t = (1/2)rho*A*v^3. If flow rate Q = Av is increased by n, then v must increase by n (assuming area constant). Thus P is proportional to v^3, so power increases by n^3.

Multiple choice commercial applications management of natural resources unequal distribution of resources protecting our environment rules are for everyone

A biogas plant works to its maximum capacity when

  1. Conditions are aerobic and sewage is supplied
  2. Conditions are anaerobic and temperature $40^oC$
  3. Conditions are aerobic and temperature $40^oC$

  4. Conditions are anaerobic and sewage is supplied
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Biogas refers to a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen. Biogas is produced by anaerobic digestion with methanogen or anaerobic organisms, which digest material inside a closed system or fermentation of biodegradable materials. There are two key processes: mesophilic and thermophilic digestion which is dependent on temperature. Research in biogas production indicates that the optimal temperature for mesophilic bacteria is around 37°C and around 55°C for thermophilic bacteria.


So, the correct answer is 'Conditions are anaerobic and temperature 40C'.

Multiple choice chemistry rocks and minerals coal and its products study of coal coal

How much coal is needed to supply enough electricity to light ten 100-watt bulbs for about an hour?

  1. 1 pound

  2. 10 pounds

  3. 50 pounds

  4. 20 pounds
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

100 watts = 0.1 KW


For 1 bulb 0.1 KW of power is required, for 10 bulbs that turn out to be 1KW.
Now 10 bulbs have to light for 1 hour that will be 1 KW multiplied by 1 hour = 1 KW

The thermal energy of coal is 6,150 KW/ton, only 40% of the thermal energy of the coal is converted to electricity. so the electricity converted per ton of coal is 0.4 * 6,150 = 2,460 KW/ton.

Number of tons of coal burned for 10 light bulbs for one hour = 1 KW / 2,460KW/ton = 0.0004065 tons 

Multiplying by 2000 pound/ton we get 0.81300 $\approx$ 1 pounds.

Multiple choice zoology meaning of life energy flow and loss energy flow in a food chain energy flows

If $5$ joules of energy is available at producer level then find the energy transferred to the lion in the following food chain.
Plants $\rightarrow$ Deer $\rightarrow$ Lion

  1. $5$ joules
  2. $0.5$ joules
  3. $0.05$ joules
  4. $0.005$ joules

  5. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
  • According to the 10% rule when energy is passed in an ecosystem from one tropic level to the next, only 10% of the energy is passed on and the rest is wasted.
  • 10% of (10% of 5 joules)=Energy transferred to the lion.
  • So, the correct choice is '0.05 joules'.
Multiple choice zoology meaning of life energy flow and loss energy flow in a food chain energy flows

If the energy produced by plants is 800,000 kJ, then how much energy will exist at the level of secondary consumers?

  1. 80 kJ

  2. 800 kJ

  3. 8,000 kJ

  4. 80,000 kJ

  5. 800,000 kJ
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

According to the 10 percent law, when energy is passed in an ecosystem from one trophic level to the next, only ten percent of the energy will be passed on.  The remaining is lost during transfer, broken down in respiration, or lost to incomplete digestion by higher trophic levels.

As per this rule, if the plant produces 800,000kJ of energy then only 10% of this is passed to the next trophic level which is the primary consumers. So, the energy obtained by the primary consumer will be 80,000kJ.
Now, only 10% of this 80,000kJ is passed on to the next trophic level (secondary consumers) which is 8,000KJ.
So, the correct answer is option C.

Multiple choice zoology meaning of life energy flow and loss energy flow in a food chain energy flows
Read the passage and answer the following question.

Assume you had a 10m$^2$ pond to study. You measured the amount of light energy bathing the water surface at 1,000kcal.


About how much of that available energy would be expected to be converted into producer biomass?

  1. $100$ kcal
  2. $1,000$ kcal
  3. $2,000$ kcal
  4. $10$ kcal
  5. $200$ kcal
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Plants absorb 1% of sun energy for photosynthesis. With each trophic level, only 10% energy is transferred and 90% is lost as heat in respiration. Here, the total available sunlight is 1000kcal, the producers will get 1% i.e. 10kcal. Correct answer is D.  

Multiple choice botany leaf - external morphology morphology of leaf structure of a leaf the leaf

A Leaf of $20 \,cm^2$ grows $5 \,cm^2$ per hour and $B$ Leaf of $25 \,cm^2$ grows $5 \,cm^2$ per hour
The relative growth rate of leaf $A$ and $B$ respectively is

  1. $25\%$ and $20\%$
  2. $20\%$ and $25\%$
  3. $50\%$ and $100\%$
  4. $25\%$ and $50\%$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Relative growth is the increase in size relative to the size of the plant leaf present at the start of a given time interval. It can be also expressed as percentage increase.

Hence, for leaf A, relative growth = 5/20 × 100 = 25
For leaf B, relative growth = 5/25 × 100 = 20.
So, the correct option is '25 and 20'.