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 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'.

Multiple choice physics spectra the electromagnetic spectrum electromagnetic spectrum electromagnetic waves

The sun delivers $10^3\, Wm^{-2}$ of electromagnetic flux on the earth's surface. The total power that is incident on a roof of dimensions $6 \,m \times 30\, m$ is 

  1. $1.8 \times 10^5\, W$
  2. $7.2 \times 10^5\, W$
  3. $0.9 \times 10^5\, W$
  4. $4.5 \times 10^5\, W$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Given, electromagnetic flux $=10^3\, Wm^{-2}$
The area of roof $A= 6\, m \times 30\, m$
$=180 \, m^2$
The total power incident on roof
$P=10^3 \times 180$
$=1.8\times 10^5 \, W$

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

Efficiency of machine is

  1. Ratio of velocity of effort to velocity of load

  2. Ratio of the load to its effort

  3. Ratio of useful work done by the machine to the work put to the machine by its effort

  4. All
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Mechanical advantage (MA) is the ratio of the load to its effort.
efficiency of machine (n) = MA/VR
Velocity Ratio (VR) is the ratio of the velocity of effort to velocity of load.

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

Efficiency is

  1. $\displaystyle \frac { Energy\quad input }{ Energy\quad output } \times 100$
  2. $\displaystyle \frac { mechanical\quad advantage }{ velocity\quad ratio } \times 100$

  3. Both

  4. None
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

As no machine is perfectly efficient.
So, we define efficiency as efficiency = $\displaystyle \frac { Energy\quad input }{ Energy\quad output } \times 100$
Also Efficiency = $\displaystyle \frac { mechanical\quad advantage }{ velocity\quad ratio } \times 100$
A petrol engine is about 30% efficient, a diesel engine is 40% efficient.

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

The ratio of the work output to the work input of a machine is

  1. mechanical advantage

  2. efficiency

  3. work

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

Efficiency is the (often measurable) ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without waste. In more mathematical or scientific terms, it is a measure of the extent to which input is well used for an intended task or function (output).

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

The mechanical advantage of each machine is determined by

  1. adding the mechanical advantage for each simple machine

  2. finding the simple machine with the largest mechanical advantage

  3. finding the average mechanical advantage for the simple machines

  4. multiplying the mechanical advantage for each simple machine
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The mechanical advantage of each machine is determined by  multiplying the mechanical advantage for each simple machine.
Option D is correct

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

In a lifting machine, the efforts required for lifting loads of $20$ N and $30$ N were $5$ N and $6$ N respectively. If the velocity ratio of the machine is $20$, determine efficiencies corresponding to loads of $20 $ N and $30$ N

  1. 25 % and 20%

  2. 35% and 55%

  3. 30% and 50%

  4. 60% and 65%
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let us take first case,


 in which $load=20N , effort =5N ,VR=20$

$Efficiency=\dfrac{load}{VR*effort}*100$

Putting above values, we get

$Efficiency=\dfrac{20}{20*5}*100=20%$

In second case we have.  $load=30N , efoort =6N ,VR=20$

$Efficiency=\dfrac{load}{VR*effort}*100$

Putting values we get,

$Efficiency=\dfrac{30}{20*6}*100=25%$

Hence$ Efficiency=25%,20%$

Multiple choice physics simple machine common machines terms related to machines introduction to simple machines

Efficiency of a machine can also be written as 

  1. $\dfrac{\text{velocity ratio}}{\text{mechanical advantage}} \times 100 $ %
  2. $\dfrac{\text{mechanical advantage}}{\text{velocity ratio}} \times 100 $ %
  3. $\dfrac{\text{mechanical advantage}}{\text{velocity of the gear}} \times 100 $ %

  4. None of the above
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$effiency=\eta=\dfrac{Machanical-advantage}{velocity-ratio}*100$