Questions tagged [efficient-energy-use]

Use the tag *Efficient Energy Use* in questions having to do with processes or engines or constructions where energy is spent or transferred and the efficiency thereof. Efficiency means both the measurement and the techniques and principles for its optimization.

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1answer
33 views

Sound from a rocket

I have been watching programmes commemorating the Apollo 11 mission. One obvious feature of the launch is the sound. It was mentioned that water was sprayed into the pit below the rocket not (...
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1answer
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Why does intercooling improve the efficiency of a Brayton Cycle?

I've read in numerous sources that intercooling improves the efficiency of a Brayton cycle. The explanation given is that it decreases the compression work by minimizing the temperature during the ...
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2answers
54 views

Why a reversible engine's efficiency must be equal to that of a Carnot engine

Apparently, in order to prove that an engine must be as efficient as a Carnot engine if reversible is because apparently for its efficiency in either direction: $\eta_E \le (1-T_2/T_1)$ $\eta_R \ge (...
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1answer
50 views

Can two quasi-static adiabates intersect? [closed]

I saw a question in my physics book that I've been trying to understand lately. The question is simple: given two quasi-static adiabatic curves, could you find a point of their intersection? Also, ...
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1answer
67 views

Is it possible to transform energy stored in anti-matter into coherent radiation?

In Travellers, a sci-fi show on netflix, they have 10g of some form of anti-matter floating around. A quick $mc^2$ calculation gives a quantity of $E = 10^{15} J$. For reference, Little Boy released $...
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1answer
169 views

How to calculate the power required by a fountain pump?

How can I calculate the power required by a fountain pump, if just the efficiency of the pump, the height of the fountain and the diameter of the water jet is given?
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2answers
688 views

Should we take the elevator instead of the stairs to save energy?

In our university there's a posted sign that encourages students to take the stairs instead of the elevator to save the university electricity during the hot NYC summers. When you climb the stairs ...
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4answers
528 views

Is it more efficient to drive fast uphill?

I know that for a rocket escaping the atmosphere, it's not efficient to travel slowly because even staying stationary consumes a lot of fuel. Does the same apply to a vehicle traveling uphill? In ...
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2answers
233 views

Natural gas furnace vs. power plant efficiency

In the USA, residential heaters often use a measurement called AFUE, which is supposed to be a percentage of input energy to output energy. It is typical for a two-stage natural gas furnace to achieve ...
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3answers
158 views

Is the “efficiency” of car brake affected by how much force is applied by the brake?

Someone argues with me that stepping on a car's brakes for long periods of time will wear out the brakes faster than applying the brakes for short periods of time. We were talking about stepping on ...
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Why is it so inefficient to generate electricity by absorbing heat?

When I turn on a heater, it's supposed to be roughly 100% efficient. So it converts electricity to heat with great efficiency, but why can't we do the reverse: generate electricity by absorbing heat? ...
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1answer
143 views

Efficiency of any machine

Why cant the efficiency of a machine be 100%? We always see that there is some energy that is lost, mostly in the form of heat. But why is it so? Is it impossible to create a machine with 100% ...
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1answer
56 views

How can I calculate the efficiency of an air-powered car?

An air-powered car is powered using high pressure compressed air stored in a tank. The pressurized tank is connected to an air-powered engine. The volume of the tank is 0.3 m3, and the initial ...
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1answer
38 views

would heating a stove to 400 degrees be more or less energy efficient if done more quickly?

Would heating a stove to say 400 degrees be more or less energy efficient if done over a shorter or longer period of time? say 5 vs 10 minutes. Which would cost more on the electricity bill?
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Efficiency given two temperatures

Not sure what formula to use for this question? I am new to physics and the regular efficiency formula doesn't seem like the one I should be using. I was thinking the carnot efficiency, since it ...
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1answer
523 views

Stacking thermoelectric generators for a better efficiency

As far as I know, a thermoelectric generator has about 5-8% of efficiency. That means it uses 5% of the heat and converts them to electricity. Rest 95% is dissipated through the other side. So I can ...
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1answer
83 views

How can energy of petrol be compared to electricity?

When driving my car, which is powered by an electrical engine, I'm consuming between 10 and 17 kWh per 100 km (measured at the charging outlet). The car I had before consumed between 5 and 8 litres ...
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1answer
65 views

How much extra fuel is needed to transport a 1kg object over a distance of 10,000 miles? [closed]

I propose the scenario: a 1,300-kilogram, base weight, vehicle is carrying a 1-kilogram textbook with a 35% efficient engine over a distance of 10,000 miles. How much fuel is being burned to carry ...
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0answers
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How to prove that an arbitrary cycle in the PV diagram has an efficiency less or equal than Carnot's Cycle? [duplicate]

The statement is: Consider an ideal gas with constant number of particles (N is constant), and an arbitrary thermodynamic cycle (closed curve with no self-interesections) in the PV diagram. Show that ...
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58 views

Understanding efficiency

This is a repost of a previous question, following the advice of an experienced user. I was told that reversible processes are more efficient than irreversible ones, and that this fact is expressed ...
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2answers
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Understanding entropy and efficiency

A few years have passed since the last time I studied thermodynamics, and I'd like to give me a chance of asking some questions, maybe trivial, I don't know, but I want to clear matters up. Since my ...
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1answer
40 views

Is it correct to find ideal engine's efficiency using celsius scale?

I am solving a problem from my book which asks to calculate the maximum efficiency of an engine whose hot body is at temperature 300°C and cold body is at 25°C. Using 1- Tc/Th, I got efficiency equals ...
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3answers
326 views

Water turbine power generation for large and small turbines

If you have a small water turbine and a larger water turbine in a river, would the larger turbine produce more power or would they produce the same because of the equal speed of the river?
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1answer
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What is the average range of voltage of lightning in sky?

I wondered how much energy is required to produce , such a loud sound? What is the average range of voltage in sky? Is there any possible way to store it ,and use when needed?
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Why is the efficiency of human cells less than the efficiency of an Otto engine?

I always used to think (I don’t know why!) that the efficiency of human (and animal and plant) cells should be equal to or near the efficiency of a Carnot engine or at least should be the highest ...
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1answer
284 views

How is the energy efficiency in bicycles accounted for? [duplicate]

Bikes are more efficient than walking, but that's only because roads are flat so wheels make sense, right? Does it make sense to say that this fact is simply gained by putting a lot of energy into ...
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1answer
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How much realistically usable energy can be obtained from matter/antimatter interaction?

The high energy density of a matter/antimatter system is well known. However, depending on the nature of the material, most of the energy from the interaction is released in the form of photons (...
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3answers
558 views

Calculating the electrical power generated from a wind turbine [closed]

Heres the question: A wind turbine has blades that sweep an area of 2000 m^2. It convrts the power available in the wind to electrical power with a efficiency of 50%. What is the electrical power ...
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2answers
132 views

Are all heaters exactly as energy efficient as each other?

I many applications, efficiency is measured in terms of how much of the input energy is wasted generating heat instead of the actual type of energy you want. For example, lightbulbs generating heat ...
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1answer
117 views

How can I get 100% fuel efficiency from an IC Engine in a car?

I understand that the current technology of internal combustion (IC) engines gives about 20% for a normal car. I want to understand what I can do to achieve 100% or near 100% efficiency in the same ...
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4answers
3k views

Why does heat engine need to reject energy to environment?

The most efficient thermodynamics cycle is the Carnot cycle, it involves 1/ Receiving heat from hot source, expanding and doing work on the surround 2/ Receiving work from environment, being ...
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2answers
167 views

How much money can I save on gas by losing weight?

I read somewhere that if you carried around a penny in your car, over the lifetime of the car, you would end up paying an extra penny in gas than if you weren't carrying that penny. I was wondering ...
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Do you know the principle which says that connecting two sources of similar kind produces a waste and destruction? [closed]

There is a great article, called commutation cells, which states that you cannot transfer kinetic energy from one container to another immediately, bypassing the potential energy storage. Otherwise, ...
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2k views

Which is more mechanically efficient, a bicycle or a unicycle?

Human physiology aside, I wonder which device is more mechanically efficient, a bicycle or a unicycle?
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1answer
95 views

Heating 2 sets of 1L water vs 1 set of 2L water [closed]

Can heating 2 sets of 1L water or 1 set of 2L water have a variation in terms of fuel efficiency? Can one be more fuel-efficient than the other? (migrated from chemistry site)
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2answers
4k views

Otto and Carnot efficiency comparison

Lets say both engines operate between same temperature limits. Carnot eff: $$ e_\text{Carnot} = 1-\frac{T_L}{T_H} $$ Otto eff: $$ e_\text{Otto} = 1-\frac{T_4-T_1}{T_3-T_2} $$ assuming ...
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2answers
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seperate engines working vs single engine

What is more fuel efficient: Two identical cars running at 50mph or one being tied to the other and only the first dragging both at 50mph? Or what requires less fuel: Two cars to start from stop and ...
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5answers
2k views

Engine fuel consumption vs power

Something that has bothered me for a long time is why a 600 hp engine uses more fuel per kilometer than a 80 hp engine. Let pretend I have two equal cars (same shape same weight etc) except for the ...
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1answer
2k views

energy efficiency - dryer vs hang+dehumidifer

When I have wet clothes coming out of the washing machine, I can either tumble-dry them in an electric clothes-dryer, or I can hang them to dry. But I can only hang them in a basement. The basement ...
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1answer
1k views

Is “eco” shower mode actually using less power? [closed]

At work we have electronic showers in the gym, they have three modes: * (A snowflake, Which I guess means no heating at all) ...
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Do magnetic fields cause ionisation of gases?

I am doing my final year engineering project on Magnetic Field Assisted Combustion and was curious to see what people thought about it. Companies sell rare earth magnetic arrangements to be attached ...
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1answer
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Maximum power transfer proof

I have the following homework problem. Consider a power supply with fixed emf $ε$ and internal resistance $r$ causing current in a load resistance $R$. In this problem, $R$ is fixed and $r$ is a ...
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3answers
2k views

Shouldn't solar cells be nonreflective?

If solar cells take energy from light, then why do they reflect it? How come they can take energy without actually affecting the source? Shouldn't reflected light (as a minimum) have a lower ...
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1answer
449 views

Pressure energy to Electric energy

Is the pressure energy can be converted into electric energy? Like pressure energy of vehicles on a bridge can be converted into electric energy.
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7k views

How much of the energy of a car is required for overcoming air resistance?

How much of the energy of a car is required for overcoming air resistance? For example, assume a car going 65 mph has a mileage of 25 gpm. If all air resistance was eliminated, approximately what ...
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2answers
2k views

Efficiency of Bicycle Pedalling

Consider a bicycle with multiple gears. Suppose that you are in a starting position with someone holding your bike upright (so when you start there's no issue with clipping in etc). It's well-known (...
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3answers
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Why is the Carnot engine the most efficient?

It seems that the only condition used in proving that the Carnot engine is the most efficient is that it is reversible. More specifically, the Carnot engine can be run in reverse as a refrigerator. ...
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1answer
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Energy efficiency for common fuels

If we take matter-antimatter annihilation as being 100% energy efficient, what would be the energy efficiency of: Uranium fission, Fusion of Hydrogen to Helium, and Combustion of Kerosene. I ...
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4answers
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How does lowering your thermostat save energy?

Say it is winter and the outside temperature is 0 degrees f. I set my thermostat to 74 degrees. When the temperature inside my home reaches 72 degrees, the furnace will kick on and warm the house to ...
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1answer
142 views

Is it possible to improve the efficacy of light bulbs with a semipermeable mirror? (The greenhouse bulb)

Let us imagine that there exists a material that reflects infrared radiation, but is transparent for visible light. Could we take an incandescent bulb and add this material to the inner surface of the ...