135
votes
Accepted
Why don't we use weights to store energy?
You can use dead weights, but you need a huge amount of weight.
For example the biggest pumped hydroelectric system in the world (the Gianelli Hydroelectric Plant in California, USA) uses water ...
- 10.2k
96
votes
Why don't we use weights to store energy?
Let's spin some numbers to further illustrate the poor energy density of gravity-based storage systems. Assume that you have a 100 kilogram lead weight that you can lower into a 10 meter deep hole in ...
- 2,421
64
votes
Why don't we use weights to store energy?
We do use mass in a gravitational field to store energy and have done for hundreds of years!
Grandfather Clocks
Grandfather Clocks have used powering weights since the 1660s. This was when they ...
- 741
62
votes
Accepted
Can a gym be built to supply electricity to homes?
The maximum continuous power that can be generated for an hour by a fairly fit person on an efficient machine like an exercise bike or rowing machine is $\sim 200$ W (olympic-standard track cyclists ...
- 136k
44
votes
Why don't we use weights to store energy?
Another way to store energy in mass is the use of flywheels. You simply take a massive wheel and spin it up to store energy, use an electrical generator as a break to take energy out.
Their main ...
- 22.8k
42
votes
Accepted
The water analogy seems to imply that power = current. Why is this incorrect?
Power to a water-wheel depends both on the current (amount of water delivered)
and the head (vertical drop of water as it turns the wheel). So, the
water analogy does have TWO variables that ...
- 10.2k
40
votes
Why is it impossible for the reactor of the nuclear power plant to turn into an explosive nuclear bomb?
A nuclear reactor cannot explode like a nuclear weapon.
For a thermal reactor -like Chernobyl or Three Mile Island- the neutron generation lifetime is too long.
For a fast reactor (and a thermal ...
- 9,441
38
votes
Why do we reduce only current to prevent power loss? Why not voltage?
You need to pay attention to where the voltage is. Increasing the supply voltage does not mean that the voltage in all parts of the circuit go up. In fact, it might go down in some parts. Let's do ...
- 2,085
28
votes
Why is "work done/time taken" correct over "force × velocity" as a definition of power? (A level)
Both seem to be correct, as another answer also points out. Meaning, both formulas can be used to calculate power.
But the fundamental definition of power is the latter, namely energy per time.
In ...
- 52.3k
27
votes
Are energy and work the same thing?
Are energy and work the same thing
No.
Work is one of the two means for transferring energy. The other is heat. But work (and heat) are not the energy itself. They are processes for transferring ...
- 77.9k
22
votes
Why don't we use weights to store energy?
We do. Just the weights we use are made not from lead, but from water. Many water reservoirs are also used to store energy by pumping water up when you have energy surplus, and letting it come down ...
- 619
22
votes
The water analogy seems to imply that power = current. Why is this incorrect?
Here is a simple way to keep this stuff straight.
Power is always the product of an effort variable and a flow variable. In hydraulic systems, the effort variable is pressure and the flow variable ...
- 97.8k
21
votes
Accepted
Understanding voltage and power in the fluid analogy for DC circuits
Now lets imagine two hoses... one has twice the resistance (meaning it has smaller physical width than the other). But we also make sure that both hoses have the same current (meaning that the smaller ...
- 105k
19
votes
How much power does it take to maintain the high vacuums in advanced physics experiments?
As someone who has worked on an experiment with an ultra-high vacuum, I can tell you an order of magnitude. However, the real answer is that asking how much power it takes isn't really the right way ...
- 8,296
18
votes
Accepted
Contradiction in Ohm's Law and relation $P=VI$
When you are going from an equation to a proportionality statement you need to be mindful of what is being kept constant.
$V=IR$ means that $I$ varies directly with $V$ if $R$ is constant.
$P=IV$ ...
- 8,920
17
votes
Mathematical Definition of Power
The work done by a force is not defined by $W=\mathbf F\cdot\mathbf r$. Work is instead defined in terms of a line integral over a path (your equation just assigns a work for a force and position, ...
- 58.5k
16
votes
Contradiction in Ohm's Law and relation $P=VI$
There is no contradiction here. In fact, the power equation is often represented in different ways:
\begin{eqnarray}P & = & IV\\
P & = & I^2R \\
P & = & V^2/R
\end{eqnarray}
...
- 1,562
16
votes
Watts vs. volts amperes
I understand that a watt is a unit of power (change in energy per unit time) that describes the rate at which physical work can be done
Right.
The key thing to observe is that energy can move in ...
- 1,181
16
votes
Why is it impossible for the reactor of the nuclear power plant to turn into an explosive nuclear bomb?
In brief, the lack of a containment structure surrounding any nuclear fission chain reaction will lead to the core (nuclear material) either breaking up or quickly decaying sufficiently that the ...
- 11.1k
15
votes
Accepted
Why doesn't increasing resistance increase brightness if $P=I^2\cdot R$
Power sources can work in two modes : control voltage (CV) or control current (CC).
In CV mode, the voltage is imposed, and the output current is adjusted depending on the load. This is for instance ...
- 1,239
15
votes
Accepted
Why do power lines use high voltage?
If the total resistance of the transmission line leading from a power station to you is $R$ and the city/town you're in demands an average amount of power $P$. Then $P=I\times V$ . This makes the ...
- 394
15
votes
How does a car gain kinetic energy?
how does that energy become the kinetic energy of the car, since friction force from road doesn't do any work?
This is something I've seen several times on this site lately, and I disagree with it.
...
- 15.4k
14
votes
Why don't we use weights to store energy?
We do, and we do it in the form of solid mass, too!
ARES uses rails to move concrete weights up and down.
Potential energy storage or gravity energy storage was under active development ...
- 248
14
votes
Can a gym be built to supply electricity to homes?
Yes, these exist. There's one here in Portland, OR, USA called The Green Microgym. But it doesn't generate much energy. People wildly overestimate how much energy a human can produce.
They claim to "...
- 4,534
14
votes
Are energy and work the same thing?
Work is a transfer of energy. So they are closely related (including the same units) but they are not the same.
- 109k
14
votes
Why isn't Neptunium used in nuclear power plants?
Neptunium-237 is fissile, with a critical mass of around 60 kg, but it has a very low cross-section for thermal neutrons. It needs fast neutrons, and so in a typical power reactor only a tiny ...
- 13k
13
votes
Why is "work done/time taken" correct over "force × velocity" as a definition of power? (A level)
It's because the question asks for the 'definition'
Definition of power
so D is the answer, even though C is a valid formula.
- 13.8k
13
votes
Are energy and work the same thing?
Work as well as heat are means of energy transfer. While you may have (or carry og contain) energy, e.g. thermal energy or kinetic energy etc., you can't "have" work nor heat.
Heat is what ...
- 52.3k
13
votes
Why do we reduce only current to prevent power loss? Why not voltage?
What is throwing you, here, is the difference between the applied voltage and the voltage drop across the transmission line.
Consider a simple circuit, with a voltage source, a resistor, and a "...
- 281
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