# Tag Info

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Suppose you are using a waterwheel to do some form of work (e.g. grind corn). You need a head of water to make the wheel move, and you could use either 1kg of water at a height of a million metres or you could use a million kg of water at a height of one metre. In both cases the water would do the same amount of work as it flowed through your wheel. The ...

22

I'm not sure what all you've read on them, but I'll try to clarify at least a few things. I would certainly disagree with several of your assertions. For starters, you say "...they don't produce anything you could feasibly use as a source of material for nuclear weapons." Thorium reactors use Thorium as a fertile fuel that transmutes into fissile U233. ...

21

I was going to comment on other people's answers, but this was going to become too long. Almost everyone fails to separate Thorium (which is a fuel type) and reactor type. Safety is a function of the reactor type, and molten salt in particular for this question. Does the fuel choice impact ultimate reactor safety? Yes, but to a limited extent. So how ...

19

It's all a question of if they need it. Most that are staying within a couple AU of the sun can get sufficient power from solar panels. It's when they start getting further away that they use an RTG. For example, New Horizons, which launched in 2006 (which is considered to be 'modern' when you only launch a few probes per year) is going to Pluto, so it ...

15

edit: I originally had some points about the inefficiency of RTGs, but after some more research prompted by @Jeremy I found that it's not really a valid point when they're used appropriately for the spacecraft's mission. The RTGs used by Galileo at Jupiter generated 300W of power, whereas the solar panels that will be used by Juno at Jupiter will generate ...

14

If you throw a bunch a uranium ore in one blob, nothing happens. If you chemically purify the ore so that the only element present is uranium, still nothing happens. The runaway chain reaction needed for a uranium-powered bomb involves U-235, an isotope having three fewer neutrons than the most common natural isotope U-238. According to Wikipedia, The ...

13

Dear Thomas, the diameter of the beams of these HeNe lasers is between 0.5 and 1 millimeter, so the power 1 mW is coming to $10^{-6}$ squared meters or so. The ratio of power and area is $10^{-3}/10^{-6} = 10^{+3}$ Watts per squared meter. On the other hand, when a 3W LED is watched from the distance 0.1 meters, the power of 3 W is divided to $4\pi R^2 = ... 11 The real problem with RTGs is that the US stopped making Pu238 in the 80s and has been very slow to start up production again, purchasing all our spacecraft Pu238 from the Russians (who have now also run out). I don't know about the byproducts from the breeder reactors, but Pu238 itself is actually not that dangerous to handle, and only toxic if ingested. 10 Within power systems such as regional or national electricity grids,$\frac{\mathrm{d}^2E}{\mathrm{d}t^2}$is called the slew rate: it's used to denote the rate of change of power demanded from, or supplied to, electricity grids. It's typically either expressed as MW/s or GW/h, being two time periods of interest in balancing electricity grids. ... 9 There is thermal expansion of metals, and in a power line the length will become greater than the installation one. Gravity takes up the slack and lowers the lowest point of the line. The shape taken by suspended strings/cables with only their own weight is called a catenary, and the length depends on the constants of the solution ( alternative analysis in ... 9 This is a cost to benefit question and can only be answered by a guess in a physics board. There is a new generation of small compact reactors that could be used for powering apartment buildings The new reactor, which is only 20 feet by 6 feet, Seems compact enough, so it is not size but weight that is important, since this weight has to be lifted. ... 9 The upper bound is easy. The solar constant through the atmosphere is about$970W \over m^2$so you're gathering at most$\pi\cdot((0.75m)^2 \cdot 970W / m^2 = 1.7kW$If your focal point is perfect (it isn't) then you'd have infinite power density. You will need to measure the actual power produced (and therefor the efficiency) because it depends on a ... 9 Any reasonably flat piece of sort-of reflective metal will function perfectly well as a heat collector, but would not be terribly suited to do astronomical observations with. In principle, you could probably pull it off. But it would require a lot more accurately shaped mirrors, with a lot better quality reflective surface. Also, there's good reasons ... 8 This would guarantee a meltdown. They're trying to get heat out of the core because---thought the fission chain reaction has been suppressed---various unstable fission daughters continue to decay. Adding hot lead would add heat to the system and not stop this behavior. Total disaster. -- If the core does slag out, it will probably end in a hot ... 8 I find this sort of thing becomes much more intuitive if you can think of an analogy in terms of water. In this case, we can think of it like this: Here we have water flowing through a hole in a bath tub, into another tub underneath. The stick figure has been given the task of keeping the water level constant, by lifting water back up into the top tub ... 8 The energy will start dissipating in the atmosphere as corona losses, also the high tension(potential difference) between the wires and the ground will make it dangerous. There maybe electrical breakdowns and electric shocks which will then fry the wires along with causing damage to nearby objects and people it will also breakdown the transmission by ... 7 As far as I know you have to worry about how much energy being deposited per surface area. And the area of the "hotspot" of the laser can be very small. So the deposited energy it enough to kill cells on your retina. Actually there is a whole article on Wikipedia about it. 7 The German THTR-300 Thorium High-Temperature Reactor operated for about 16,000 hours and the IAEA produced a report on its shutdown. So there are no physics barriers to thorium reactors: there is an existence proof for thorium reactors. That ends the relevant answer for this site. There are economic, engineering, social, political, technical, and ... 7 Short, short version: It's complicated. Slightly longer version: Internal combustion engines have at least two relevant performance characteristics: power and torque. Furthermore the maximum attainable values for both are functions of the current engine speed (RPM). Acceleration will cease if the current requirement for either power or torque equal the ... 7 Here's my guess: As you know, internal combustion engines burn fuel. The power output of the engine is a function of both the current RPM and the amount of fuel you inject. But there's a catch: the engine can burn a limited amount of fuel per cycle, and therefore the higher the RPM, the more fuel can be combusted. So (as @dmckee stated), at a given gear the ... 6 Nuclear power is the only way to make submarines work underneath the water for long distances without coming up because of the oxygen that all other types require. And batteries for electric engines are too heavy. Historically a submarine had to go up to just below sea level to get air through a snorcel for the engines that charge the batteries. Then it ... 6 I would add, however, that any modification of the environment near the transmission lines changes the net impedance (the effective AC resistance) of the circuit carrying the power. So, even while for the most part you are just collecting leaked energy, you are also lowering the amount of transmitted energy. If you build an electric circuit containing a ... 6 Energy is conserved, and it is presumably not building up anywhere prior to hitting the water. Thus whatever power hits the dish is the same as is delivered to the water, with two catches: Nothing is a perfect reflector, especially over a broad range of frequencies, so the dish will only transmit a fraction$\alpha < 1$of the incoming light. Hopefully ... 5 This is a graph of Binding Energy per nucleon for the various nuclei. The fission of heavier nuclei would lead to daughters with higher binding energy than the parent (graph slopes downward towards the high mass end). Daughters will be stable, hence energetics favors fission for heavy nuclei. In Response to comment: This answer address fissionable ... 5 As an ex-physicist who now works as a quant in power markets I think it's safe to say the physics of the matter will be swamped by the economics in commodities and how power markets work. Two things to note: power prices are set by markets and not by the viability of the technology (prime mover) solar is hard to make money with w/o a long term Power ... 5 Hmm, lets see. The melting point of lead is fairly low, 327.46 °C and it is a good absorber of radioactivity. I think the problem with the reactors is not the heat per se, but the exposure of the fuel rods to the air without cooling because of escaping steam not being replenished by cooling water. The remaining steam etc may blow out the thick container ... 5 It is not the most straightforward viewpoint to say that a device "needs 1500 watts". This is more a consequence than a condition. What happens is that you create an electric circuit by plugging in a device into the outlet. That circuit follows Ohm's law: $$V = I R$$ So for a given voltage and resistance a certain current$I\$ will flow. The power is simply ...

5

Power consumption is about linear with frequency. The processor contains millions of complementary FETs as shown. When the input goes low the small capacitance gets charged and it will hold a small amount of energy. A same amount is lost during the charging. When the input goes high again the charge will be drained to ground and be lost. So with each ...

4

Probably not, lighting uses a decreasing proportion of the power load these days. In the UK the grid used to carefully monitor TV, during the commercial breaks in popular soaps the power draw would go up by giga-watts as everybody made tea! This was in the days when 20M people watched the same show and in a country with electric kettles.

4

The laser is most likely not dangerous at all when used normally, because it is very low power and within the visible portion of the spectrum. The idea is that it is sufficiently low power that your blink reflex will protect your eye before any damage occurs. The danger of a given laser source depends on its power, power density, and wavelength, as these ...

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