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Comment by droithomme | Hacker News Reader

But did you know that it was over 1 kW and under 1 GW?

(And if no, were you enrolled in a Physics Electricity & Magnetism course at the time.)

I don’t know why you’re getting downvoted. 1kW is a microwave, and 1GW is a nuclear power plant. It seems not unreasonable to expect people to understand this.

Maybe, but the fact that a high end commercial microwave is about the same amount of power as my car rolling along at about 30mph(45kph) on a level road would have surprised my intuition initially. I would have pegged a car for MUCH higher power output.

So, while I probably wouldn't have pegged a dragster at GW, I might have pegged it at 50-200MW assuming I put my car at an incorrect 200kW on average as opposed to top end (assuming a dragster is at least 100x a bog standard car wouldn't be out of bounds). In addition, assuming that a dragster has an instantaneous (or short lived) power output on the same level as a diesel power plant (designed to be continuous) probably also isn't out of whack.

My estimates are still hilariously out of whack--simply because my errors are compounding in the same direction.

I have had this happen at other times.

RF level strength loss due to antennas is one of those. I was hunting a problem and saw the signal but something like -90dbm down. I mumbled that "Wow, did we get our antenna match that wrong?" An experienced RF engineer looked at me and laughed "If you have an antenna that can actually kill a +20dbm signal to better than -90dbm, you should patent it. Check the firmware. Power amp probably isn't turned on." He, of course, was completely correct because he knew that killing an RF signal more than 50dbm or so gets really hard.

A car driving 100 kph (62 mph) on a highway, uses 2 or 3 ml (0.07 to 0.1 US liquid oz) of gasoline per second. That sounds also like about the size of a fire that you can use to heat up food.

Interesting. A US gallon per hour is roughly a milliliter per second.

100km/h ~ 30m/s

3cm^3/s / 30m/s = 3cm^3/s / 3000cm/s = 0.001 cm^2 = 0.1mm^2

Fuel efficiency can be measured as an area, and 0.1mm^2 corresponds to approximately 30mpg. See https://news.ycombinator.com/item?id=9504215

"Maybe, but the fact that a high end commercial microwave is about the same amount of power as my car rolling along at about 30mph(45kph) on a level road would have surprised my intuition initially. I would have pegged a car for MUCH higher power output."

Well, I think most people don't have an intuition for whatever a "high end commercial microwave" is. I would expect a car on a level highway to be using on the order of 10 hp. I'm sure my microwave uses far less.

Air resistance is proportional to speed squared, meaning (air resistance component of) energy per distance is ~ speed², and energy per second is ~ speed³.

45 km/h vs. 90 km/h is almost an 8-fold difference of power to sustain. Air resistance is not the dominant factor on lower speeds, so I'd guesstimate it at 6-7x.

At 130km/h, with 33.6 MJ/l gasoline energy content, at 7l/100km, you get: 9.1l/h -> 300 MJ/h -> 84kW. At 30% efficiency you get 25kW or 33hp. That's about a dozen microwaves.

At 45km/h, air resistance power use should be about 1/24 of 130km/h. Guesstimating total consumption as 1/10, you get around 2kW, or a single (240V 10A) microwave/kettle.

Going at a steady 30 mph on a straight, level road is not really something people do in my part of the world, so I can't say that I have experience to know what sort of gas mileage you'd get.

Edit: More research suggests that gas mileage should be similar between 30 and 60 mph, it kind of plateaus, but I guess that means power required would still go up proportionally. 10 hp is something I was remembering from reading something about driving at around 60, so I guess 30 mph should take 5 hp.

In about the 80s, cars were rather more efficient at 50mph. Depending on brand, market segment, and transmission, I'd believe they are roughly as fuel efficient at higher speeds. My car (2.0L turbocharged hatchback) uses about 7-14kw to maintain 60-70mph on level ground at around 30-33mpg. The unfortunate uncertainty in my figures is due to poor memory

It's a meaningless question if the person answering it simply doesn't have some qualitative frame of reference to gain intuition from...and gaining intuition on EM phenomena grounded in reality isn't easy; there'd be a lot more EEs and physicists out there otherwise.

Besides, if you're walking into a time-constrained exam of toy questions, I fail to see what incentive a student may have to corroborate toy answers against a reality that it may or may not be an approximation of.

A lot of household appliances have power measured in watts (microwave ovens, electric heaters, etc). And megawatts and gigawatts are commonly used in discussion of electric power plants. My electricity bill is in kilowatt-hours, and that's what my electricity meter says on the front as well. Isn't that enough background knowledge to guess that an electric car would likely be more than a kilowatt (my microwave oven is more than that) but less than a gigawatt (a decent sized power station?)

No, not offhand. If someone, at some point, had said that kiloWatts and Horsepower are the same order of magnitude, I might have been a little better about that.

Gigawatt would have set off my "Huh?" detectors as I have heard that in correspondence with power plants. However, given that diesel generators sometimes power buildings, it would not have set off too much worry.

> However, given that diesel generators sometimes power buildings

How much power do you think a building typically draws?

To give a few examples for scale:

- typical U.S. home draws between 1KW to 25KW of electricity.

- typical U.S. large shopping mall around 1MW

- typical large data center with tens of thousands of servers + cooling is in the 5 to 15MW range

As a bonus: a fully loaded 747 weighing 400 tons on takeoff is around 90MW (aka 120,000HP).

And to circle back to the diesel generator, while size can very, they are fairly commonly around 1,000 - 1,500hp which translate to around 1MW.

OK, how much intuition do you have for dynes?

And that's metric, not some odd British system.

Metric, but not SI, and obsolete.

I own many things labelled in watts: laser pointer, phone charger, light bulbs, speakers, computer, fridge, tumble drier, oven.

That covers mW, W and kW.

Hundreds of MW to 1 or 2 GW is familiar as the output of a power station.

Vehicles are obviously in between.

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