## Section1.5Electrical power (WIP)

How much work can an electrical circuit do? This leads us Power, calculating power with Watt's law - Marble wheel analogy - drop (energy/marble) * flow (marbles/second) = energy/second - Following the analogy, voltage * current joules/coloumb * coloumbs / second = joules/second = watt - Examples: Power in various electronic devices

### Subsection1.5.1What can you do with a Watt?

It can be hard to get a sense of how much a Watt is, so let's nail down a few reference points.

You're probably already aware that light bulbs are labeled with their power consumption. In the old days of incandescent bulbs, a 60W bulb was standard, a 100W bulb was "bright", and a 200W bulb was enough for a whole room. Modern LED bulbs produce similar light output for roughly one tenth the power --- somewhere between 5W and 30W depending on the light output.

My electric bill states my average power consumption every month, which is generally in the range of 1kW (1000 Watts).

Suppose you want to heat an 8 fluid ounce (~236g) cup of water in 2 minutes (for example, in a coffee maker or hot water kettle). How many Watts is that?

Let's assume we're going from room temperature water (say 25C) up to boiling (100C). One calorie is the energy required to heat 1 gram of water by 1 degree C, so we need $$(100-25) \cdot 235 = 1.8\times10^4 \mathrm{calories}\text{.}$$ One calorie = 4.1868 Joules, so this is $$7.4\times10^4\,\mathrm{J}\text{.}$$ 1 Watt is defined as 1 joule per second, so if we need $$7.4\times10^4\,\mathrm{J}$$ over 120 seconds, the average power needs to be at least 620 W.

Nearly every electronic device is labeled with its power consumption. Go look around and see what you can find!