Explain power - Page 1
Forum section: electronics / beginners

author: MrBond
author_rank: Contributor
timestamp: January 02, 2022, 01:30:08 am
content: Hello everyone, i need some help understanding power.So the definition i was given is that power is watt per time unit, yet the formula is P=U•I or P=W/t or P=U•I•t/t.So my question is, if power is watt per unit time why does the formula literally eliminate the notion of time?

author: bdunham7
author_rank: Super Contributor
timestamp: January 02, 2022, 01:35:37 am
content: Quote from: MrBond on January 02, 2022, 01:30:08 amThe expression P = W/t is wrong. I don't know where you got that or where you are coming from, but:P = W. Power is expressed in watts.P = E / t. Power is energy per unit time. A watt is a joule (energy) per second (time).P = U * I While it is true that this expression appears to have no time element, the I (current) is C (charge, in coulombs) per t (seconds).

author: themadhippy
author_rank: Super Contributor
timestamp: January 02, 2022, 01:49:42 am
content: Quotemaybe getting mixed up with joules, were watts= joules / time

author: MrBond
author_rank: Contributor
timestamp: January 02, 2022, 02:00:19 am
content: Well, energy is joules, which is watts/second.

author: bdunham7
author_rank: Super Contributor
timestamp: January 02, 2022, 02:17:01 am
content: Quote from: MrBond on January 02, 2022, 02:00:19 amNo, energy is joules which are watts * seconds.

author: xrunner
author_rank: Super Contributor
timestamp: January 02, 2022, 02:30:19 am
content: Quote from: bdunham7 on January 02, 2022, 02:17:01 amRight,1W = 1 J/sW = J/stherefore,W * s = J

author: george.b
author_rank: Frequent Contributor
timestamp: January 02, 2022, 03:30:28 am
content: Quote from: bdunham7 on January 02, 2022, 01:35:37 amIt isn't. It's just a notation mix-up.W here is a variable - work - not the unit. Power is the rate at which work is done, so P = W/t[ s], in a Variable notation.Quote from: MrBond on January 02, 2022, 01:30:08 amIt isn't - power is given in watts,period- and it doesn't: I = Q/t (charge over time), so time is built in.Consider the joule as defined by the work (W) required to move 1 coulomb of charge (Q) through a potential difference of 1 volt (V), so W = QV, or, equivalently, V = W/Q.By substituting, we can see that P = VI = W/t: P = (W/Q)(Q/t) = W/t.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 02, 2022, 08:25:50 am
content: Forget voltage, current, charge etc. first. Get the very fundamentals, energy, time and power, correctly worked out first. These apply in everything; electronics, mechanical systems, etc.Power is SPEED. Rate of change of energy.Power is about how quickly energy is consumed (or produced).Unit of energy is Joule. Everything else follows. Watt is just short-hand name for 1 joule per second.Backwards units like Wh (watt-hours) are really confusing for laymen and beginners. Really, we should just avoid them and talk about Joules when talking energy. Then, the speed doesn't really need a separately names unit. We talk about km/h or miles/h. We could just talk about MJ/h as well, but by convention, we just don't do that.(That being said,knotsis an example of a specialized unit for speed. Watt is similar.)

author: IanB
author_rank: Super Contributor
timestamp: January 02, 2022, 09:13:17 am
content: Quote from: MrBond on January 02, 2022, 01:30:08 amI think this has now been correctly explained by others, but here is a recap:Power is defined as "the rate of doing work". So ifwork(measured in joules) is given the symbolW, and time is given the symbolt(measured in seconds), then you have power (watts) = work (joules) / time (seconds), or in symbols:P=W/tIn electrical terms, you haveP(watts) =U(joules/coulomb) *I(coulombs/second)From this you see in each case that: 1 watt = 1 joule/second

author: golden_labels
author_rank: Super Contributor
timestamp: January 02, 2022, 10:11:17 am
content: Quote from: Siwastaja on January 02, 2022, 08:25:50 amA note: see two posts below before reading this response.In mechanics, speed is not a change in energy. At constant speed and mass, energy remains constant.You can see that in dimensional anaysis, askg·m·s-2(that is: force) is missing:Energy: J = kg ⋅ m2· s-2Power: J / s = kg · m2· s-3Speed: m · s-1The rate of change of energy in mechanics is also power: kg · m2· s-3.

author: IanB
author_rank: Super Contributor
timestamp: January 02, 2022, 10:19:27 am
content: In colloquial English, "speed" can refer to "how fast something is changing" or "rate of change" in a general sense. For example, if I take a hot pan off the stove, I can talk about the speed that it is cooling down. That doesn't mean the pan grew legs and is walking away

author: golden_labels
author_rank: Super Contributor
timestamp: January 02, 2022, 11:50:41 am
content: I know the common meaning, but I assumed it’s meant to be a comparison with mechanical systems: that “speed” is later referenced with units of km/h used.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 02, 2022, 12:07:12 pm
content: OP needs basic understanding with most understandable, colloquial terms.If they could already work with the exact physical definitions, they wouldn't have the problem at all.Let's say rate, then. Which, to be fair, I did say in my post.Speed in km/h is rate of change when the goal is to get from place A to place B; i.e., rate of change indistance. Similarly, speed in energy consumption is rate of change of energy.

author: Capernicus
author_rank: Frequent Contributor
timestamp: January 02, 2022, 12:13:11 pm
content: km/h is a rate.a rate is a derivative.if you sum the watts up over time its the "integral" I wonder what its called, is it the Joules?

author: Siwastaja
author_rank: Super Contributor
timestamp: January 02, 2022, 12:23:27 pm
content: Quote from: Capernicus on January 02, 2022, 12:13:11 pmYes, you got it right. If you sum watts every second, you get energy in watt*seconds, which is exactly the definition of Joule.Like, if you sum the speed in m/s every second, you get integral as (m/s) * s = m. This is the distance you have traveled.The confusion arises from the fact these two examples work the opposite way. If James Watt never came up with a specific unit for rate of change in energy, maybe we would talk about joules per time unit instead. Like we talk about km/h.

author: MrBond
author_rank: Contributor
timestamp: January 02, 2022, 12:26:07 pm
content: Quote from: george.b on January 02, 2022, 03:30:28 amThis is a very good answer, thank you very much! I had huge troubles understanding it.

author: george.b
author_rank: Frequent Contributor
timestamp: January 02, 2022, 03:59:30 pm
content: Quote from: MrBond on January 02, 2022, 12:26:07 pmYou're welcome! I'm glad I could clear that up

author: rpiloverbd
author_rank: Regular Contributor
timestamp: January 03, 2022, 11:48:14 am
content: In simple expression, P=IVP=Power,I=current, V=Voltage

author: themadhippy
author_rank: Super Contributor
timestamp: January 03, 2022, 03:27:53 pm
content: Quotesometimes

author: mcz
author_rank: Contributor
timestamp: January 03, 2022, 05:52:05 pm
content: Quote from: rpiloverbd on January 03, 2022, 11:48:14 amQuote from: themadhippy on January 03, 2022, 03:27:53 pmI believe the correct thing here would be p(t) = i(t) * v(t)Simple expressions only work for a short time and some day they suddenly appear to be wrong.. Which they most of the time are not, just incomplete or simplyfied too much.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 03, 2022, 06:08:07 pm
content: Power in Watt is a rate but it's not W/s but rather W=J/s just like Knot is a speed unit and it's Nautical mile per hour.

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 19, 2022, 12:42:21 pm
content: At school we learned it this way up:s  - distance, measured in metert [s ] - time, measured in secondsv = s/t [m/s] - speed = distance per timea = v/t [m/s²] - acceleration = speed change per timeF = m a [kg m/s² = N] - force = mass multiplied by acceleration, measured in NewtonE = F s [kg m²/s² = N m = J] - energie = force multiplied by distance, measured in JouleP = E/t [kg m²/s³ = J/s = W] - power = energy per time, measured in WattGoing backward:E = P t [W s] energie = power multiplied by time; if you run a hairdryer of 1000W or 1kW power over 1 hour time, you use energy of 1000Wh or 1kWh.and P=U I [W = VA] voltage multiplied by current, measured in Volt Ampere or Watt.