Explain power

[MrBond]

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?

[bdunham7]

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?

The 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). 

[themadhippy]

I don't know where you got that or where you are coming from

maybe getting mixed up with joules, were watts= joules / time

[MrBond]

Well, energy is joules, which is watts/second.

[bdunham7]

Well, energy is joules, which is watts/second.

No, energy is joules which are watts * seconds.

[xrunner]

Well, energy is joules, which is watts/second.

No, energy is joules which are watts * seconds.

Right,

1W = 1 J/s

W = J/s

therefore,

W * s = J

[george.b]

The expression P = W/t is wrong.

It 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] = W[J]/t[ s], in a Variable[Unit] notation.

So my question is, if power is watt per unit time why does the formula literally eliminate the notion of time?

It 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.

[Siwastaja]

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, knots is an example of a specialized unit for speed. Watt is similar.)

[IanB]

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?

I think this has now been correctly explained by others, but here is a recap:

Power is defined as "the rate of doing work". So if work (measured in joules) is given the symbol W, and time is given the symbol t (measured in seconds), then you have power (watts) = work (joules) / time (seconds), or in symbols: P = W / t

In electrical terms, you have P (watts) = U (joules/coulomb) * I (coulombs/second)

From this you see in each case that: 1 watt =  1 joule/second

[golden_labels]

Power is SPEED. Rate of change of energy.

A 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, as kg·m·s^-2 (that is: force) is missing:
Energy: J = kg ⋅ m² · s^-2
Power: J / s = kg · m² · s^-3
Speed: m · s^-1

The rate of change of energy in mechanics is also power: kg · m² · s^-3.

[IanB]

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 

[golden_labels]

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.

[Siwastaja]

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 in distance. Similarly, speed in energy consumption is rate of change of energy.

[Capernicus]

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?

[Siwastaja]

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?

Yes, 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.

[MrBond]

The expression P = W/t is wrong.

It 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] = W[J]/t[ s], in a Variable[Unit] notation.

So my question is, if power is watt per unit time why does the formula literally eliminate the notion of time?

It 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.

This is a very good answer, thank you very much! I had huge troubles understanding it.

[george.b]

This is a very good answer, thank you very much! I had huge troubles understanding it.

You're welcome! I'm glad I could clear that up

[rpiloverbd]

In simple expression, P=IV
P=Power,I=current, V=Voltage

[themadhippy]

In simple expression, P=IV

sometimes

[mcz]

In simple expression, P=IV
P=Power,I=current, V=Voltage

In simple expression, P=IV

sometimes

I 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.

[BeBuLamar]

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.

[Peter_O]

At school we learned it this way up:

s [m] - distance, measured in meter
t [s ] - time, measured in seconds
v = s/t [m/s] - speed = distance per time
a = v/t [m/s²] - acceleration = speed change per time
F = m a [kg m/s² = N] - force = mass multiplied by acceleration, measured in Newton
E = F s [kg m²/s² = N m = J] - energie = force multiplied by distance, measured in Joule
P = E/t [kg m²/s³ = J/s = W] - power = energy per time, measured in Watt

Going 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.