This is just too good

[Glom]

I'm watching The 100 Greatest Movie Stars.

In the top twenty is Robin Williams. One commentator, stressing her admiration for Williams, said, this is a direct quote, "I cannot measure his energy in megavolts."

Neither can I, megavolts measure potential difference.

I know it's just an excessive nitpick, but it was just a brilliant bit of bad science in it's execution. It loses something in the write-up I'm sure. You had to see it yourself.

[Wingnut Ninja]

Well, she was technically right... :roll:

[tracer]

Maybe she meant mega electron-volts. ;)

(In which case she still wouldn't be able to measure his energy, because a mega electron volt is such a small unit that it can hardly be detected with macroscopic equipment.)

[kilopi]

I'm watching The 100 Greatest Movie Stars..
Also, Hollywood has co-opted the technical term "stars" and ... cheapened it. :)

[Beelzebob]

they have a perfect right to call them 'stars', haven't you seen the light shining out of their.........*ahem*

[Pinemarten]

P=IE
Power = Current x Electromotive force or Wattage = Amps x Volts
Power is energy and can be measured in watts, horsepower, joules, calories, etc.
Voltage with no current flow doesn't produce power, but a battery can be measured in stored energy. Amphours x voltage equals potential power.
If we new Robin's capacity, we could measure his power.

[tracer]

Amphours x voltage equals potential power.
You sure you don't mean Amphours x voltage equals potential energy?

[Pinemarten]

Power and energy are interchangeable terms, according to my education.

[Glom]

P=IE
Power = Current x Electromotive force or Wattage = Amps x Volts

Firstly, your use of dimensions on one side of an equation and units on the other is questionable at best for an equation of definition.

Secondly, electromotive force is a crappy term for induced potential difference. My Physics teacher says never use it fully although EMF is acceptable. Potential difference is not wattage, wattage is the quantity measured in watts, being power.

Power is energy and can be measured in watts, horsepower, joules, calories, etc.

No power is not energy. Power is the rate of transfer of energy.

P = dE/dt, where P is Power and E is Energy transferred

Power is indeed measured in watts and the crappy unit of horsepower, but joules and calories are units of energy, not power.

The Watt is defined as the amount of power drawn when 1J of energy is transferred over the interval of 1s.

Therefore 1W = 1J/s.

Voltage with no current flow doesn't produce power, but a battery can be measured in stored energy. Amphours x voltage equals potential power.

From the eqn P=VI, without a current, there is no transfer power. That much is right. Charge times potential difference equals potential energy, not potential power.

Power and energy are interchangeable terms, according to my education.

No according to mine. Power is the rate of transfer of energy.

P = dE/dt

1J = 1kgm²/s²
1W = 1kgm²/s³ = 1J/s

[Glom]

Some homegenity tests to show the nature of the watt.

Consider P=VI.

This eqn gives us 1W = 1V × 1A

The amp is a base unit, but what is the volt? There are a number of equations we can use.

First, the definition of the volt. 1 volt is the potential difference across which 1 coulomb of charge does 1 joule of work traversing. Or V=E/Q, which gives us 1V = 1J ÷ 1C.

1 coulomb is the charge passed by a current of 1 amp in 1 second so 1C = 1A × 1s. Therefore 1C = 1As. Sub this into the equation for pd and we get 1V = 1J ÷ 1As = 1J/As.

Sub this into the eqn for power: 1W = 1J/As ×1A. The amps cancel to give 1W = 1J/s. In other words power is energy over time, which means the rate of transfer of energy.

But if you don't agree with some of the steps of that derivation, here's another using electric fields. For two flat parrallel charged plates, the electric field strength is equal to the potential difference between them over the distance between them. e = V ÷ d where e is the electric field strength and d is the distance between them. The definition of the strength of a field is the force per unit of whatever the field acts on. In other words, since E-fields act on charge, e = F ÷ Q. Therefore F ÷ Q = V ÷ d. Therefore, V = F × d ÷ Q so 1V = 1N × 1m ÷ 1As = 1Nm/As.

Now since, work is the energy transferred when force of 1N acts on an object over a distance of 1m, 1J = 1Nm. Subbing into the previous equation for the volt, 1V = 1J/As as before.

How about using magnetic fields? For the force acting on a conductor carrying a current in a B-field, F = BIl, where B is the magnetic flux density measured in teslas and l is the length of the conductor in the B-field. From this eqn, B = F ÷ (I × l) so 1T = 1N/Am.

Magnetic flux, Ø = BA, where A is the area through which the B-field runs. So units of Ø = 1N/Am × 1m² = 1Nm/A.

Then use Faraday's eqn. V = dØ/dt. 1V = 1Nm/A ÷ 1s = 1Nm/As. 1Nm = 1J, so 1V = 1J/As.

So as you see, many routes all lead to same conclusion that 1W = 1J/s and therefore power is not energy but the rate of transfer of energy. Of course, it all depends on you accepting various definitions for things like the amp, magnetic flux and the joule. Just stating these definition does feel slightly circular when trying to prove another definition, but proving that something is defined as it is can be a circular activity.

[Pinemarten]

I stand corrected.

W = IV
In order to calculate power, time is needed in the equation.
P = IVt

I have always been lazy in my math and substitute variables for units, and therefore make sure I write the unit designation in brackets.

A battery can be 'rated' in power since time is included in the stored potential: amphours. If the battery is drained in 10min or 10hr the power is the same.

I should not have used the term EMF. It DOES refer to induced voltage ONLY! There are 4-6 other ways to produce voltage without induction.

And I do accept all your definitions as fact. I remember all too well studying flux densities until I puked. Including other units , such as Maxwells, Webers, and a metric thing that was so horrid you don't want to hear about it.

[Pinemarten]

As an afterthought I think Volta defined the volt as a mole of electrons?
I am probably wrong.
Another afterthought.
Horsepower is rated @ certain RPM. My motor for instance is 335 hp @ 2500 rpm. That is where car companies confuse people. I should do the math on some of these ads and post a true comparison of the claims. I doubt it would be a topic on this site though.

[Glom]

Glad we got that sorted. I only wish I was allowed to study flux densities until pukage but our course considers Ø=BA to be only just within the grasp of A-level students. :roll:

One problem, your equations are the wrong way round. P = dE/dt. P = IV. E = IVt.

[Pinemarten]

I remember the discussion about V=IR instead of E = IR. Volts = Amps x Ohms
It was recommended that we use that, instead of E=IR because E is accepted as energy as opposed to voltage.
I think they assumed that electricians may eventually study science, and therefore save us some embarassment.
Watts= Amps x Volts is W = IV
Power = Amps x Volts x time is P = IVt
Does this make sense?

[Glom]

A capital epsilon is sometimes used for EMF.

Watts = Amps × Volts, but Watts measure Power

Joules = Amps × Volts × Seconds and Joules measure Energy.

[tracer]

I stand corrected.

W = IV
In order to calculate power, time is needed in the equation.
P = IVt

Um ... I think you've got that backwards.

Watts are a unit of power. If P is power and W is wattage, then P= IV and W = IV are both correct.

However, to calculate energy, you need to multiply the power by the amount of time the power is applied. Energy = IVt.

I always learned it as "Power is energy per unit time." I.e. one Watt is, by definition, one Joule per second.

[Pinemarten]

You are right. I don't think I am doomed to confuse power and energy forever. The who and whom thing; is another story.
I remember doing the math on how long it would take a 3500W hot water tank to raise 160L of water from 15C to 50C. The horrid memory is returning, I actually got the right answer.

[Hale_Bopp]

One of the students at my school asked me what is the work done by a force of 1 Newton applied through a distance of 1 meter. That is the definition of a Joule. I asked her why she wanted to know (she was not in any of my classes).

She said her boyfriend was giving her hints about what her birthday present would be and now she knew :) (Her boyfriend was in my physics class).

Rob

[tracer]

Her boyfriend got her a Newton-meter for her birthday? ;)

[tracer]

I remember doing the math on how long it would take a 3500W hot water tank to raise 160L of water from 15C to 50C. The horrid memory is returning, I actually got the right answer.
Must ... resist ... temptation ... to ... solve ...

Ah, heck.

It takes 1 calorie of energy to raise 1 mL of water 1 degree C. 160L is 160,000 mL. To raise 160,000 mL of water 35 degrees C would require 160,000 x 35 = 5,600,000 calories of energy.

1 calorie = 4.18 Joules. Thus, 5,600,000 calories of energy is 23,408,000 Joules of energy. (Rounding to 3 significant digits, this is 23,400,000 Joules.)

A 3500 Watt heat source is emitting 3500 Joules of energy per second. It would take (23,400,000 / 3500 = ) 6686 seconds for this 350 Joule/sec heat source to emit 23,400,000 Joules. That's 1 hour and 51-and-a-half minutes.

So, if you start right now, you'll have just enough time to watch Star Trek II: The Wrath of Khan up to the closing credit crawl before your water's done.

[Glom]

Her boyfriend got her a Newton-meter for her birthday? ;)

Be careful. Energy and moments are homogeneous with respect to units. The difference is that work is the dot product of force and displacement, while a moment is the cross product. Since energy has a unit of its own, Newton-metre is reserved for moments.

[Pinemarten]

Must ... resist ... temptation ... to ... solve ...

Ah, heck.

It takes 1 calorie of energy....................................1 hour and 51-and-a-half minutes.

So, if you start right now, you'll have just enough time to watch Star Trek II: The Wrath of Khan up to the closing credit crawl before your water's done.

I'm impressed. You are now officially a 'bad electrician'.
By the way seek 'white pepper'. I couldn't help but to check out your site.

[kilopi]

Her boyfriend got her a Newton-meter for her birthday?
My wife loved the Newton meter I got her last birthday.

[RichField]

Must ... resist ... temptation ... to ... solve ...

Ah, heck.

It takes 1 calorie of energy....................................1 hour and 51-and-a-half minutes.

So, if you start right now, you'll have just enough time to watch Star Trek II: The Wrath of Khan up to the closing credit crawl before your water's done.

I'm impressed. You are now officially a 'bad electrician'.

Actually, it's just thermo, the original problem didn't specify that the hot water tank was electrical, could have been oil or gas. There are more ways than one to get a Watt. Very nice explaination tracer.

Speaking of which:
My wife loved the Newton meter I got her last birthday.
Just think of what she would have said if you had given her 1 Watt!

[kilopi]

One what?

[Glom]

A Newton-meter a second? That'll mean a lot of work.

[Dickenmeyer]

How many figs in a newton-meter?

[kilopi]

How many figs in a newton-meter?
Figs? Isn't that mixed up? Don't you mean, gFis, "gigafeet inverse seconds"? But that's the speed of light, or close to it, not work.

Fd=0 is my motto.