EEVblog #1009 - Voltage vs Power vs Energy

[EEVblog]

Fundamentals video explaining the difference between voltage, power, and energy. The difference between Power & Energy is probably the most misunderstood and misused terminology in engineering!

[EEVblog]

There will be a re-upload shortly with an additional clip.

[Brumby]

Just a comment to help with the power being 'instantaneous' while its definition has a time component...

This same class of measurement applies to speed/velocity.  When a speed camera snaps a photo of you - it captures, what is in effect an instant in time (let's leave shutter speeds out of this).  But at that instant in time you were traveling at a rate that can be precisely - and correctly - stated.

The definition of speed is distance over time - but even though the picture may look like you're not moving, your speed at that time is well defined.

[max_torque]

Energy conversion is Power integrated (added up)

So, say we convert energy to power at a constant rate of 10kW.


In 100 hours we convert a total of 1000kWh of energy  (1000/100 = 10)

In 10 hours, 100kWh (100/10 = 10)

in 1 hour, 10kWh  (10/1 = 10)

in 1min  (1/60th of an hour), 0.1667 kWh  (0.1667 / (1/60) = 10)


Every time we reduce the interval, we also reduced the divisor, so the result is the same, 10kW of power. 

And that, fundamentally what integration is, it's every infinitesimally small chunk added up.  However small that chunk is, as long as it is more than zero.

However, our brains and our language, unlike our mathematics, finds these concepts difficult to address.  So, in everyday life we tend to lump things into standard sized chunks, chunks that are able to be realised and imagined on a human scale.

For example, we often describe the velocity of an object in Miles Per Hour.  Here, we have chosen a convenient and finite unit of time, that as humans we can articulate.  It makes sense to use an hour, because for typical objects in everyday conversation, typical velocities result in sensible distances travelled over that hour.  For example, your car might do 60 miles per hour, an fast train 100 miles per hour, or an aeroplane maybe as much as 600 miles per hour.  In these cases, we use "per" as our spoken language avatar for the mathematical function of "divide"

In effect, these parameters become ratiometric, and as such, you can use a divisor of any magnitude and end up back at the same value you started with!

[Lodditin]

I just thought of a look alike analogy for your dam/voltage. If the picture got flipped upside down, so the hole is at the bottom. Maybe the explanation of potential energy would come through better and it would be easier to explain a coulomb first.
Else i think it's a fine video which may get some people to think over what they mean to say

[bjcuizon]

  This video should be showed to some people who say: "My Power bill is.. yadi yadi yada" instead of saying Energy Bill.

[German_EE]

The way it was explained to me (a number of decades ago) was a piano being pushed out of an apartment window.

The voltage is the floor of the apartment, the higher the drop the higher the voltage or potential difference

The current is the size of the piano, a Casio keyboard is going to have a lot less effect than a concert grand

The size of the crash (power) is voltage x current or height x instrument size

Things however fall apart when we consider power over time as this experiment will produce a short lived pulse rather than a steady 'current'. That would require a large supply of instruments.

Don't try this at home 

[boggis the cat]

I appreciate that you're trying to get these points across to a wide audience, but shying away from "physics" units on principle was a mistake.

Working back from energy is probably the easiest approach to explaining these relationships.

• Energy: 1 J = 1 Ws (and 1 kWh is 3.6 million Ws, or 3.6 MJ)
• Power: 1 W = 1 J/s (establishes that power is energy per unit time)
• Voltage and current: 1 V x 1 A = 1 W (establishes that you can't calculate power or energy just from voltage, or current, alone).

Very simple derivation at each step, with asides to include common units as necessary (e.g. one kWh is 3.6 MJ).

The way you presented this was over-complicated, and potentially confusing.  Introducing energy in joules, with the 1 W = 1 J/s definition then changing to kWh for energy was a lost opportunity.

[RGB255_0_0]

Nice video Dave. Could have just explained what "Ah"s are for completeness.

Next video: Apparent Power vs Real Power, phase angles etc please 

[BobC]

And how about in the AC domain?  What about Watts, VA and VAR?  What's a "power factor"?

More to the point, it would be great to see a physical demonstration of (not just measurement of) the above AC power measurements.  Where do the VARs "go"?

The first demo I saw used a PAVM (phase-angle voltmeter), an elegant instrument rarely used any more.  These days I'd use the math functions in an o'scope.

Power factor correction is another great topic, but perhaps a bit beyond what a fundamentals video should cover.  Many of us have seen various plug-in electronic gadgets that correct the power factor, but perhaps the neatest trick EVER is how power utilities minimize reactive power losses on their massive multi-phase distribution networks by using over-excited synchronous AC motors.

But that would require an introduction to multi-phase power and induction motors, not a topic many of us need to bother with very often.  But, still, some very cool fundamentals.

[Brumby]

Shying away from physics terms is not the wrong thing to do here.  There are some things that are necessary - but to include any more than is necessary is just going to confuse.

Really confuse.

This is why I wanted to keep calculus out of the discussion - to keep it simple.

This is why I gave the speed camera example.  It is a common reference point that pretty well everyone can understand.  Using words like "calculus" and "integration" - while technically accurate - will instantly trigger the "run for the hills!" response in a whole lot of the audience that could otherwise have really gained an understanding.

This is why Dave stuck to the three terms - and went no further.  This is a teaching exercise.  He understands the lesson and what the objective is - and keeps what is being presented deliberately limited to achieve that.

[Brumby]

Re-watched to see what was added.  It was the dam analogy.

I think that worked quite well - but I would have liked to have seen it presented at the beginning and used along the way.

[ggchab]

Very good video  Thanks.
I would also be interested by a video about apparent power and real power.