Its been a while since I have made an addition here, so i will do so now with a discussion on Power.

If your not sure of both Current and Voltage, i suggest you review it before you go futher because they play a big role in determining just what Power is.

In short, Power is a measurement of Energy used in one Second.

Here are some basic units that may help you see this:

Newton (Unit of Force)
1 kg*m/(s^2) "Kilogram meter per square second"

Joule (Unit of Energy)
1 N*m "Newton meter"

Ampere (Unit of Electromagnetic Curent Flow)
1 C/s "Coulomb per second"

Volt (Unit of Electromagnetic Force)
1 N*m/C "Joules per coulomb"

And finally:

Watt (Unit of Power)
1 J/s "Joules per second"

So how does this relate to Current and Voltage?
Well lets look at the math first:

1 Watt = 1 J/s = 1 N.m/s

If we take Current and multiply it with Voltage well find:

1 V * 1 A = [ 1 N*m/C ] * [ 1 C/s ] = 1 N.m/s

Hense we now have shown that:
1 W = 1 V * 1 A

To now understand this, i will divert your attention to the properties observed from a rotating tire on a motor. We can observe that a very large tire on the motor will have a very fast rotating speed on the outer edge, however it will have very little pull strength. Alternately, if we put a very small wheel on the motor we would find that its rotating speed is much slower, but shows considerably more pulling strength.

These properties are both a result of the Torque equation:

Torque (rotational energy) = Force * Distance

Since in both cases the motor never changed, the amount of energy applied never changes. This means that in both cases for one property to increase, the other must be reduded by an equal factor such the end result is always the same.

In the sense of electonics, the same prinicpals are also able to be observed. A lightning strike and a static shock both have very high voltages, however a static shock only annoys you in the winter. A lightning strike however, will annoy your family, cause it will KILL you!.

The reason here is the POWER is not the same for the to effects. Lightning bolts have tremendous amounts of power (Gigawatts), where a static shock would only have a couple watts at best. It takes approximately 10,000 V for electricity to jump one inch in air. So your static shocks in the winter mean your charged to perhaps 5,000V just by walking across the room in you fuzzy slippers. Conversly, the amount of current flowing from a static shock would have to be very small! After all, it doesnt take much energy (and hence power) to walk across the room! If you accumulate 5,000V upon yourself, you can be sure the current felt is in the micro-Amp range!

In all cases Energy/Power is conserved. Be reminded in this example, the Power you put into walking across the room is determined by Energy exerted divided by the time you took to do so.

Power helps you realize the amount of "punch" you are actually giving your electronic devices, and batteries that supply this punch! Most resistors you deal with are low power devices! You put perhaps 24V across them, but the amount of current your actually exerting may be only 10 mA at best, hence 24V * 0.010A = 0.240 ~ 1/4 Watt Resistor (have your heard this before ;) ? ) Notice we dont care just what the resistance is, only thing here that matters is How Hard (voltage), and How Fast (current)! To be honest, the resulting current will be a function of the resistance, but this is more of a means to an end, than a direct function there of resistance itself. Other things such as silicon devices also have power ratings and tollerances, where resistances within them are not explicity shown. In these cases you will find that Power and Current is the two factors that will determine if you about to destroy the device, or run within acceptable limits of its ability.

Power has other ways of being found as well:

Power = 1 Volt * 1 Amp
Power = 1 Volt * [ 1 Volt / 1 Ohm ] = 1 V^2 / Ohm
Power = [ 1 Amp * 1 Ohm ] *1 Amp = 1 I^2 * Ohm

In all cases they reach the same conclusion!

A side note here:

Wires *do* have resistance. And since wires are inherantly series elements (rope like), the resistnace will get larger with distance of wire.

Power companies sell you power to do your every day life. They want to be sure you get your Power with minimal losses in doing so. If they were to give you 1000W for your toaster on a 120V outlet directly over 5 km of wire haveing 0.1 ohms resistance per meter, you would find the power sourced is:

Power Sourced = ?

Total Wire Resistance = 5000m * 0.1 Ohm/m = 500 Ohms
Current used by toaster = 1000 W / 120 V = 8.333 Amps
Power consumed by wire = I^2 * R = (8.333 A)^2 * 500 Ohms = 34,722.2 Watts

Power Sourced by the power company then must be:
1000W + 34,722.2 W = 35,722.2 Watts!!!

Thats alot of power just to run your toaster from a direct 120V feed from your power company!

The power companies realized this inefficiency a long time ago! The culprate causing the wasted power is the wire resistnace. But you cant really change physics cause you dont like its outcomes, you instead have to work with physics. Seing the looses were generated by "I square R losses" (I^2 * R = W lost) and we cant change R all to much, we must go after changing the current (I) itself.

If the voltage was adjusted to 500,000 V, then consequently the current would be reduced considerably for the same amount of power! To do so, power companies use transformers (those ugly stations that parks tend to be made outa) to do just this. They work both ways, ramping voltage up and current down, and vice versa.

If the same above problem was analized with the same copper wire transmitting at 500,000 V over 5000m distance, and then ramped back down to 120V for your toaster you'll find:

Power Sourced = ?

Total Wire Resistance = 5000m * 0.1 Ohm/m = 500 Ohms
Current used by toaster = 1000 W / 120 V = 8.333 Amps
Voltage Tranforming factor: 500,000 V / 120 V = 4166.67
Current Tranforming factor: 1/Voltage Transforming = 0.00024
Current on wire = 8.333 Amps * Transform Factor = 8.333 * 0.00024 = 0.00199 A

Power consumed by wire = I^2 * R = (0.002 A)^2 * 500 Ohms = 0.002 Watts

Power Sourced by the power company then must be:
1000W + 0.002 W = 1000.002 Watts!!!

Ahh, now this is much better! Power companies can now transfer power to your home via very high voltage power lines!

If you want to picture what is happening here. There is very little "Movement" of coulombs along these high voltage lines cause "I" is so small. However, there is a tremendous pressure upon them (500,000V) giving them alot of potential to do alot of work if allowed. Since the bottom line is nothing is moving much on power lines, the result is that there is not much "friction" from electrons bouncing off copper atoms, which would create heat and drain POWER from the source (as we saw happening in the first example)!

I hope this gives you a good feel for what Power is and how it ties into electronics.
Posted on 2003-05-09 00:38:26 by NaN