Basic Theory Question

Resistance!

 

Resistance, or more correctly, impedance. Measured in Ohms.

Sounds like you can calculate power using the power triangle:
P
VxI

There is also the resistance triangle:

V
IxR

 

Impedance for AC
Resistance for DC

 

Although he was talking AC

 

Just getting the ball rolling lec

 

Do you own a car ?
Have you the keys ?
Go to car.
Lock yourself in.
.....That's it.
Are you still reading this ?
RS.....

 

So I'll read "juice" as power, and try and give a simple analogy.

If you're a school kid looking with help with their homework, or just curious, then the explanation below might help.

If you're planning on working on real electrics without understanding Ohms law then please stop. There are less painful ways to kill yourself.

Electricity is the movement of electrons.
Power is how many electrons are moving every second (current) multiplied by how hard they're being pushed (voltage).*

So if the voltage is the same on two devices, but the power is different then it must be the current that's different.

As has been pointed out above, if the push (voltage) is constant then the number of electrons that can get through the circuit every second depends on the resistance.

Here's a video walking you through some calculations.



*how many electrons per second is really measured in coulombs of electrons per second - a coulomb is like a bucket of electrons. So one amp is a coulomb sized "bucket" of electrons per second.

 

Hi Ding.

Sometimes the analogy with water flowing through a pipe works well.

Voltage is the 'pressure' or 'force' behind the flow. The Oomph. It's what 'pushes' the leccy through the wire. With all your appliances, this 'force' - 240V - is the same for them all.

So, imagine a tank full of water up to the brim. Now drill a wee hole in the bottom of the tank and fit a short length of wee hose to it - how much water flows out? Now drill a much larger hole beside it and fit a much larger hose to that one. Compare the two flows - are they the same?

Hopefully you'll notice (if you build this tank) or realise (if you chust think about it) that a lot more flow comes out the larger hole and hose.

If you don't 'see' that as being the case, take it to it's logical conclusion - remove the whole bottom of the tank. I think you'll agree that a lot more water comes out now?

Cool. So the bigger the hole the greater the flow - for the same amount of stored water (voltage).

So how does this work with leccy? The appliances that draw less current/power are the ones with much thinner holes in them. By that I mean they have a much larger resistance to the leccy flow, just like a wee hose has a much larger resistance to the water flow.

And if you were to measure that resistance to water flow in the pipe, and measure the resistance to leccy flow in an appliance, you would find that same correlation.

If you want more - like HOW does this happen inside an appliance, then you have to look at what is inside that appliance. The hair drier than uses a surprisingly large 2,000 watts has a 'hose' (wire) inside it that is surprisingly 'large' and with low resistance to flow.

Whereas a bulb which is typically only drawing 60 watts has a very thin wire inside it which has a much greater resistance to flow.

(That is putting it very simply... It's not chust the thinness or thickness of the wire, or even how long it is (the longer = more resistance, chust like a long hose would reduce your water flow), but also what the wire is made off - they often add various chemcials/other metals to it to control resistance.)

 

Well put DA. Except the amount of stored water is analogous to the amount of energy, no? Height of the water is like the voltage.

 

AC DC

 

Any device needs to have the power to do the work its is designed for. As an example, people use 240v power tools at home. On site they are 110v. But you get the same performance with either tool. ie you can get the same amount of work done with either voltage. They have the same power capability.

So things are made to do what's needed of them, by the manufacturers. They will offer a range of products with different power capabilities. From the bog standard to the deluxe. You would need some good maths, science and engineering to comprehend the true nature of what's being done to make such things.

Ok the 240 and 110 are fine for jobbing tradesfolk. But in the timber yard they use 3 phase machinery, because they need more power to do more, bigger things. They need to do them just as easily as the scaled down version on site. So such humungous machines are made to do this. Again, top boffins are at work doing this as we speak.

But you can't buy a three phase hair dryer, no sir.

 

Yeah, volume = 'capacity' or 'amount' (eg as in a battery).

And, yeah height of stored water over outlet = voltage.

You pedant, you...

 

It is actually IMPEDANCE in both AC and DC circuits, just that in a DC circuit there is no reactive component and the impedance matches the resistance.

 

Sorry, Bodge, for calling you a pedant...

 

Someone always has to go one better don't they DA!

 

Let's not over complicate at this stage

 

Yup. Ding comes on here asking how a HUGE fridge and a MEDIUM food processor and a TINY hair drier can have such differing power consumptions, rarely in proportion to their size.

And they walk right in to the path of Pollo and DW and...


(Hey! Ding! A food processor will pull a lot MORE current than a fridge...)

 

I'm just a bodgy pedant.

 

I prefer the water analogy where you are trying to push water though a pipe, if you put a restriction on the pipe, resistance, and still apply the same pressure, volts, then you will reduce the flow, amps.