Author Topic: "What is voltage?" (Read 4266 times)

mrcharly-YHT · « **on:** 19 October, 2012, 01:47:39 pm »

This question was chucked at me over the dinner table.

I used the 'move unit of water up a hill' analogy. Then we moved onto current.

Still seemed to leave them a bit perplexed. I think it's because they can't 'see' electricity.

Jurek · « **Reply #1 on:** 19 October, 2012, 01:50:44 pm »

Isn't it the diameter of the pipe you're using?

Ham · « **Reply #2 on:** 19 October, 2012, 01:51:11 pm »

No it's the speed, the width is the current

Wowbagger · « **Reply #3 on:** 19 October, 2012, 01:51:58 pm »

Voltage is one of the aspects of Mysterious Physics for which life is too short.

mrcharly-YHT · « **Reply #4 on:** 19 October, 2012, 01:56:36 pm »

diameter of pipe affects resistance.

volume of water/time = current

I've always used height of drop analogous to voltage - especially as you can talk about one raindrop falling from a height starting off with a great voltage, but there not being much current. Then relate that to static electricity.

I guess change in speed when passing a restriction in pipe diameter = voltage drop.

Kim · « **Reply #5 on:** 19 October, 2012, 01:58:54 pm »

Water and hills and pipe diameters and flows is how I've always described it. I spent many of my formative years messing about with buckets and lengths of hosepipe, though.

My physics teachers always like to use "Electro-motive force" rather than "voltage". The idea that something (be it chemistry, or spinny magnets, or capacitor plates moving in space, or whatever) is putting a force on the charge carrier to make it want to move. A good understanding of static electricity and charges attracting/repelling probably helps.

Electrolysis of something with a coloured ion (potassium manganate was the one we did at school) is a good way to 'see' electricity. Though by the time you understand what you're seeing, I suspect the demonstration is redundant...

Kim · « **Reply #6 on:** 19 October, 2012, 02:09:31 pm »

Quote from: mrcharly on 19 October, 2012, 01:56:36 pm

I guess change in speed when passing a restriction in pipe diameter = voltage drop.

I think the speed of the water would be analogous to drift velocity, which we tend not to concern ourselves with.

The overall flow is current, and will be constant in a closed circuit (run of plumbing from header tank to gutter).

The voltage drop for a given resistance (length of pipe of a given diameter) would be how much potential difference (height difference) is needed to give an electro-motive force (pressure) that would produce a given flow (current) in that pipe.

You can take this too far. Generally once you have more than one transistor (a sluice gate operated by the pressure on a flap), and start requiring water to flow up the hill while losing potential, escher style.

LEE · « **Reply #7 on:** 19 October, 2012, 02:31:35 pm »

Quote from: Kim on 19 October, 2012, 01:58:54 pm

Water and hills and pipe diameters and flows is how I've always described it.

Same here...like an old fashioned flushing toilet with the header tank at ceiling level was how I was taught.

A header tank 1 mile in the air (high voltage) wouldn't do you any harm if the pipe pointing at you was 1mm diameter and only held a gallon BUT a 20 million gallon header tank and a pipe 20feet in diameter would kill you.

Same voltage (height of header tank) but less resistance (big pipe) = more current (errr..current)

It's been a long time though

Pickled Onion · « **Reply #8 on:** 19 October, 2012, 02:43:54 pm »

Quote from: mrcharly on 19 October, 2012, 01:56:36 pm

diameter of pipe affects resistance.

volume of water/time = current

I've always used height of drop analogous to voltage - especially as you can talk about one raindrop falling from a height starting off with a great voltage, but there not being much current. Then relate that to static electricity.

I guess change in speed when passing a restriction in pipe diameter = voltage drop.

The other way of looking at it is voltage is like the "pressure". The speed of the water isn't different either side of the restriction, but the pressure is. The pressure difference is the potential difference is the voltage drop.

So in the analogy your cell or power supply is like a pump, as the water comes out of the pump it has a pressure which reduces passing through each constriction (resistance) until it gets back to the intake of the pump where the pressure is zero.

mrcharly-YHT · « **Reply #9 on:** 19 October, 2012, 02:47:13 pm »

The pump description is good. I'll try that out.

Biggsy · « **Reply #10 on:** 19 October, 2012, 02:55:57 pm »

Restricting the flow of water doesn't reduce its pressure. Cover most of the end of a tap with your thumb......

mrcharly-YHT · « **Reply #11 on:** 19 October, 2012, 03:07:27 pm »

Yes it does. In your example, the water (that has passed your thumb) is under a lot less pressure than it was before it passed your thumb.

Biggsy · « **Reply #12 on:** 19 October, 2012, 03:13:23 pm »

A plumber told me the water is still under the same pressure; it's just the flow rate that is reduced. Does he need to go back to plumbing school?

mrcharly-YHT · « **Reply #13 on:** 19 October, 2012, 03:38:24 pm »

Exit Stage Left · « **Reply #14 on:** 19 October, 2012, 04:20:44 pm »

It's like when you're trying to soak a sibling with a hosepipe. You put your finger over the end of the pipe to restrict the aperture. The jet of water goes further because the velocity has to increase to maintain the flow rate. The pressure increases and the water hurts more when it hits you. The amount of water flowing remains the same.

Change the resitance and the relation of current to voltage changes. Current is how much water there is, voltage is how far you can spray it, and resistance is your finger over the pipe. If you get a high enough voltage the current can travel through air.

perpetual dan · « **Reply #15 on:** 19 October, 2012, 05:35:22 pm »

I think the coulomb-beetle analogy is rather fun. Its a long time ago, so I may be mangling this but...
Beetles carry mars bars around a circuit. The amount of mars bar eaten per beetle to keep the them going is voltage.
The number of beetles per second is current.

clarion · « **Reply #16 on:** 19 October, 2012, 05:47:24 pm »

I always like to think of voltage as potential difference, which provides an instant mental link to potential energy.

mrcharly-YHT · « **Reply #17 on:** 22 October, 2012, 12:49:04 pm »

Quote from: clarion on 19 October, 2012, 05:47:24 pm

I always like to think of voltage as potential difference, which provides an instant mental link to potential energy.

That's the approach I took. The kids are pretty familiar with, and understand potential energy. It links into 'how does a battery work', because you can explain how electrical energy (from charging source) is converted to potential energy by reversing a chemical reaction.

Biggsy · « **Reply #18 on:** 22 October, 2012, 01:13:20 pm »

I'd better stick to electricity rather than water. Don't call me if your taps are dripping!

Here's a question for the kids:

What happens when you connect the "+" wire of a 12V DC fan to +12V power, and the "-" wire to +5V? What is it equivalent to?

I didn't realise the answer until I wasn't a kid any more.

To have a go in a PC, the Molex power wires are usually: yellow +12V, red +5V, black ground. Fan wires are often: red plus, black ground. Don't try with a PWM (pulse-width modulation) fan.

mrcharly-YHT · « **Reply #19 on:** 22 October, 2012, 01:19:29 pm »

Quote from: Biggsy on 22 October, 2012, 01:13:20 pm

I'd better stick to electricity rather than water. Don't call me if your taps are dripping!

Here's a question for the kids:

What happens when you connect the "+" wire of a 12V DC fan to +12V power, and the "-" wire to +5V? What is it equivalent to?

I didn't realise the answer until I wasn't a kid any more. To have a go in a PC, the Molex power wires are usually: yellow +12V, red +5V, black ground. Fan wires are often: red plus, black ground. Don't try with a PWM (pulse-width modulation) fan.

I wouldn't call your plumber, either

The answer depends on whether you are regarding the fan motor as inductive (which it is, really) or resistive.

If you are talking resistive, it's just like a simple series circuit with fan in series with a resistor.

(waits for Kim or Clarion to announce he is talking bollocks)

Biggsy · « **Reply #20 on:** 22 October, 2012, 01:26:10 pm »

The question is really, simply: how many volts is the fan getting? It's for your kids, not you!

The fan is a typical PC case fan.

(I'm probably not wording it correctly in formal terms - because I don't have sufficient formal education, sorry).

Kim · « **Reply #21 on:** 22 October, 2012, 02:08:54 pm »

Quote from: mrcharly on 22 October, 2012, 01:19:29 pm

If you are talking resistive, it's just like a simple series circuit with fan in series with a resistor.

(waits for Kim or Clarion to announce he is talking bollocks)

Assuming no shenanigans with the power supply and that ground is common, it's simply a fan in series with a 7V voltage source.

Standard trick for quietening computers.

mrcharly-YHT · « **Reply #22 on:** 22 October, 2012, 02:23:28 pm »

A 'gamer's trick used to be to have a BFO fan wired in series with a potentiometer. Normally the pot would be turned up to keep fan quiet.

If you were hammering the processor in a hard gaming session, you'd turn the pot down until the fan was screaming like a guitar at a thrash metal concert.

Biggsy · « **Reply #23 on:** 22 October, 2012, 02:31:35 pm »

It's still common for hardcore gamers to use pots. It's ironic not to use their powerful computers to adjust the fans automatically (via PWM). But we digress.

Author Topic: "What is voltage?" (Read 4266 times)

LEE