Help With 14 Year Old's Physics Questions

[ericmark]

Although the current is double it uses 4 times the power.
A hair drier designed to run on 110volt at 450W would have a resistance of 110²/450 = 26 ohms plugged into 230volt then 230²/26 = 2Kw so with 4 times the power something has to go pop very quickly. (1967.355W is nearly 2Kw and by time you allow for the fact that the element is coiled and the frequency has dropped from 60 to 50Hz it would likely be 2Kw) also just to compound the problem even further the fan will also be going slower.

But if you got a UK 2Kw electric heater and connected this in series with the 450W USA hair drier the hair drier would work OK. So the work required to overcome the voltage is not hard to correct but if we take a clock designed to work in USA to get that to work in the UK would be a lot harder so the frequency is a much bigger problem than the voltage.

 

[blondini]

As yet I haven’t touched ohm’s law which explains cable sizes but I would think there is enough to confuse the kid without including that.

Stop giggling at the back!

 

[ban-all-sheds]

As yet I haven’t touched ohm’s law which explains cable sizes

I am trying to explain to my sister all about Ohm's law.

Why is that?

but I would think there is enough to confuse the kid without including that.

Job done eh? You chose to put all that material in..

 

[securespark]

Oh dear! I think I've muddied the waters a bit....

I was explaining how if voltage goes down, the current goes up for any given load. Also, I was explaining that that is how the national grid distributes electricity at high voltage & low current, meaning they don't have to string cables round the country that are as thick as tree trunks.

But she can't square that idea with:

Increase the pressure ( voltage ) and more water ( electrons ) flow through the same pipe.

Or this:

As V increases, so I increases.

 

[bernardgreen]

Oh dear! I think I've muddied the waters a bit.... I was explaining how if voltage goes down, the current goes up for any given load.

It doesn't go up unless the resistance is reduced. If the resistance is constant the current also reduces as the voltage reduces.

Watts = Volt x Amps so if a constant wattage is to be dissapated at the load then the current must somehow be increased if the voltage is reduced.

The only way to increase current with reducing voltage is to reduce the resistance of the load to allow the lower voltage to push more current through the load.

But she can't square that idea with:

Increase the pressure ( voltage ) and more water ( electrons ) flow through the same pipe.

The amount of water flowing is not the power.

A small jet of water at high pressure will do more work cutting through dirt than a bucket full poured gently over the same dirt.

Or this:

As V increases, so I increases.

Only true with constant resistance loads As V increases I increases and Power increases. To keep constant Power the resistance has to be changed. The resistance of the UK 450 watt hair drier is different from the resistance of the American 450 watt hairdrier

 

[blondini]

Or this:

As V increases, so I increases.

Only true with constant resistance loads As V increases I increases and Power increases. To keep constant Power the resistance has to be changed. The resistance of the UK 450 watt hair drier is different from the resistance of the American 450 watt hairdrier

R is assumed to be fixed, it is the resistance of the appliance.

Just keepin' it simple

 

[securespark]

It doesn't go up unless the resistance is reduced. If the resistance is constant the current also reduces as the voltage reduces.

So what is the best way of explaining how the NG works?

And how can it be best explained how a 50W lamp running at 12V draws in XS of 4A?

 

[ban-all-sheds]

There comes a point when analogies of hosepipes and water break down.

There comes a point where you just have to accept that conductors have resistance, that a voltage across one will cause current to flow, and that the relationship between voltage, resistance and current is as described by Herr Doktor Ohm.

I'd suggest some simple experiments/demos to demonstrate.

Voltage? Yes it's a bit like water pressure - more volts is stronger pressure. Here - this is a 1.5v battery, with a wire on each end. Put them on your tongue. Here's a 9V one. Put that on your tongue. Feel the difference?

The following will be all you need to actually show Ohm's law in operation:

1 x 6V or 12V battery
1 x voltmeter
1 x ammeter
1 x 1kW radiant electric fire element, not quartz enclosed
1 x 6V/12V lamp in a holder
1 x metal rod with a threaded end and a couple of nuts to fit
wire to connect it all up.

The metal rod slid up and down the fire element will give a nice visible variable resistance, you can show the voltage changing and the current changing and the lamp glowing brighter and dimmer.

 

[Adam_151]

I always thought going back to the unit definitions made sense.

If you remember that a watt is defined as one joule a second, and a joule is a measurement of enegy that can do a bit of work (eg. 4.19 of them can raise the temperature of one millilitre of water by one degree celesus)

Along similar lines, an ampere is define as one coloumb a second, and a coloumb specific number of electrons

Again, on similar lines, a volt is one joule per coloumb.

So thats were AmpsxVolts = watts comes from

If you have a specific amount of watts to transfer, you can either do it with lots of columbs with not many joules each (high current, low voltage), or less coloumbs with more joules each (low current, high voltage)

If an anology is needed, consider that you have fruit to move, you could send it on many trays spread out (low volatage) in the back of many trucks (high current), but the disadvantage of this is it takes lots of trucks and lots of fuel and is inefficent)

Or you could pile it high (high voltage) and send it in less trucks (low current), this is more efficent, but if someone knocks it over when uploading, they might be injured by the high voltage (fruit) falling on them (electric shock)

.... Its the best I can come up with at this time of night

 

[ericmark]

The current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.

To make it easy given translated version. Original will have been in German of course.

You are of course correct in saying I had muddied the waters there are no formulas in ohm's law they came latter.

 

[plugwash]

Do bear in mind that ohms law is true if and only if the load is purely resistive, many loads aren't.

 

[ericmark]

Are you are returning to "War of Currents" and yes that is of course the whole point and Nikola Tesla did after all win this war.

Ohm's law is really all to do with DC. And I am sure if one was to put enough thought to it one could introduce some turbulence into the old tank with two pipes demonstration to show how with AC there are more factors.

But where would you stop? Kirchhoff's laws, Thevinin's or Norton's? I have already muddied the water now I am trying to pour on a little oil. With as few PCB's as possible!

 

[blondini]

Where to stop?

The OP is about GCSE physics, that eliminates all the fancy stuff.
If you connect a 3 volt bulb to a 6 volt battery it will blow the bullb. Same thing will happen if you connect a 110 volt bulb to a 230 volt supply, and for the same reason.