Help With 14 Year Old's Physics Questions

[securespark]

I am failing in my uncle-y duties.

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

My niece has a question for her homework which goes,

Why won't an electrical appliance designed to be used in the US work in the UK?

I am finding it difficult to explain without getting too technical!

Can anybody help her, please?

 

[robertito]

Basically because the USA mains is 110v, compared with our 230v. So at best a fuse would blow, at worse it could be really dangerous.

 

[bernardgreen]

The easiest way would be to use the water analogy of electricity to demonstrate ohms law

The amount of water ( electrons ) that can flow through a pipe ( wire ) depends on the pressure ( voltage ) the difficulty ( resistance ) the water ( electrons ) has in getting through the pipe.

The smaller the pipe ( higher resistance per metre ) and the longer the pipe ( more metres of resistance ) the greater the difficulty ( resistance ) and so less water ( electrons ) pass. Increase the pressure ( voltage ) and more water ( electrons ) flow through the same pipe.

Ohms law applies to resistive loads such as heaters where the resistance is constant. ( it isn't in practise but lets not confuse the person )

As mentioned the USA equipment is designed to work on 110 volts and the UK supply is 230 volts. So twice the voltage ( pressure ) means twice as much current ( water ) will flow which will damage the equiment.

Also twice as much pressure could cause a pipe to burst ( insulation to break down ) letting water ( electricity ) go where it shouldn't and do more damage.

Some electrical equipment is designed to work at both 110 and 230 volt. In these the "resistance" inside is automatically adjusted to allow the right amount of electricity to flow so Ohms law while still applicable is confused as the resistance is not constant.

 

[securespark]

Thanks bernard.

Can you explain in layteenager's terms what would happen to a US appliance (for the sake of argument, a hairdryer) when plugged into 230V?

 

[wotan]

Basically because the USA mains is 110v, compared with our 230v. So at best a fuse would blow, at worse it could be really dangerous.

Also equiptment in the USA operates at 60 Hertz we in the UK operate at 50 Hertz.

Wotan

 

[Taylortwocities]

That's correct, Wotan but I do not remember there being an F factor in ohm's law

Can you explain in layteenager's terms what would happen to a US appliance (for the sake of argument, a hairdryer) when plugged into 230V?

The US appliance is designed to work on 110volts and we have 230volts

so a hairdryer's heating element will get over twice as hot as its supposed to and will probably burn out.

The motor will also probably burn out for the same reason - its speed is a factor of frequency but the windings will be expecting only 110volts, not 230..

 

[bernardgreen]

Can you explain in layteenager's terms what would happen to a US appliance (for the sake of argument, a hairdryer) when plugged into 230V?

Assume 450 watts heater in the American hairdryer. So at 110 volts that will need approx 4 amps Watts = Volts X Amps

To get 4 amps at 110 volts the resistance of the heater has to be about 25 ohms. Volts = Resistance x Current

Plug the hairdryer into 230 volts.

Current is about 9 amps ( which hopefully would blow the fuse )

Power in watts = 9 x 230 = approx 2000 watts, more than 4 times the power intended.

Double the voltage means 4 times the power.

 

[blondini]

For schoolboy er schoolgirl physics just use the Ohms Law equation I=V/R.

R is assumed to be fixed, it is the resistance of the appliance.
V is either 110 or 230.
I is the current drawn.

As V increases, so I increases. If a US appliance designed to operate on 110 volts is connected to a 230v supply it will draw way too much current, and as Bernard says wayyyyy too much power, It will overheat and fail.

 

[Taylortwocities]

Current is about 9 amps ( which hopefully would blow the fuse )

No it won't, its a US appliance, no fuse..........There might be one in teh adaptor but that would be 13A..

 

[Deleted member 18243]

The answer, simply, is:

"because of the different supply voltage".

 

[Deleted member 18243]

Incidentally, does anyone know if it is true that Americans don't have electric kettles?

I heard that because of the low supply voltage, the current required to heat a kettle of water would mean having an unfeasibly large cable.

Whenever I've been in the USA I've never seen an electric kettle, in homes or hotels, but there is usually a stove top kettle.

 

[rebuke]

Talking about US to UK adapters reminds me of a rather scary one someone showed me once: http://www.cl.cam.ac.uk/~mgk25/exploding-plug/

 

[Taylortwocities]

That's a fairy tale

http://www.amazon.com/Chefs-Choice-...6?ie=UTF8&s=home-garden&qid=1235233261&sr=1-6

for example.

But note its only 1500watt so it'll take much longer to boil than ours.
Thats how they get around having too big a cable. & limits the current at the socket.

 

[VanSolo]

Loved the 'Exploded' and 'Detonated' in that article. Far more dramatic than the usual 'blew' or 'popped'.

 

[ericmark]

Disregarding voltage as in the USA they do have a split phase supply so some items are 220 volt and in the UK 110 volt transformers are readily available we have to look at the frequency of the supply. Except for some odd exceptions where the USA still using DC. The USA uses 60Hz where in the UK we use 50Hz this means motors including those used in clocks will run slow.
To correct this is very hard. One could turn the AC to DC and back to AC, which is done with some items, but it is very costly.
A secondary problem is voltage as many items used in the USA are designed for 110 volts to earth so when used on a UK 110 volt supply the switches and fuses do not protect the item should a fault occur so there is a high risk of fire if the item becomes faulty.
Many items can cope with the different voltages and frequencies sometimes with change over switches and sometimes, with switch mode power supplies, which are able to run on AC or DC from 100 volts to 250 volts. Because of this adaptors are made so a standard USA plug and be plugged into a UK socket and visa versa. Since not all items can handle this though it means it is easy to burn out USA equipment by powering it on the wrong voltage.
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.

The thing I find odd is how everyone seems to realise about the voltage but miss the frequency which is a lot harder to correct and also seem to gloss over the dangers with single pole switching and fusing.

There are quite a few auto-transformers which will correct the voltage and with earthed equipment they work well. But the USA plug in the same way as many of the continental plugs is not polarity linked when used for double insulated items without an earth, so using these the wrong way around can again result in a un-fused and un-switched leg which can be 110 volts to earth.

Now are you any better? And we still have not redressed the different supply types with “Hot legs” etc.

I am sure the kid will have had it explained and there will be something, which the teacher will expect as an answer, and the problem is a little knowledge is dangerous.

When my boy was at school he was asked what were the two types of transistor to which he answered Bi-polar and Field Effect. Which he was told was wrong and you have NPN and PNP. Yet in another class he was doing at the same time to get his amateur radio licence he was asked how to calculate watts for which he answered volts x amps and was marked as wrong as he had missed out power factor correction. So sorry to say how ever hard we try it is bound to be wrong!

He did however pass his RAE at 14 so he at least could understand it at 14 even if some of his teachers didn't! He is now an electrician and 29 years old so maybe teachers have been re-trained now?