Electricity to heat experiment

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melissa
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Electricity to heat experiment

Post by melissa »

:coffee: A teacher ordered 15cm of Nichrome wire per student to construct an element. Students were to coil wire into a spring shape , attach alligator clips to either end suspend it in a 50ml beaker of water and hook up to transformer (DC 12 volts) and a lamp. Lamp came on but no change in water temp after 1 hour. Not sure if it actually was Nichrome (looks the same) also tried Cu wire without success. Does anyone have any ideas? :coffee:
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sunray18
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Re: Electricity to heat experiment

Post by sunray18 »

we have just done this. I made the coils and checked them by connecting them, in air, to the power at low voltage. They heated up immediately. Either the lamp is the problem or the wire is not nichrome. We didn't use a lamp.
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DavidPeterson
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Re: Electricity to heat experiment

Post by DavidPeterson »

We do this regularly and never have any problems - heats water very quickly.

We never have a lamp in the circuit. Other than to prove to the students that there is a current running through the circuit I see no point. It would, however, eliminate any chance of short circuiting if the wire coil you used was not providing enough resistance (eg too short, or not thin enough).
Maybe the lamp is drawing to much current of it's own and there is not enough resistance in the wire to heat the wire and hence not heat the water. What size globe was in the circuit? - when I use lamps for simple circuits pracs, they are normally 12V 0.1A.

If I was you I would try without a lamp in the circuit.

A side note: The prac in the text book we use says to not use a beaker but a polystyrene cup :? - problem is, as soon as the kids realise the wire is heating, they use the heated wire to cut through the polystyrene - so I always give glass beakers!
Last edited by DavidPeterson on 17 May 2012, 10:42, edited 2 times in total.
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dime
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Re: Electricity to heat experiment

Post by dime »

Yep, we have never put the lamp in the circuit either. It has always worked.
As a side note to the polystyrene cups, that is how you cut polystyrene, with a hot wire (power going through a wire). The resulting smoke is toxic. Just goes to show that text books aren't always right or practical.
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DavidPeterson
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Re: Electricity to heat experiment

Post by DavidPeterson »

Yes, I know about the polystyrene and they banned cutting it with hot wire years ago. I don't like the texts we have been using for many reasons, but now with the Aust Curriculm coming it gives us an excuse to ask for more money (outside our budget) for new texts.

Let's hope that the new ones are better - I redesign or 'tweak' 2/3 of the ones in the current books.
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Ocker
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Re: Electricity to heat experiment

Post by Ocker »

The reason for Polystryene cup is for insulation
You are making a calorimeter sit glass beaker inside cup
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Re: Electricity to heat experiment

Post by Lorikeet »

Ok - So I am jumping on this thread - cause the concern is how hot the wire gets whether in glass or polystyrene ...
Anyone have a good alternative for this prac ...
With thanks Kate :coffee:
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Ocker
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Re: Electricity to heat experiment

Post by Ocker »

So you have thick copper or brass rods through to the water and connect with screw lugs, to the resistance element.
The heating only occurs in the element, P=IE thats power in watts equals current in amps times voltage in volts
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Re: Electricity to heat experiment

Post by bigmack »

Ocker wrote:So you have thick copper or brass rods through to the water and connect with screw lugs, to the resistance element.
The heating only occurs in the element,
Brilliant idea Ocker :thumbup:

Just so happened to find some of these screw connectors in the back of the "nuts and bolts " draw .
Works a treat . ( water not added for clarity)
setup.jpg
Now when it comes to Nichrome wire , we have 4 different sizes in our storeroom . The size of the wire has a dramatic effect on the length that is needed for the prac to work . One issue is that if the length of wire is too short or is of too lower resistance then the power supplies tend to trip out which is a problem .
So by finding a suitable length that will pass about 5 Amps , the power supplies will not trip but enough current will flow to allow the water to heat sufficiently to get a good result in a short amount of time .

Here is a length of 28SWG ( Standard wire guage ) Nichrome wire .To limit the current to 5 Amps it is only 165mm long .
You can see that it creates a huge amount of heat .Note that the heat is evenly spread over the full length of the wire.
28 SWG.jpg
Here is length of 23 SWG nichrome wire . it is 1125mm long to pass 5 Amps ....but...it is creating as much heat as the 28SWG wire , its just spread over a longer length .......that's why it doesn't look so hot .
The point here is that if you had to bend these wires over and out of a beaker of water , the "bent " bit would be cooler than had you used the 28 SWG which is obviously very hot on the same length the "bent bit" would be
23 SWG.jpg
So from what I found in my store room .
28SWG (0.375mm ) needs to be no shorter than 165mm long for 5 amps
23SWG (0.610mm) " " " " " " 1125mm " " "..............( Note that this wire come from a different supplier and has different electrical characteristics , that's why it needs to be way longer than the 22SWG )
22SWG (0.712mm) " " " " " " 710mm " " "
18SWG (1.22mm) " " " " " " 1600mm " " "

As general rule the thicker the wire , the longer it will need to be . But also I found that Nichrome wire from different places has slightly different resistance per meter .

The 22SWG stuff from Serrate seems a good size that doesn't need to be so long that it tangles and shorts .
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Ocker
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Re: Electricity to heat experiment

Post by Ocker »

Yes I always used the 22# wire and rolled it round a pencil, now the heating effect is P=I E Watts = Amps × Volts this is constant regardless of what size wire/length you use
bigmack
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Re: Electricity to heat experiment

Post by bigmack »

Ocker wrote: the heating effect is P=I E Watts = Amps × Volts this is constant regardless of what size wire/length you use
That is an undeniable fact .....but as the Current is directly related to Voltage and resistance ( I = E/R ) , and resistance is determined by length and diameter of conductor ( R = p x L/A , where R= Resistance ,p = specific resistance of the conductor , L = length ,A = Cross sectional area )then these have a direct relationship to Power .

At the end of the day , we need to provide a suitable length of a suitable sized wire so that the class can carry out the prac in a reasonable time frame and obtain a worthy result . IE , the piece of Nichrome wire should have a resistance around 2.5-3 ohms .This will provide the most heat in the shortest amount of time when connected to a typical school Power pack on 12V .

If the resistance of the nichrome wire is too low , then the power supplies continually cut in and out . This upsets the time it takes to heat the water .....not to mention overheating the power packs . :whistling2:
If the Resistance of the Nichrome wire is too high , the power will be so low that the period may well be over before a notice in temp change occurs .
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