Thursday, June 11, 2015
Electromagnets
A coil of wire is called a ‘solenoid’. A solenoid with a magnetic core is an 'electromagnet’.
We wound insulated copper wire round a steel nail, making 25 turns, and connected the ends to a power supply - 2 x 1.5V batteries. Our electromagnets could pick up 2 drawing pins. We investigated how to make them stronger. We found that increasing the number of turns off the wire, or increasing the voltage of the power supply, made the electromagnets stronger. The strongest could pick up 18 pins.
The steel nail is the core of the electromagnet. Ideally this would be made of iron, or another 'magnetically soft’ material, so that it was not magnetic when switched off.
To test how much difference the coil and the core made, we tried separating them. Our champion electromagnet could pick up 18 drawing pins, but when we removed the core and simply ran current through the wire, it couldn’t pick up any pins. The core alone, with no current, retained enough magnetism to pick up one or two pins. However, once reassembled, the electromagnet was much stronger. This demonstrates the purpose of the magnetic core of an electromagnet, which is to increase the strength of the overall magnetic field.
Thursday, May 21, 2015
Properties of Metals - thermal expansion and thermostats
Today we were investigating properties of metals. We looked at the classic properties of metals first. Metals are:
BBC Bitesize Properties of Metals
Quizlet - quick quiz on properties of metals to revise .
- Lustrous - shiny when polished or freshly cut
- Flexible - they bend before breaking, when hit.
- Ductile - they can be drawn into wires
- Malleable - can be hammered into sheets
- Sonorous - make a ringing sound when hit, rather than a dull thud.
- Electrical conductors - electricity can flow through them
- Thermal conductors - energy can flow through them, causing heating.
Things to look at:
BBC Bitesize Properties of Metals
Quizlet - quick quiz on properties of metals to revise .
Friday, April 17, 2015
Density, Thermal Insulators, Metals
Density
Density is an important concept and we used it to sort and identify polymers in our last session. This time we explored the idea further. First, Alison placed a can of Coke and a can of Diet Coke in water. The Coke sat lower in the water than the Diet Coke, showing that Coke is more dense, ie has more mass per unit volume. Each cubic centimetre of Coke weighs more than Diet Coke.
We discussed how to find densities of other objects. Measuring the mass is easy if you have a good balance, but for density we also need the volume - the amount of space the object takes up. With a regular object like a cube, it’s easy to calculate volume by measuring the sides and multiplying height x width x length, but what about irregular objects like the lumps of Mystery Metal that Angie had found? We tried to calculate the density by placing samples in a displacement vessel and measuring how much water was displaced. This could work well but we found that, if the sample was very small, the water in the displacement vessel just “bulged” and didn’t overflow. This means the surface tension on the water was able to absorb the extra volume. So, we ideally would need to know what range of equipment was accurate for.
Properties of metals
We had twelve rectangular pieces of sheet metal, each labelled with a letter. We passed them round and tried to work out what each mystery metal was, and how we might investigate that without damaging the sample. Using magnets, observing the colour, and comparing the masses and feeling how easily they conducted heat, we were able to deduce most of them - but there were some tricky ones, eg non-magnetic steel. Our samples included tin, copper, brass, steel, iron, zinc, lead, bronze.
Thermal Insulators
Thermal insulators are materials which don’t allow heat to pass through them easily, whereas thermal conductors do. We wanted to investigate which was the best insulator :- bubble wrap, woollen cloth, or aluminium cooking foil. Each group was provided with three identical empty cans and the insulation materials, hot water, thermometers, and sticky tape. Groups designed their own method of testing and chose how to write up their results. For example, one group produced bar charts showing how much the temperature changed over 30 seconds, while another took several measurements over 10 minutes and charted the rate of cooling. We discussed how our tests might be biased or inaccurate, eg :
- was it easier to coat the can in one insulating material compared to the others?
- Did we have to remove insulation to take temperature readings, and if so, did heat escape while we did it?
Our findings were consistent overall, that bubble wrap was the best insulator, followed by wool, and then cooking foil last of all. This was a surprise for some people because aluminium foil is used in cooking, but it’s used for a different purpose - usually to help distribute heat evenly over the surface of food, to stop it burning.
For a recap on thermal insulators, and a quick quiz, see BBC Bitesize on Heat .
Thursday, February 12, 2015
Polymers and Hydrogels
Hydrogels are polymers which can absorb large quantities of water. They are alsol known as “smart” polymers, because they can change shape quickly in response to changes in their environment. Examples include hair gel, water-retaining gel, eg in disposable nappies or plant pots, and soft contact lenses. Alison demonstrated hair gel’s excellent qualities on Tiny. One
source explains:
Method: We had Boots own brand, which cost 18.5p per nappy, Pampers, which cost 26p each, and Naty “natural” nappies, which were 28.2p. We pulled the nappies apart and extracted the polymer granules, then added distilled water and measured how much water each collection of granules could absorb.
Results: Boots own brand absorbed 650ml ( so the cost of nappy divided by the liquid absorbed gives us 0.028p per ml), Pampers absorbed 800ml (0.0325p per ml) and Naty absorbed 680ml (0.041p per ml), although one group had an unexpected result of 900ml.
Conclusion: If all you’re concerned about is absorbency, the Boots own-brand nappies were best value for money. However, there might be other factors which would influence your decision, such as whether the nappy fitted well, or environmental factors.
RSC experiment on hydrogels - discusses the chemistry behind the hydrogel.
“Hydrogels are networks of long polymer chains that attract and store large volumes of water. It is not uncommon for 90% of a hydrogel to be water. As water molecules are attracted and stick to the polymer chain network, this causes it to swell forming a gel.”
Hair Gel Collapse Experiment
We added salt to hair gel and found that the gel shrank and just a little liquid was left.
Salt replaces water in the polymer structure of the gel, because it’s more strongly attracted to the polymer than water is. This is why you should choose your styling products carefully if you want to look cool in the sea.
University of Warwick : Hair Gel collapse experimentPolymers : Disposable Nappies
We looked at a type of hydrogel, found in disposable nappies, and tested which of several brands of nappy was best value for money in terms of absorbency.Method: We had Boots own brand, which cost 18.5p per nappy, Pampers, which cost 26p each, and Naty “natural” nappies, which were 28.2p. We pulled the nappies apart and extracted the polymer granules, then added distilled water and measured how much water each collection of granules could absorb.
Results: Boots own brand absorbed 650ml ( so the cost of nappy divided by the liquid absorbed gives us 0.028p per ml), Pampers absorbed 800ml (0.0325p per ml) and Naty absorbed 680ml (0.041p per ml), although one group had an unexpected result of 900ml.
Conclusion: If all you’re concerned about is absorbency, the Boots own-brand nappies were best value for money. However, there might be other factors which would influence your decision, such as whether the nappy fitted well, or environmental factors.
http://lavenderpondnotes.tumblr.com/post/118965025616/polymers-disposable-nappies
Catalyst -a magazine for teenage chemistry students - has a nice article about hydrogels which discusses medical applications.
Things to look at:
RSC experiment on hydrogels - discusses the chemistry behind the hydrogel.
Identifying Polymers
We used density to identify polymers:
Last month we synthesised a polymer, nylon, and this time we investigated some other types of polymer. We talked about what polymers are - long molecules formed when lots of smaller molecules join together, end-to-end. They occur in nature, eg rubber, DNA, hair and nails are polymers. We often think first of man-made polymers though, and these are known as “synthetic polymers”, such as nylon, PVC etc.. These can present challenges in waste disposal because they don’t break down, hence the concerns about the environmental hazards of plastics.
We tried to identify some common polymers by comparing their relative densities. Each group had six liquids of known densities in test tubes, and a selection of small pieces of polymer - most of which were raided from the recycling bin. We placed a sample of each polymer into each liquid and noted whether it floated or sank. If it floated, it was less dense than the liquid, and if it sank, it was more dense. In this way we were able to identify a density range for each sample, in grams per cubic cm (g/cm3). We looked at a table of common polymers and their densities, and tried to work out which each was. Finally, we looked at the recycling symbol which had been secretly kept back from each sample, to see if we were right.
The RSC provided the following table::
Polymer Density range/g cm -3
EPS - expanded polystryrene 0.02 - 0.06
PP - polypropylene 0.89 - 0.91
LDPE - low density polyethylene 0.91 - 0.93
HDPE - high density polyethylene 0.94 - 0.96
PS - polystryrene 1.04 - 1.11
PVC - polyvinyl chloride (PVC) 1.20 - 1.55
PET - polyethylene terephthalate 1.38 - 1.40
When we checked our results, we’d correctly identified many of the polymers, but some were a little outside the ranges stated in the table. The RSC say: “But note that materials made of polymers may also contain other substances as fillers, plasticisers, stabilisers, etc., which may make the density of a particular sample fall outside the ranges indicated; note the wide range of PVC densities in the table above.”
It’s important to be able to correctly identify polymers in order to recycle them, and in fact industrial mixed recycling facilities use a process a little like this to sort polymers. This BBC podcast about carbon, polymers and recycling includes a visit to a recycling plant to find out how it’s done.
Some other links to recap and take you further on this topic:
RSC Identifying Polymers - Classic Chemistry Experiments
The Polymer Party - short video about polymers, good fun.
From DNA to Silly Putty: the diverse world of polymers. - TED Ed video , including environmental concerns.
BBC Bitesize on Polymers - recap and activities
Fantastic Plastic - from Catalyst, a science magazine for teenagers. “ This Catalyst article looks at the use of polymers in the manufacture of household items. The exciting thing about polymers is that it is possible to make polymers behave in so many different ways by organising their long chain molecules in different ways – polymers are the ultimate designer material. The article looks at their molecular chain and also the future for polymers in manufacturing.”
Robert Krampf : Polymers and Slime - video from the popular science teacher with fun activities.
Subscribe to:
Posts (Atom)