Yearly Archives: 2012

Why kettles boil slowly in the US

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I saw a tweet recently that intrigued me:

http://twitter.com/#!/yoz/status/191445005414567937

The voltage of mains electricity varies from country to country: the majority of countries use between 200 and 240 volts, but a small minority (most notably the US, Canada and Japan) use between 100 and 127 volts.

Countries using 100-127 volts are shown in red; countries using 200-240 volts are shown in blue. Countries with a mixture of the two systems are shown in purple.

The voltage* of an electrical supply is what pushes electrons around in a circuit. The higher the voltage, the faster the electrons move and thus the higher the current (one amp is equivalent to about six billion billion electrons flowing past a point per second). With a low voltage the rate of transfer of electrical energy is therefore much slower. In the UK, with a mains voltage of 230 V and a limit of 13 A per socket the maximum possible power to one appliance is 2990 watts (2990 joules per second). In the USA, with a mains voltage of 120 V and a limit of 15 A per outlet the maximum possible power is reduced to only 1800 watts, which is why in the US many large appliances (e.g. washing machines, tumble dryers) have to be connected to a separate high-voltage circuit.

To raise the temperature of one litre of water from 15°C to boiling at 100°C requires a little bit over 355 kilojoules of energy. An “average” kettle in the UK runs at about 2800 W and in the US at about 1500 W; if we assume that both kettles are 100% efficient† than a UK kettle supplying 2800 joules per second will take 127 seconds to boil and a US kettle supplying 1500 J/s will take 237 seconds, more than a minute and a half longer. This is such a problem that many households in the US still use an old-fashioned stove-top kettle.

* As a physicist I would normally use the term “potential difference” in place of “voltage” but voltage is better understood by the general public. Looks like the engineers (who prefer “voltage”) won that battle.

† As electric kettles actually use the joule heating effect that is responsible for most of the energy wasted in other electrical devices this isn’t a terribly unfair assumption.

Poverty and the wind

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A map of poverty* in London clearly shows a clustering of poorer areas to the north-east of the city.

There is a very simple reason for this, and it’s the same reason that poorer areas are found towards the north and east of most large and old towns in the UK: the prevailing wind.

Because of its position to the north-east of the Atlantic Ocean, the prevailing wind in the UK is from the south-west (i.e. blowing north-east). Any atmospheric pollution produced in London – and in the 1800s and 1900s that was be a lot of pollution – would be blown to the north-east, making that area less attractive and therefore cheaper to live in.

You can explore poverty in the UK using the interactive Google Map below, which I found via a story in The Grauniad:

* The data used is the 2007 Index of Multiple Deprivation, and the mapping is by London Profiler.

Why you can’t open aeroplane doors in flight

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There have been many stories of people trying to open aeroplane doors whilst the aeroplane is in flight. Below is an explanation of why you shouldn’t worry if this happens during your flight.

The cruising altitude of most transport aeroplanes is about 38 000 feet (11 600 metres). At this altitude there is less air above the aeroplane pushing down upon it and thus the air pressure is lower. At 38 000 feet the air pressure is about 21 kilopascals (21 000 newtons per square metre) compared with 100 kilopascals (100 kPa) at ground level.

At this altitude there wouldn’t be enough oxygen present in the cabin air for people to breathe (this is why aeroplanes carry oxygen masks, in case the cabin depressurises for some reason). Therefore the cabin has to be kept pressurised to a greater level, usually to the equivalent of about 8000 feet (2440 metres). At 8000 feet the air pressure inside the cabin is about 75 kPa, more than three times the exterior pressure at cruising altitude. There is therefore a difference in pressure between the interior and exterior of the aeroplane of 54 kPa, or 54 000 newtons per square metre.

The passenger doors on a 747 (for example) are 1.19 metres wide by 1.93 metres tall, giving the door an area of 2.3 square metres. This means that there is a force of 124 000 newtons (54000 N/m2 × 2.3 m2) pushing the door closed. To put this in perspective, a force of 124 000 newtons is equivalent to the weight of 12.6 tonnes; so unless a passenger is some sort of Superman, capable of exerting a force bigger than this, the door will remain closed.

Aeroplane doors are wedge-shaped so that the fuselage bears this force, relying on the pressure differential rather than on some sort of internal locking mechanism to keep the door closed and maintain a good seal. The same is also true for spacecraft doors.

Thanks to JU for asking the question that prompted this post.

What are contrails?

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Contrails are artificial clouds that form behind aircraft flying at high altitude.

Jet fuel (also known as AVTUR for AViation TURbine fuel) is made up of long-chain hydrocarbons, from nonane (C9H20) to hexadecane (C16H34). When these long-chain hydrocarbons are burnt they combine with oxygen to form carbon dioxide and water. The water vapour in the plane’s exhaust condenses out (sometimes as ice) to form trailing artificial clouds known as contrails.

Military aircraft often have to be careful to avoid leaving contrails as it makes them very easy to spot. This can be done by adding chlorosulphuric acid to the exhaust, as the chlorosulphuric acid reacts with water to form sulphuric and hydrochloric acid which doesn’t condense out in the same way as water. However, it is usually easier for the pilot of the aircraft to simply decrease altitude until contrails cease forming.

Three different types of magnetism

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When you think of magnetism the chances are that you’re only thinking of one type of magnetism: ferromagnetism. But there are two other types of magnetism: paramagnetism and diamagnetism, that are less well known.

Ferromagnetism is the only type of magnetism that produces forces large enough to be easily felt, and ferromagnetic materials are the only ones that demonstrate spontaneous magnetism – magnetism outside of an applied magnetic field. The most common ferromagnetic materials are those that contain iron, cobalt and nickel but other elements such as dysprosium and gadolinium and compounds such as chromium oxide and manganese bismide also demonstrate ferromagnetic properties.

Paramagnetic and diamagnetic effects only exist in the presence of an applied magnetic field: paramagnetic materials such as tungsten and aluminium create an attractive force when exposed to magnetic fields and diamagnetic materials such as pyrolytic carbon and mercury create a repulsive one.

A small sheet of pyrolytic carbon levitates above an array of neodymium-iron-boron magnets.

Water is weakly diamagnetic, about forty times less diamagnetic than the pyrolytic carbon shown above, but this is enough that light objects which contain a large amount of water can be levitated if placed in a very strong magnetic field.

This frog was levitated using a 16 tesla Bitter electromagnet at the High Field Magnetic Laboratory at the Radboud University Nijmegen in the Netherlands.