Category Archives: Learning & Teaching

Safety first!

1 Reply

cornerMost of us walk up and down stairs at home without a second thought and often without holding the handrail. It’s a personal choice to hold the handrail or not. However, for some when you are at work it is no longer a personal choice but a health and safety rule. You must hold the handrail and in many organisations you are expected to politely ask visitors to do so. This is justified on the basis that trips/slips and falls are the most common sources of workplace injuries accounting for 40% of serious injuries. For managers it is about managing risk and reducing costs.

Risk is the probability of something happening multiplied by the consequences when it does happen. Many of us subconsciously calculate risk when we make decisions in everyday life. The consequences of the aircraft crashing on the way to your holiday destination is very serious, if not fatal, but the probability is extremely small so that overall the risk is acceptably low. We make lots of risk assessments in our personal life but as soon as an organisation gets involved and feels that it might be liable for the consequences then our freedom of choice is eroded quickly. Hence, the instruction to hold the handrail on the stairs. However, the equation is changed when the cost of reducing the risk involved in an essential or profitable activity is too high or perceived to be so. A simple example would be being free to stand on a platform within half a metre of a passing express train. It would be too expensive and probably impractical to install railings or remove everyone from the platform. However, at least we have platforms and are not allowed to wander around on the track; that would be really dangerous with both a high probability of being hit and fatal consequences as the Liverpool MP William Huskisson found out at the inauguration of the first scheduled passenger train service on September 15, 1830. When the train stopped on the way from Liverpool to Manchester, he got out and walked down the track to the Prime Minister who was in the next carriage to enthuse about the service and he was killed by the train going the other way. There are easier ways to get a street named after you, not to mention a town in Australia!

Source: http://www.workplacesafetyadvice.co.uk/common-injuriescauses-accidents-work.html.  BTW – according to this website, the finance is the safest sector in which to work and agriculture the most dangerous sector.

Photo credits: Sarah & CharlesPicture8

Dream Car?

Leave a reply

f1When you are young most people would like a car with the performance of a racing car. When you get a little older and have children then you want something with the strength of an armoured personnel carrier in order to keep them safe. By the time you are old enough to have grandchildren, you are worried about whether the world’s resources will still be round for them and you would like a car with the fuel efficiency of the ETH PAC Car II which at 15,212 mpg, or 0.01857 litres/100 km holds the world record for fuel efficiency. In my case the compromise is a Volkswagen Golf, which is an example of the engineering conflict resolution between cost, structural integrity, performance and sustainability discussed in last week’s post [‘Conflict Resolution’ on June 25, 2014].

My colleague who lives in Zürich has an alternative resolution to the conflict. He does not own a car. Instead he has bought shares in a car sharing scheme, Mobility. If he needs a car then he selects a car on-line from those available in the nearest car-park and there 2500 available from 1200 in Switzerland. When I visited him a few weeks ago he picked up a sporty BMW 1 series by using a smart card in his wallet to open the car and said if he had been on his own then he would have taken the little SMART car parked next to it. As my Swiss colleague said if you are the car you drive then he can be a different person everyday.

Energy efficiency

2 Replies

We were sent a summary of our annual gas and electricity consumption recently by our local utility company. The utility quantified our consumption of both gas and electricity in units of kilowatt-hours (kWh). It is usual to be sold electricity in kilowatt-hours but most people are confused by this unit. Perhaps because they learnt at school that the units of energy are Joules in the SI system and the power rating of appliances is usually given in Watts. They might know that a Watt is a Joule of energy per second, so what is a kilowatt-hour? Well it is about 3.6 x 106 Joules or 3.6 MJ, because it is 1000 Joules per second (= Watt) for one hour. So, I think the utility company should be telling me how many MegaJoules (MJ) we have consumed. After all we are used to seeing the energy content of our food quoted in kiloJoules (kJ), as well as calories.

The situation with our gas consumption is rather different because the utility does not supply energy but gas. The amount of energy that I get from it depends on what I do with it. If I burn it under conditions of constant volume, e.g. in a closed rigid container with exactly the correct concentration of oxygen then it will generate more energy in terms of heat than when it is burnt in constant pressure conditions, such as at atmospheric pressure in air. This is because in constant pressure conditions some of the energy released by combustion is used to expand the exhaust gases against the constant pressure, i.e. to do work, and only what is left is released as heat. So the utility should sell the gas by weight. If they sold it by volume then I would be paying more for the same amount of gas (i.e. number of hydrocarbon molecules) when the supply pressure was reduced.

Oil companies don’t sell gasoline or diesel in Joules for the same reason but they can sell by volume because it is always supplied to our cars at atmospheric pressure and the volume of a liquid is essentially constant.

We like to compare the efficiency of cars in terms of miles per gallon, or kilometres per litres. Efficiency can be loosely defined as what you want divided what you have to put in [See my post entitled ‘National Efficiency‘ on May 29th, 2013]. So for a car, what you want is kilometres travelled and what you put in is litres of fuel. However, when we are all driving plug-in electric cars then we will probably talk about how many kilometres per megajoule our car achieves [see my post entitled ‘Are electric car back?‘ on May 28th, 2014] . Unfortunately, while we are in transition with plug-in hybrids, car manufacturers like to quote very attractive kilometres per litre and ignore the electricity supplied via the plug – as if it were free!

Image courtesy KKN Liebstadt NPP from http://www.nucleartourist.com/systems/ct.htm

Watched kettle never boils

Leave a reply

boiling kettleThe phrase ‘a watched kettle never boils’, or a watched pot as Americans might prefer say, is a familiar phrase.  We have probably all stood waiting for water boil thinking it is taking a long time.  This might be in part because the rate of boiling does indeed slow down during the heating process and then speed up towards the end.

When an electric kettle is first switched on the element in the bottom of the kettle heats up causing heat to be transferred by conduction to the water.  The water adjacent to the element rises in temperature becomes less dense, moves towards the surface and transfers heat by natural convection to the contents of the kettle.  As the temperature of the water rises, tiny bubbles form on the element due to local boiling.  Bubbles are dislodged by new ones forming and float up to the surface giving the appearance that complete boiling is imminent.  However, as the temperature rises further the element becomes completely covered by a film of vapour that insulates the element from the water and slows down heat transfer to the water.  This delays boiling until the element has pumped enough energy (heat) into this film for heat transfer to occur across it from the element to the water. Sections of the film tend to break away and belch onto the surface of the water.  This process of large bubble formation and belching on the surface usually establishes itself fairly quickly once the first one has broken free and we see the familiar violent boiling of the kettle.

So the watched kettle has boiled but only after what might have seem like an interminable delay.  If you have a transparent electric kettle then you can watch this happen, otherwise you could watch a YouTube video – possible the most boring video on YouTube?

The process described above is known as the Liedenfrost effect and is illustrated graphically in the chart below, which is based on Figure 6.16 in ‘The Design and Simulation of Thermal Systems‘ by NV Suryanarayana and Oner Arici published by McGraw-Hill.  There are a number of more comprehensive explanations available, for example by Jearl Walker.  The Leidenfrost effect is also responsible for the way water disperses in liquid droplets across a very hot surface instead of evaporating as steam, see this Youtube clip for more explanation.

boiling graph