One Way Street

I’m a fan of walking, but as such that means that I’m very often crossing the road. Now you’d think that the main danger would be cars, but it’s not! It’s bikes. It’s very rare you see a car going down a one way street the wrong way. And if they do, it’s very often with a look of cringing embarrassment with a side of well-I’m-pointing-this-way-now!

Bikes on the other hand are a different matter. They go everywhere, whether it’s the wrong way down a one way street or not, regardless of how many half asleep people there are making their way to work! Now if only there was a better way of making sure that people didn’t do things like that than just putting up signs.

If only there were diodes for people. Diodes are an important electronic component, and they work by only letting electrons flow through them in one direction, so they are effectively a one way street for electrical current. This means that you have to insert them into your circuit the right way round, otherwise it will never work!

You can see from the electrical symbol for the diode which way it works, and when you see a diode, there is a flat side to it that corresponds with the flat side of the triangle, or a direction written on it. Diodes work by having a very low resistance in the correct direction, and a very high resistance in the opposite direction. These days, the type of diode most commonly found is the light-emitting diode, normally shortened to LEDs:

Ah, you say, LED TVs are the next big thing! So now you know that inside these new pieces of technology are lots of diodes, whose one-way system I would like to borrow for our streets!

Helpful Decay

It seems like every time that radioactivity hits the news it’s a bad story, with nuclear power plants bearing the brunt of the public’s distrust of all things radioactive. But you’ve probably got at least one thing in your house that works by radioactive decay, and there are loads of other useful applications too.

I wrote before about radioactive decay here but I didn't calrify the three types, alpha, beta and gamma. They are all created by processes which are very similar, just the end product is slightly different. These rays are characterised by how easy it is to stop them. If you think about a light beam, it’s not stopped at all by thin glass, but a really thick piece of glass will make it much dimmer. And it can’t get through walls or anything solid like that.

These rays are just the same, there are things that they can pass through, and things that stop them. Alpha rays are the weakest, and are stopped by less than a mm of lead, or even a very thick piece of paper. Beta rays need a few cm of lead to stop them and gamma rays need even more. Lead is used as the standard measurement here because it’s a very dense material, offering plenty of bulk to absorb all that radiation.

 

Firstly, this is good to know if you want to be protected from radiation when you’re working in close quarters with a source. But that’s not all it’s useful for.

Alpha radiation, because it’s so easily stopped, is very often used in smoke alarms as the detecting device. This works with an alpha-source on one side of an air gap and a detector on the other. In normal use, the radiation reaches the detector just fine. But if the air gap fills up with smoke particles, this is enough to stop the rays, and the alarm sounds.

Beta radiation is more commonly used for testing the thickness of things during manufacturing processes. The source is placed one side of the product and the detector on the other. If it gets too thin then too much radiation will get through, and if it gets too thick then not enough will get through. The manufacturing line can then be adjusted accordingly either way.

So, whilst we can easily be led to believe by the news that radiation is bad, in fact it can be quite useful! It’s just a matter of treating it with respect and making sure that nuclear waste is very carefully looked after!

Compare the Meerkat

Those comparison websites are everywhere these days, with varying degrees of annoyingness I have to say. But when it comes to comparing electricity prices, or working out the amount your appliances use it’s pretty simple to do the calculations yourself, you just need a few tools to decode the information you’ll get from the companies.

We’ve previously come across energy being measured in Joules (J) but those canny electricity providers give their prices for kWh, that is, the price for an hour of electricity if you use 1000 Watts. It’s simple to convert Joules to kWh though using the simple equation:

Energy (in kWh) = power (in watts) x time (in seconds)

1 kWh = 1000 x (60 x 60)

            = 3 600 000J

Now where it gets tricky are all those crazy numbers and dials on the electricity meter, so it’s usually best to wait for a bill to arrive to get all the details off that. On there you'll find a current meter reading, a previous reading, the units used, plus the cost per unit. Nowadays when it’s all worked out you’ll also have a nice huge cost amount as well!

So, it’s simple to work out the price once you’ve got all the details:

Now let’s see how much it costs to run a lamp for 15 minutes. If you want to work this out for one of your items, have a look round it and check out the label. There should the a value on it in watts (W).

So now you have it, you’re as good as a price comparison site, if not better, since you can work out how much it costs to use any of your appliances in your own house! Well done little meerkat.

The Sea Breeze

Ok, so it’s a little late in the year to be going to the beach but I just can’t resist the salty charms of the seaside! I love feeling the sand between my toes, hearing the gentle crashing of the waves and feeling the wind in my hair. Well, actually most of the time the wind is really unnecessary. It’s not like we very often get to sunbathe and it gets so hot we’re glad for a breeze. But that doesn’t matter to the weather!

It’s always windy at the beach, even when it’s quite calm inland, because of the way that air moves when it gets hot. In the daytime the sun heats the land up faster than it heats the water up. This in turn heats the air above the land. Hot air is lighter than colder air since the molecules are moving faster, making it less dense, so it rises. Cold air from out to sea then rushes in to fill this gap.

As evening falls and the sun starts to lose its heat, the opposite happens. The land cools down a lot faster than the water, so now it’s the warm air out to sea that rises. This creates a breeze in the other direction.

This is an example of convection, which is the way that hot areas of gases and liquids move around. This all happens because of a change in density as the molecules get warmer, as well as the need to keep the space all filled up – there can’t be any unfilled space!

Mirror Mirror On The Wall

We take it for granted that the mirrors in front of us show us a true reflection of the world, but it’s not always the case. Like in the funfair’s hall of mirrors, just a small deviation from a true and flat surface can squish, stretch or otherwise change the appearance of the reflected object. But thankfully in Physics we like to assume that our mirrors are always perfectly flat. That is to say we think about the fact that they might not be, but then to simplify our own lives we’ve decided to assume they are.

As an aside, an assumption is not the same as neglecting to think about certain aspects of your experiment that may have an effect. It’s merely a way of simplifying complicated things, based on deciding to ignore, temporarily, the effect of something. Always state your assumptions, so that you can justify them, or go back and revisit them easily.

But back to mirrors, now we have a perfectly flat one in front of us, what can we see? Our own faces and the objects around us are crystal clear, only in reverse, but how far away are the things we can see? Are they true to the world around you, or just a projection, like a TV screen?

If we imagine looking at a mirror more from the side we can perform a simple experiment to see what happens.

So the real object and its virtual image appear identical, and as far away from the mirror as each other. This is because the angle of incidence (incoming ray) and the angle of reflection (reflected ray) are always the same with a pure and flat surface. The object distance u is therefore equal to the image distance v. There are loads of ways to test this our yourself, so why not have a go?

A Perfect Copy

Photocopiers are a bit of a mystery aren’t they? A big, hot box with flashing lights and whirring noises that can churn out copies of your document almost faster than you can blink! But how does it work? Well, we’re back to that old favourite, electricity again!

It’s all about static electricity once more, which is the small charge which can make balloons stick to the wall (or cats) and can even make your hair stand on end! It’s easy to create a static charge on plastic items like rulers, and if you’ve ever felt a spark from a metal object that’s thanks to static too.

Sometimes these charged objects are fun, but they can also be hazardous too, like if you were to experience a spark from a static charge when you were near to fuel, for example. This could get very explosive very fast. But it can also be useful and in this case, it’s vital to making copies – fast. Let’s lift the lid and see what happens inside.

A plate inside the photocopier is negatively charged all over, and this is the centre of the copying system. That bright light you see when the copier makes the copy is because the machine relies on a projection of the image you’re copying. And that image gets projected onto the charged plate. It’s made of a special kind of material so that the light areas lose their charge, but the dark areas keep it.

The powdered ink (toner) is then applied to the plate, and it sticks to the negatively charged bits, where the dark on the image is. A piece of paper is then pressed onto the plate. It’s heated so that the powdered ink melts and sticks to the paper.

All of this happens in super quick time, so it seems incredible that so many processes are involved! And it’s all thanks to the humble electric charge.

Testing Testing 1....2....3

In the past, when things like huge steel bridges were the latest technology, sometimes they wouldn’t quite work how they were meant to. Every now and then it was a disaster. It was a puzzle for engineers, who were trying their best to make things safe. One man decided to make a difference, a Scot called Kirkaldy.

He built the world’s first testing machine that could be used to create like-real-life testing situations on large bits of the construction materials. And on Sunday I went to the museum where it’s kept. The Kirkaldy Testing Museum is a small and unassuming place, remarkable only if you happen to notice the “Facts not Opinions” motto above the door.

By all accounts, he was a rather opinionated guy, with everything he said (well, relating to testing of these large building parts anyway) meticulously based on the results of his tests, which he wrote down in a huge leger so he could check back and show people how right he was. He even kept all the pieces he’d tested to destruction until the sheer weight was in danger of bringing down the building!

The tests he carried out were of two types, one a test to stretch the item (tensile test) and another to squash the item (compressive test). These are the two main types of forces that area on major beams in construction, particularly in bridge building:

The clever part was, that although he could only make his machine go one way, as it was all about water pressure, he managed to both push and pull the samples. He did this with an ingenious method, adding an extra “space” for testing the items in compression (red), the other side of the stretching part (blue).

For a time, Kirkaldy was the go-to man for any sort of material testing, with his all-singing all-dancing machine the star of the show. It was important to test whole girders, or whole joists, because you couldn’t be sure of the material quality, because manufacturing methods for making the metals were poor. But as these improved, tests could be done on smaller and smaller samples, making this beast obsolete.

The museum has a great collection of such testing rigs that are smaller and more compact, for testing littler samples. And today we have electronic machines, capable of getting results so accurate Kirkaldy himself may have had to concede defeat. But none are quite as important as that gigantic pioneering machine!