How does your Quartz Watch Work?

Developments in technology have a profound impact on society which I am sure you all know about. However, at the beginning of its research, these fields are often incredibly esoteric so today I am going to talk about a piece of technology that everyone owns and you may even be wearing one right now: a quartz watch.

Tick-tock, tick-tock

Every single time piece relies on something oscillating with a constant time period. In a mechanical watch, there is a swung balance wheel, when in a quartz watch, it is a tiny quartz crystal, otherwise known as silicon dioxide or just sand. The crystal has piezoelectric properties, so when it is mechanically deformed, whether that is being squashed or bent, a small electric charge accumulates causing a small electric currrent - this is how we create a spark with barbeque lighters. However,  like most things in physics, it can also work in reverse: when we pass a current through the crystal, it will deform. This is what causes the quartz to oscillates and ultimatly keep time.

Piezoelectricity In Detail

This is all a very nice description of what happens, but what is the mechanism that actually causes the quartz to deform when a current is passed through? When an asymmetrical crystal is subject to mechanical stress, there is a change in the overall polarisation and dipole density of the crystal. This is actually quite similar to how capacitors work, which I have currently been studying about in physics. Any spatially separated charge, such as in a dielectric in a capacitor will result in an electric field, therefore an electric potential. In a piezoelectric device, mechanical stress, instead of an externally applied voltage is what causes this charge separation in the individual atoms of the material. When an AC current is applied, the crystal will mechanically resonant and the frequency of the resonance is determined by the physical dimensions of the material, the "cut angle" with respect to the crystalline axis of the original crystal and the ambient temperature.

Why use Quartz?

Many materials exhibit piezoelectric properties, but why was is quartz specifically chosen to be used inside millions of watches today Well it is incredibly abundant in the Earth's crust and apart from its piezoelectric properties, it also has many other attractive properties that make it ideal for use in a wrist watch. The accuracy of a quartz watch is due to its high mechanical and chemical stability. As mentioned above, changes in ambient temperature can change the frequency of the resonance, however, quartz actually has a low temperature coefficient that means that is relatively little change of physical property when the temperature changes. Quartz also has a high Q a resonance which basically means how under damped the oscillator is - A higher Q factor means that there is a lower rate of loss of energy relative to the stored energy in the oscillator and oscillations die out more slowly. Since quartz is also incredibly hard and relatively inert, it you can rely on the  crystal not faulting during the lifetime of the watch.

Measuring Time

So now that we understand why the quartz was chosen and why it vibrates, how do our wrist watches count seconds and tell us the right time? The quartz crystal is cut by a laser into a 3mm tiny fork shape which means that the current from the battery makes it vibrate at 32768hz +/- 0.06hz. This specific number is chosen as it is exactly 2¹⁵ hz (oscillations per second), meaning some simple circuitry can easily use this to determine the time interval between each second. This is also why there are regular pulses per second with the second hand in a quartz watch whilst a mechanical watch moves continuously. Then it's all the simple matter of gears in the correct ratio to move the minute and hour clock.

Conclusion

Once owning a horological device was one of prestige and privilege, but since the perfect piezoelectric properties of quartz crystals were first ulitised in a wrist watch in the 1960s, the price of watches have dropped to the point of disposability. They are  incredibly accurate to within 5 seconds/month, more accurate to an order of magnitude when compared to mechanical watches and only surpasses in long term accuracy by primary atomic stands such as Caesium and Rubidium. I believe that they have truly revolutionised the world we live in and that is ultimately what the whole point of manipulating and using these weird quirks in the natural world to improve human life is all about.

Written by Rebecca Wang

Why Air Bazookas are endless fun

"I'm having so much fun."

Something that I have been curious about for a while are air bazookas. As you can see from my picture from the Big Bang 2012 Young Scientists' and Engineering fair, the air vortex cannon or bazooka is a toy which fires doughnut shaped rings of air - or smoke if you fill up the cavity with smoke - that are strong enough to ruffle hair, knock over cans and scare neighbours pets after travelling several meters. It has always intrigued me how these doughnuts of air can travel so far, are formed so easily and are stable, so I am going to attempt to answer these questions in this blog post today.

Toroidal vortices
One of the main reasons why Air Bazookas are so fun is due to their funky doughnut shaped smoke clouds they produce. When you force a large volume of fluid through a small hole, you will get a toroidal vortex. This sounds very technical and esoteric, but a torus is just the mathematical description of a doughnut shape and a vortex is the spinning motion of a fluid - it's the doughnut rings of air that are fired out of the air bazookas.

How are they formed?

1. The diaphragm of the air bazooka is pulled back and released, suddenly pushing air out of the bazooka. As the air approaches the hole, it has a uniform velocity.
2. As it passes the hole, the air on the edge is slowed down by the drag from the surface of the hole, as well as coming into contact with the stationary air outside the bazooka. This forms a velocity gradient with the air in the centre of the hole/ inner layers having the greatest velocity and the air at the circumference/ outer layers with the lowest velocity.
3. This velocity gradient causes the inner layers of air to "roll" around the outer layers, forming the vortex. Imagine an air molecule towards the outer layer - it is being pushed harder from the inner layers as they are travelling at a faster velocity than the air on the outer layers, giving the air angular momentum and causing it to spin.

Why are they so stable?

Another reason why Air Bazookas are endless fun is due to the fact that the air doughnuts hold their shape for a relatively long time before dissipating. Toroidal vortices are incredibly stable due to the fact that moving fluids exert less pressure on their surroundings than still fluids. Air molecules are constantly colliding with each other and their surroundings as they are a gas, meaing that there is air pressure. When you disturb air, there is still the same amount of kinetic energy, but more is in the direction of motion and less on the surface, meaning the pressure is lower for fluids with higher velocity. I'm not sure if that is a 100% correct derivation, but what we are discussing is Bernoulli's Principle: the fast air moves, the lower the air pressure.

Since the velocity of air inside the vortex is greater than the velocity of air outside, there is also higher pressure surrounding the torus, which squeezes and maintains its shape. This net inwards pressure acts with the same principle as vacuum packaging to make the toroidal vortices very stable.

Why do they travel forward?
A further reason why Air Bazookas are so fun is that you can aim and fire them at other people - for us to understand why we can do this, we must appreciate why they travel forward in the first place. As the flow of air in the vortex "rubs" against the still air, friction causes the torus to move itself forward. This is exactly like how a bicycle wheel works: if you imagine a spinning bicycle wheel which is then dropped on the ground, the friction between the spinning wheel and the ground would cause it to move forward. Another analogy is like how a swimmer pulls himself/herself along through the water.

The energy required to move the ring forward and to keep the vortex rotating comes from the momentum of the rotating air inside the vortex. Air has momentum as it has mass. The spinning air inside the torus is where energy is stored - once all this energy is used up through work done to overcome the frictional forces, the doughtnut of air dissipates.

Why do they travel so far?
The last reason why air bazookas are so fun is that you can aim and fire air doughnuts at people several meters away. The reason why this is possible is the poloidal flow of air in the vortices - the flow of air around the "width" of the doughnut. Since the air is moving, the friction between the vortex and surrounding stationary air decreases, meaning that longer distances can be travelled with little loss of mass and kinetic energy with air doughnuts than if it was just a jet of fluid.

So in conclusion, it's all a matter of fluid dynamics.

I feel like I should start referencing where I get my information from. Here are my main sources:

• Skulls in the Stars
• Arbor Scientific
• San Francisco State University
• ABC
• Wikipedia

Written by Rebecca Wang

Why does boiling water have a lower splashy sound compared to cold water when it is being poured?

During study leave for my AS exams, I have made myself plenty of cups of tea. Something that caught my attention and made me curious was that when you pour boiling water for a cup of tea into a mug, the splashing sound that is created is lower in pitch than if you were to pour cold water.

After typing this question into Google, I did not manage to find a conclusive answer to this conundrum, so I decided to do some research myself. Here a series of questions that I asked myself when trying to work out the reason behind the different splashy sounds.

Why are we hearing a splashy sound in the first place?
I did a quick Google search and found this article by NASA's Earth Observatory: How do Raindrops Make Sound Underwater? The sound generated when a splash of water being poured into a mug is pretty much the same sort of idea as when a raindrop hits a puddle so this article was very useful. The article states that "there are two components to the sound generated by a raindrop splash:  the splat (impact) of the drop onto the water surface and then the subsequent formation of a bubble under water during the splash".

What causes variation in pitch?
The pitch of a sound depends on the frequency of the vibration that causes the sound. Therefore, the higher the frequency, the higher pitched the sound emitted. Also in this particle, they also mentioned the Minnaert Resonance, the acoustic resonance frequency of a single bubble in an infinite domain of water. Obviously the water already in my mug is not an infinite domain of water, but if we ignore the effects of the mug, I reckon Minnaert's equation can still be used. It stated that the frequency of the sound emitted depends on bubble radius, local pressure, local water density and a geophysical constant.

Which one of these variables is causing the variation in pitch? 

• Bubble Radius

From my initial observations, I did not think there was any variation in bubble radius with changing temperature. However, then I thought about it a bit more and realised that water is more "runny" when it is hotter. In more scientific terms, water has a lower viscosity at higher temperatures, due to less hydrogen bonding. Therefore I concluded that at higher temperatures, the bubble radius must be smaller - is this a correct assumption to make?

• Local Pressure

I was mostly pouring the water into my mugs of tea at the same spot in the kitchen so there was probably no significant change in local pressure in the ambient space. Pressure, therefore, becomes a constant.

• Local water density

Water density decreases as temperature increases, so with hotter water, there is a lower local water temperature.

What is my conclusion?
As temperature increases, the bubble radius decreases as it has a lower viscosity. Furthermore, local water density also decreases as temperature increases and that gives us our answer! The bubbles get smaller and the water becomes less dense, giving a higher frequency. But this is the opposite conclusion to the observations so therefore this conclusion is invalid. So, in conclusion, I have not reached a conclusion! I will continue pondering with this conundrum tomorrow. Anyone have any better explanations? I would love to know below.

Written by Rebecca Wang