Author Archives: Alan

Gravitational constant G, the one value that behind “everything”

New equipment for the measuring the gravitational constant G is reported by Li on Nature using two techniques TOS and AFF.

As we all have studied in High school science class or physics class, the reason that an apple will fall from trees, a rocket needs to thrust hot air to the ground to take off and even how can astronauts can ‘fly’ in the middle of air all have to do with gravitational constant G.  The gravitational acceleration is often been mistaken as gravitational constant just as mistaking gravity as the only gravitational force. The gravitational force is the attractive force between any two objects and the force is proportional to the weights of the two objects(assume the distance is a constant) and this proportion is the G. Just like the most common noticed gravitational force we are experiencing, gravity, is actually the attraction between us or an object with earth.

Nowadays, even though there are still some strong arguments on G should not be treated as a constant, it is generally been accepted that Newton’s law of universal gravitation is ‘true’ and gravitational constant can be measured.  Starting from this point of view, getting an accurate gravitational value is crucial since this value has been used for lots of daily life technology and precise aerospace calculations in astronomy.

Uncertainties of current and previous experiments. Made by Stephan Schlamminger

This passage will compare the traditional way of measuring G and a new improved way of doing it developed by a research group lead by Qing Li. The measurement of G is affected by lots of factors such as air, magnetic field and more importantly other objects that are near the equipment. For the reason of presenting so much factors, the uncertainty of the results is very large as reported by Mohr the uncertainty is 47 parts per million. While in Li’s group, they achieved recorded the smallest uncertainty of 14 parts per million while the largest uncertainty is 550 parts per million larger.

In the early days, the first successful measurement of G was done by Cavendish in 1798 and the part that is hanged by string is two connected spheres in a dumb-bell shape as you can see from the video below.

But in Li’s group, they built a “two plate-containing torsion balances” which uses two plates to replace the spheres to improve the precision. Also what worth mention is they used a fused silicon dioxide (silica) fibers with high-quality factor of the torsional oscillation mode (Q) to reduce the anelastic effect. And with all the other improvements together they managed to obtain the smallest uncertainty.

The instruments made by Li’s team. Source

This experiment can potentially benefit a lot of area of work by providing a more accurate fundamental constant value. The accuracy of work and research from the benefited field can also be improved.

Battery for future electric cars

In recent years, the car industry got a huge revolution for the blossoming of electric cars. More and more people put attention on electric cars for its quietness while running and amazing acceleration ability. Also, since electric cars are driven by electric motor, means no refuel on the gas station thus no direct CO2 emission, this environment-friendly property has been greatly spread by major media.

Source: C.-X. Zu & H. Li Energy Environ. Sci. 4, 2614–2624 (2011)/Avicenne

 

However, there are two major problems dragging the wide application of electric cars back. First, the cost of making the batteries for electric cars are extremely high. Winfried Wilcke, heads of IBM’s nanoscience and technology division got interviewed and said: “battery packs for electric cars cost more than $500 kWh−1“. While, in comparison, it is reported by The Union of Concerned Scientists, the cost of making electric cars are comparable to gasoline-powered cars if the cost for battery packs is between 125 and 150 kWh−1. Second, the range of electric cars from one charge is significantly smaller than gasoline cars. The range of electric cars is averaged at around 150mi while gasoline cars are around 400mi.

An electric car made by Tesla Photo by Jp Valery on Unsplash

 

For solving those problems mentioned above, research groups around the world are trying different solutions and developing new batteries. Magnesium-ion battery and  Sodium-oxygen battery are two of the approaches scientists working on. The Magnesium-ion battery uses magnesium ion that can carry the double amount of electrons of lithium carries and it migrate in pairs. In total, magnesium ions can carry four times more charges that currently used lithium ions.

For the sodium-oxygen battery, it can only provide half of the range supported by lithium-oxygen battery but 5 times more than the lithium-ion battery. Also, sodium is cheaper than lithium, therefore the cost of making battery pack to provide the same amount of range can be cut down to as low as 1/5 as the lithium-ion battery.

Adapted from Tao Liu, Gabriella Bocchetti and Clare P. Grey

As mentioned above, the lithium-oxygen battery has an amazing energy density to make the long-range electric car become reality, but as reported by a group in the University of Cambridge, the impurities in the air can clog the electrode of the lithium-oxygen battery and this damage the battery after few dozen charges. But researches are investigating the reasons behind it and finding solutions to solve it.

Image