We start with the puzzle. One of the photos in this video is wrongly labelled, deliberately. Can you spot which one it is? The solution will be given near the end of the video. Welcome to this Bethany Birdie video on orders of magnitude. For IB Physics, you're expected to use powers of 10 and to compare quantities using ratios. Here is the statement from the IB Physics Guide. And here is a typical multiple choice question on this topic. What is the best estimate for the ratio of the diameter and atom to its nucleus? You should be able to answer this by the end of the video. The rest of this shark is 19,000 kilograms approximately. When we're talking about orders of magnitude, we only look at the powers of 10, in this case 10 to the 4. In other words, whale sharks are measured in tens of thousands of kilos. You may have realised that if a whale were 90,000 kilograms, it would actually be closer to 10 to the 5 kilograms. In fact, the dividing line is between 3 and 4 times 10 to the 4, but IB will usually give you values which are clearly one or the other. Here are two orders of magnitude. A typical animal cell is 10 to the minus 5 metres across, while our galaxy, the Milky Way, is 10 to the 21 metres. One of the advantages of orders of magnitude expressed in powers of 10 is that you can quickly compare two quantities. In this case, our galaxy is 10 to the 26 times larger than the cell. There are many relevant orders of magnitude in physics, and it's hard to know all of them. You would find it very useful to know these different sizes though. A nucleus has the same order of magnitude as a proton or a neutron, 10 to the minus 15. The atom is typically 10 to the minus 10 metres, also known as an angstrom. The earth and sun are respectively 10 to the 7 and 10 to the 9 metres across, and it's useful to realise that the sun is at 100 times larger than the earth. The distance to Proxima Centauri is 10 to the 18 metres, and the universe that we can detect is 10 to the 27 metres across. The examples in this video have all been distances measured in metres. IB exams will sometimes ask you to state some other quantities. You can't memorise them all, but there are other ways. Some information is found in the data booklet, such as the rest mass of an electron. Or you can calculate a quantity, for example by converting 13.8 billion years to seconds. Or you can estimate a quantity, for example by assuming a 5 watt charger takes an hour to charge a mobile phone. Here is the solution to the question we asked earlier. IB expects you to know the orders of magnitude of atoms and subatomic particles, namely 10 to the minus 10 and 10 to the minus 15. The ratio is therefore 10 to the 5 with the larger number on top, and you know what 10 to the 5 is. In other words, an atom is this much larger than its nucleus. And the solution to the puzzle here, as to which photo is wrongly labelled, is that the image of the Milky Way was in fact the Andromeda Galaxy, which is quite similar so far as we know. But we are in the Milky Way, so we can't take pictures of it.