Units and Sig. Figs…

With my holidays almost at a close (but you still have masses of time so enjoy it!) I thought it would be worthwhile posting this from Hank Green. It is the second video of a series on Chemistry from the YouTube channel – Crash Course. It is a new channel to me but looks really interesting as the production values seem higher than many.

In this video Hank explains the importance of units and then gives you some rules for how many significant figures your calculations should give. It is good stuff.

(N.B. – before you correct the video, since 2019 the kilogram has been defined as the mass of the Plank constant measured using the Kibble balance – but the IPK is fine!)

I really like Hank – he is also a rather accomplished musician. Remember the ‘does it make sense’ test – think about the number you have produced and check that it seems to fit with what you know about the world.

At 6 minutes 44 seconds into the video, Hank says, “I only know the speed of my car to 2 decimal places” – did he mean 2 significant figures or perhaps nearest whole number? I think so.

At 10 minutes he does the sum 60 x 5.0839. He says that we know this to 2 significant figures therefore his answer should be 310 because the 5 rounds up. 60 could be to one significant figure but he says that there are two. I had a play with this significant figures calculator. (If you want to put in scientific notation you can write 6.0 x 10¹ as 6.0e+1). It tells you how many significant figures there are in the question and calculates the answer.

Hank’s rules for significant figures…

• When adding or subtracting it is only the numbers after the decimal point that count and they should be the same as the least in the question.
• When multiplying, your answer should have as many significant figures as the least in the question.

There are a couple of other posts on similar topics you might also read…

Degrees Of Confidence

Standard Form

Question…

1. How many cells are there in a 9 volt battery (a PP3 like the one that Hank pushes against his tongue).
2. Why can you feel the tingle from a battery on your tongue but if you touch the terminals with your hands you don’t feel anything?
3. What is the SI unit of work?
4. When we did the “Who is the most powerful?” experiment, how did we calculate the work done?
5. In UK English, what is the difference between metre & meter?
6. Hank also gets the Graphic Design team’s name wrong – he calls them Thought Bubble, what is their correct name?
7. Write 6012 in scientific notation.