# I Units and Concepts

1. Jul 28, 2016

Hello, I'm new here. I have a question that I don't think many people can answer. In my Engineering Design class last school year, we use physics on a daily basis. My teacher goes over units and where they come from. He also goes over fundamental measurements like mass and time. He stressed to us the difference between units and concepts, like how the unit for gravitational potential energy is the Joule (J) in the SI system, but the concept is represented by a capital U. Another example is the Coulomb (C) and (I think its concept is) Q.

So after all that, what I'm asking is does anyone know all or most of the units and their matching concepts, or know of a chart that shows it? I've looked with no luck. Thanks.

2. Jul 28, 2016

### Staff: Mentor

What you call concept seems to be a physical quantity like energy or charge. Their magnitudes are called dimensions (not to be confused with the mathematical term, although related). The units are the degrees in which these measurements are compared to.

quantity: energy - dimension: $ML^2T^{-2}$ (mass M, length L, time T) - units: $J = NM = \frac{kg\cdot m^2}{s^2}$
quantity: electric charge - dimension: $IT$ (current I, time T) - units: $C = As$

Here are listed some: https://en.wikipedia.org/wiki/Physical_quantity

3. Jul 28, 2016

### Andy Resnick

Speaking as an instructor, I caution you against this approach for a variety of reasons. Assigning a particular letter/symbol to either a unit or physical quantity is essentially arbitrary; the same letter is often used for multiple quantities. Concepts first, names second.

I like fresh_42's reminder that you should be able to deconstruct any unit into a combination of M,L,T, and sometimes C. Being able to do this will help you understand the underlying concepts: *any* unit of energy *must* have units of ML2T-2, and if you forget that, all you need to remember is that kinetic energy is 1/2 mv2. Similarly, the units of magnetic field B must be MT-1C-1 (why?)

4. Jul 28, 2016

Okay, I've never heard it called a physical quantity. That's exactly what I'm talking about. Thanks.

5. Jul 28, 2016

I understand derived units and where they came from. Thank you. And yes, I don't fully understand the purpose of physical quantities other than to represent them in equations. Otherwise it's pretty arbitrary.

6. Jul 28, 2016

### Staff: Mentor

Units are arbitrary, e.g. miles per hour versus kilometers per hour or Celsius versus Fahrenheit. Velocity and temperature, however, are basic and not at all arbitrary. Wait until you're driving too fast through a curve or burn your hands on the oven. I once had to do an emergency break while driving. I can tell you, momentum and energy aren't abstract or arbitrary either.

7. Jul 28, 2016

I'm not sure I understood what you said then. I saw it as you calling physical quantities arbitrary, not the units themselves. Trust me when I say I understand and will never underestimate momentum or any other form of force or energy.

8. Jul 28, 2016

### Staff: Mentor

As you see at the example, measurements are arbitrary. No physical meaning forces us to use Kelvin. And Celsius was originally defined upside down: boiling at 0°C and freezing at 100°C. This is pretty arbitrary. Of course there are pros and cons to the conventions we made. Otherwise they wouldn't last very long. Or did you ever have heard about Delisle?

The temperature on the other hand is a physical quantity. It can be experienced, and measured. Whether we measure it in Kelvin or Delisle is a matter of choice. Units are only the scales on our rulers, resp. apparatus with which we compare things. Whether you achieve 10 yd or 9.144 m doesn't matter for a new down, it's the same physical quantity length.

9. Jul 28, 2016

### houlahound

10. Jul 28, 2016

### David Lewis

Unit names, such as joule and coulomb, are not capitalized. The symbols for units named after individuals are upper case or (when the symbol consists of two letters) capitalized.

11. Jul 29, 2016

Okay, I understand what you're saying now. To slightly change the subject, people have said that time is the only true measurement, because things like length were determined by the things we had around us. But at the same time, isn't that true for how we measure time? Now, excuse me for not taking the proper time to research this, but who's to say noon starts when it does? Besides the fact that the sun is highest in the sky around that time, why is noon where it is? Who determined how long an hour is? I get it that 60 is an easy number to work woth, but that doesn't excuse my point. Why do we measure in 24 hours (approx.)? No one told us when a certain time has reached, or to count in 24 hours. It seems like it was just a convenient way to measure time. I get that without time we don't exist, but doesn't this mean that the way we measure/experience time is fake?

12. Jul 29, 2016

### houlahound

Convenience based on celestial events.

We don't measure time, things happen in time that we measure.

13. Jul 29, 2016

### symbolipoint

This is very good post.

14. Jul 29, 2016

### davenn

xx

15. Jul 29, 2016

### Staff: Mentor

To be pedantic: symbols are only uppercase when derived from a proper name: e.g., J for joule and K for kelvin, but m for meter and s for second

16. Jul 29, 2016

### davenn

Hahaha, I pretty much wrote that and then deleted it in my previous post

Thankyou for the confirmation

Dave

17. Jul 29, 2016

### Staff: Mentor

To quote from The International System of Units (SI) published by the Bureau International des Poids et Mesures:

18. Jul 29, 2016

So we've basically made everything convenient for ourselves. Thanks.